Atomic Structure — Print · Grade 8 Science

Atomic Structure

Learning goals

I can identify the three subatomic particles and describe their charge, mass, and location in the atom.
I can use an element's atomic number and mass number to find its numbers of protons, neutrons, and electrons.
I can distinguish molecular and extended-structure models.

Readings for this unit

A container that works once

Before this unit

Count positive and negative charges; distinguish an object from the particles making it.

Use three teaching sessions of about 40–45 minutes: first observe the phenomenon, draft a model and study Parts 1–2; next explain Parts 3–4 with the figures and evidence; then work the application, examples, practice, model revision and exit check. Divide any difficult part into smaller steps and revisit after feedback. Investigations, review and quizzes have separate sessions. Study the four visual parts across the scheduled sessions, then use the illustrated worked case and independent transfer questions. Extend or revisit a session when the learner cannot explain the figure; completion time is a proposed allocation, not a mastery guarantee.

Start with something to explain

What would explain the observations in atomic structure?

Illustrative evidence case. Two supplied particle records contain A: 8 protons, 8 neutrons, 8 electrons; B: 8 protons, 10 neutrons, 10 electrons. Neither record is a microscope image.

This oxygen-16 counting model locates protons and neutrons in a nucleus and assigns electrons to a cloud region. It is not to scale: the relative sizes and distances, as well as electron behavior, are omitted. The numbers describe one neutral isotope, not every oxygen particle.

Notice and wonder

If only electrons rise to 10, what entries change? Write two observations from the supplied record without guessing their cause. Include a quantity and its unit when given. An observation is what was recorded; an inference is what you think explains it.

Write one question about why the pattern occurs and one question about a comparison that could distinguish competing explanations. A testable question identifies what could be observed or measured.

Your first model

Before reading the model answer, sketch the named objects or particles and label what you think changes. Compare element identity, mass number and charge. Explain what changed and what stayed the same; name one limitation of a particle drawing. Keep this first version so you can compare it with your later explanation.

Build the language from the idea

atom: The smallest unit of an element that still has the properties of that element.
proton: A positively charged particle found in the nucleus.
neutron: A particle with no charge (neutral) found in the nucleus, alongside the protons.
electron: A negatively charged particle that moves around the nucleus in an electron cloud.
nucleus: The dense, positively charged center of an atom, containing the protons and neutrons.
atomic number: The number of protons in an atom; it identifies which element the atom is.
mass number: The total number of protons plus neutrons in an atom's nucleus.
isotope: An atom of an element distinguished by neutron count; isotopes share proton count.
ion: An atom or group of atoms with net electric charge because positive and negative charges do not balance.
model: A representation that explains selected features and makes predictions; it leaves other details out.
evidence: Relevant observations or records used to support or challenge a claim.
system boundary: The chosen limit separating the objects being studied from their surroundings.
assumption: A condition taken as true for a model or calculation and needing to be checked against the case.

All ordinary materials, including the air around us, are made of atoms or groups of atoms. An atom is a constituent of an element. Its nucleus, the very small central region, contains positively charged protons and, for most isotopes, electrically neutral neutrons. Negatively charged electrons are described by a surrounding electron cloud. Charge is a property associated with electric interactions; a proton’s positive charge and an electron’s negative charge have equal magnitudes. A neutron has no net electric charge. Hydrogen-1 is a useful exception with one proton and no neutron.

The nucleus contains nearly all the atom’s mass and is much smaller than the atom’s overall extent. A simplified stadium comparison illustrates a size difference but is not a photograph. Electrons are not tiny planets following visible circular tracks, and atoms are not literally hard empty containers. An electron-cloud model describes where an electron may be detected; a classroom dot drawing is mainly a counting aid.

What makes an atom a specific element — carbon, oxygen, gold — is simply the number of protons it holds. That count is the atomic number, and it never changes for a given element: every atom with 6 protons is carbon, and anything with 8 protons is oxygen, no exceptions. Change the number of protons and you literally change what element you have. In a neutral atom, the number of electrons exactly matches the number of protons, so the positive and negative charges cancel out and the atom carries no overall charge.

The mass number counts the protons and neutrons together, since those two supply essentially all of an atom's mass. Once you know the atomic number and the mass number, you can work out all three particle counts for a neutral atom: the protons equal the atomic number, the electrons also equal the atomic number, and the neutrons are simply whatever is left over — mass number − atomic number.

Neutrons are the one particle that can vary without changing the element itself. Every carbon atom has 6 protons, but one might carry 6 neutrons and another 8 — these different versions of an element are called isotopes. Getting comfortable with counting protons, neutrons, and electrons is the foundation for almost everything else in chemistry, from understanding why each element sits where it does on the periodic table to predicting how atoms will bond and react.

Both shown carbon atoms have six protons and six electrons. Their neutron counts differ, so their mass numbers are 12 and 14. An ion instead differs in electron count. This counting model distinguishes an isotope from a charged atom and is not a drawing of literal electron paths or relative nuclear sizes.

Mass number counts protons plus neutrons in one atom. Atoms of the same element can have different neutron counts: these are isotopes. The decimal mass on a periodic table is an average for a mixture of isotopes, not a count of particles in one atom.

A model drawing is useful for counting particles, but its circles are not actual paths followed by electrons. Label the nucleus and electron cloud, and explain what the model leaves out. Use proton count first when identifying an unknown atom.

Build your understanding

Where this fits

This unit helps answer the chapter question: How can the properties of a material help us explain it and choose a useful application? Keep a notebook table with the question, evidence, explanation and one unresolved issue. Revise a previous model only when you can name the new evidence or idea that changes it.

Part 1 · Use a model at an invisible scale

An atom has a very small nucleus containing protons and neutrons, surrounded by an electron cloud. The cloud describes where electrons are likely to be found; electrons are not tiny planets following the circles on a classroom drawing. A model helps us discuss something too small to see with an ordinary microscope.

Read the visual model

What useful information can a bead model show, and what does it leave out?

Visual reasoning — after your attempt

It can show particle counts, locations, and charges. It does not accurately show relative sizes, distances, electron behavior, or literal particle colors.

Draw a nucleus and label the three particle types and charges. A large circle around it is a counting aid, not a photograph. Real atoms do not have the colors used in a bead model, and particle sizes and distances are usually not drawn to scale.

OpenSciEd diagram linking a solid bath bomb and water before mixing to gas bubbles and liquid after mixing, with empty magnification circles for the learner’s particle model. — This OpenSciEd diagram connects visible substances and bubbles with magnified spaces for a particle model. The circles are places to represent an explanation, not photographs of atoms. Use it to distinguish what can be observed from what a particle model proposes; the bath-bomb reaction is explained later in the chemistry chapter. Bath-bomb matter model at visible and particle scales · OpenSciEd · 7.1 Chemical Reactions & Matter, Lesson 1, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.

Pause and explain

What useful information can a bead model show, and what does it leave out?

Check your reasoning after answering

It can show particle counts, locations, and charges. It does not accurately show relative sizes, distances, electron behavior, or literal particle colors.

Part 2 · Identify an element by its protons

The number of protons determines the element. Every oxygen atom has eight protons, regardless of how many neutrons or electrons it has. Atomic number is this proton count. A neutral atom has equal positive and negative charges because it has as many electrons as protons.

Carbon and oxygen are compared with their proton, neutron and electron counts labeled. Six protons identifies carbon; eight identifies oxygen. The cloud and nucleus sizes are deliberately schematic and electrons are not drawn on fixed planetary tracks. A neutral atom balances proton and electron counts.

Read the visual model

Read p⁺ as positively charged protons, n as neutral neutrons and e⁻ as negatively charged electrons. Element identity and net charge answer different questions. Changing only electrons cannot turn oxygen into carbon.

A particle has 8 protons, 8 neutrons and 10 electrons. Is it carbon, oxygen or a new element? Explain its charge.

Visual reasoning — after your attempt

It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

For a neutral atom with six protons, six electrons cancel the six positive charges. Six protons identify carbon. Removing an electron changes its charge, but the nucleus still contains six protons, so the element is still carbon.

Pause and explain

A particle has eight protons and ten electrons. Is it oxygen, and is it neutral?

Check your reasoning after answering

It is oxygen because it has eight protons. It has two excess negative charges, so it is an ion with charge −2, not a neutral atom.

Part 3 · Count neutrons without confusing two masses

Mass number counts protons plus neutrons in one atom. Subtract atomic number from mass number to find neutrons. Isotopes of an element share proton count but differ in neutron count. The decimal atomic mass printed on a periodic table is an average for naturally occurring isotopes, rather than the particle count of one atom.

Count model for the supplied records: all three have eight protons. A and B differ in neutrons; A and C differ in electrons. Boxes are records, not atom sizes or electron paths.

Read the visual model

How do carbon-12 and carbon-14 remain the same element?

Visual reasoning — after your attempt

Both contain six protons. Carbon-12 has six neutrons and carbon-14 has eight; the different neutron counts make them isotopes of carbon.

A carbon atom with mass number 14 has six protons and 14−6=8 neutrons. A neutral carbon-14 atom also has six electrons. Do not round a table average and assume that every atom of that element has the resulting neutron count.

Pause and explain

How do carbon-12 and carbon-14 remain the same element?

Check your reasoning after answering

Both contain six protons. Carbon-12 has six neutrons and carbon-14 has eight; the different neutron counts make them isotopes of carbon.

Part 4 · Keep element identity during ordinary changes

In ordinary chemical reactions, atoms rearrange and electrons can be shared or transferred. The nuclei keep their proton counts. This is why an oxygen atom in water and an oxygen atom in oxygen gas are still oxygen, even though the substances have different properties. Nuclear changes are a different kind of process.

Read the visual model

Use the nucleus count for the element and mass number. Use proton minus electron count for net charge. Keep these accounts separate: a heavier isotope is not necessarily a more positively charged atom.

Starting from the C-12 record, what changes if it gains one electron? What changes if it instead gains two neutrons?

Visual reasoning — after your attempt

One added electron gives 7 electrons and a −1 charge while keeping carbon and mass number 12. Two added neutrons instead give carbon-14 with mass number 14 and unchanged neutral charge.

Use the same color and label for each oxygen atom when drawing a reaction. Changing an oxygen bead into a carbon bead would falsely suggest a changed nucleus. Track atom identity while allowing new groupings.

Pause and explain

Why is changing proton count a poor model of dissolving salt?

Check your reasoning after answering

Dissolving separates and disperses salt ions in water; it does not convert their nuclei into different elements. Proton counts and element identities remain unchanged.

Molecules and extended structures

A formula can name a small separate group or the ratio in a repeating structure. The same drawing rule does not describe both.

An element contains atoms with one proton-defined identity. A compound contains different elements chemically combined in a characteristic composition. A molecule is a distinct bonded group of atoms, such as one water molecule with two hydrogen atoms and one oxygen atom. A chemical bond is an interaction that holds a combination together. Drawing a line represents that connection; it is not a literal stick between colored balls. Subscripts count atoms in one represented group, while a coefficient counts whole groups.

The water side shows one separate molecule with a two-to-one H:O count. The salt side is a small slice of a repeating ionic arrangement with alternating charge signs. The displayed edge is not the edge of every crystal; symbols and distances are schematic, and a visible patch need not contain equal edge counts. NaCl describes the bulk ratio.

Some solids are extended structures: a connected or repeating arrangement continuing beyond the few particles drawn. Ordinary sodium chloride forms a lattice, a repeating arrangement of positive sodium ions and negative chloride ions. NaCl states a one-to-one ratio, not a separate two-atom molecule floating inside the crystal. A polymer is a material whose molecules include long chains of repeated groups. A repeating-unit drawing can show part of a chain without claiming every chain has identical length.

Composition and arrangement help explain why substances differ in properties. An oxygen atom in water remains oxygen, but water has properties different from oxygen gas because the combinations differ. A molecular or lattice model should name its represented substance, count or ratio, and scale limits. The few drawn particles stand for many actual particles. Ordinary chemical changes rearrange existing atoms; changing an element’s nucleus is a different process, so ordinary chemistry does not explain the creation of all elements in the universe.

Explain and apply

Draw two water molecules and a larger repeating salt patch. Compare total atom counts with a formula ratio, and explain why a two-ball salt molecule is an incomplete crystal model.

Distinguish molecule from extended arrangement.
Count water atoms correctly.
Explain formula ratio and a model limit.

Reasoning and marking guidance — after your attempt

Two water molecules contain four H and two O atoms. A salt patch represents part of an extended arrangement; its bulk Na:Cl ratio is one-to-one rather than a claim that the crystal is a collection of separate NaCl molecules. Retain element/ion labels and note that colors, bonding lines and distances are conventions.

Apply the whole unit · Identify three particles

Illustrative particle records: A has 8 protons, 8 neutrons, 8 electrons; B has 8 protons, 10 neutrons, 8 electrons; C has 8 protons, 8 neutrons, 10 electrons. Oxygen has atomic number 8.

Your task

Identify the element, mass number, and overall charge of each record. Draw a labeled counting model, identify an isotope pair, and explain why the drawings cannot represent true electron paths.

What your response should show

Use the supplied observations and measurements accurately; include units where needed.
Explain the science behind your answer, rather than giving only a number or label.
Show a labeled model, calculation, or evidence comparison as requested.
State an assumption or limitation and suggest a useful next measurement.

Response guidance — after your attempt

All three are oxygen. A has mass number 16 and charge 0; B has mass number 18 and charge 0; C has mass number 16 and charge −2. A and B are isotopes. A and C differ in electron count. Label the nucleus and electron cloud; show counts without implying scale or circular electron trajectories.

Scientific references and adapted teaching resources

Parent / teacher teaching notes

Begin with counters and a particle-count table. Ask the learner to identify the element before calculating mass number or charge. If the learner changes element when an electron changes, revisit the proton rule with a second example. Adaptation credit: OpenSciEd middle-school teaching routines and public task structures by OpenSciEd, CC BY 4.0. This independent version adds original explanatory text, data and figures, terminology support and a home-study schedule. It does not reproduce the complete teacher editions or their classroom validation. The illustrated study “Molecules and extended structures” is required curriculum content. Teach it before the associated application or distribute it over the assigned unit sessions; it is not an optional enrichment. Its practice/review/quiz response provides evidence of understanding. Model/data work must not be recorded as an actual physical experiment.

The key idea

Protons and neutrons occupy the nucleus; electrons are described by an electron cloud around it. Proton count identifies the element.

Number of Neutrons

neutrons = mass number − atomic number

atomic number = number of protons = number of electrons (in a neutral atom); mass number = protons + neutrons.

Worked examples

Example 1

An atom of carbon-12 has atomic number 6 and mass number 12. Find its numbers of protons, neutrons, and electrons.

Protons = atomic number = 6.
Neutrons = mass number − atomic number = 12 − 6 = 6.
Electrons = protons (neutral atom) = 6.

Answer: 6 protons, 6 neutrons, 6 electrons

Example 2 · Worked application

Two neutral atoms each have 7 protons. One has 7 neutrons and the other has 8. Compare their identities, masses and electron counts.

Use proton count first: both have atomic number 7, which identifies nitrogen.
Add protons and neutrons separately: 7+7=14 and 7+8=15.
Neutrality requires 7 electrons in each atom; the extra neutron adds no charge.

Answer: They are nitrogen-14 and nitrogen-15, isotopes with 7 electrons each.

Check the reasoning

Check that changing neutron count leaves both proton count and charge unchanged.

Example 3

An atom of oxygen has atomic number 8 and mass number 16. Find its numbers of protons, neutrons, and electrons.

Protons = atomic number = 8.
Neutrons = mass number − atomic number = 16 − 8 = 8.
Electrons = protons (neutral atom) = 8.

Answer: 8 protons, 8 neutrons, 8 electrons

Example 4

An atom has 8 protons, 9 neutrons and 10 electrons. Identify the element, mass number and charge.

Atomic number 8 identifies oxygen.
Mass number is 8+9=17; 8 positive and 10 negative charges give a net charge of −2.

Answer: An oxygen-17 ion with charge −2.

Example 5 · Find and fix the mistake

Context: Two neutral atoms each have 7 protons. One has 7 neutrons and the other has 8. Compare their identities, masses and electron counts. A student claims: “An atom gains an electron, so it becomes the next element on the periodic table.” Evaluate the claim and write a corrected explanation.

Test the claim against the stated conditions: Check that changing neutron count leaves both proton count and charge unchanged.
Element identity depends on protons. Gaining an electron changes charge, not atomic number. The result is an ion of the same element.

Answer: Element identity depends on protons. Gaining an electron changes charge, not atomic number. The result is an ion of the same element.

Check the reasoning

Check that changing neutron count leaves both proton count and charge unchanged.

Example 6 · Interpret the visual model

A particle has 8 protons, 8 neutrons and 10 electrons. Is it carbon, oxygen or a new element? Explain its charge.

Identify the represented objects, axes, labeled quantities and stated conditions before choosing a rule.
Read p⁺ as positively charged protons, n as neutral neutrons and e⁻ as negatively charged electrons. Element identity and net charge answer different questions. Changing only electrons cannot turn oxygen into carbon.
It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

Answer: It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

Check the reasoning

Keep the model’s stated limits explicit: Carbon and oxygen are compared with their proton, neutron and electron counts labeled. Six protons identifies carbon; eight identifies oxygen. The cloud and nucleus sizes are deliberately schematic and electrons are not drawn on fixed planetary tracks. A neutral atom balances proton and electron counts.

Avoid these mistakes

Changing electrons does not create a new element.

Check your understanding

An atom has 8 protons, 9 neutrons and 10 electrons. Identify the element, mass number and charge.

Discuss the answer

An oxygen-17 ion with charge −2.

Why is the periodic-table atomic mass often a decimal?

Discuss the answer

Atomic number counts protons. A neutral atom has the same number of electrons as protons, so its charges cancel. If it gains an electron it becomes a negative ion; gaining an electron does not change its element. Mass number counts protons plus neutrons in one atom. Atoms of the same element can have different neutron counts: these are isotopes. The decimal mass on a periodic table is an average for a mixture of isotopes, not a count of particles in one atom.

Return to the phenomenon

Two supplied particle records contain A: 8 protons, 8 neutrons, 8 electrons; B: 8 protons, 10 neutrons, 10 electrons. Neither record is a microscope image. Compare element identity, mass number and charge. Explain what changed and what stayed the same; name one limitation of a particle drawing. Revise your first explanation using a named idea from the lesson. Mark the changed part and the evidence that supports it.

Compare explanations after your attempt

Both are oxygen because each has eight protons. A has mass number 16 and is neutral; B has mass number 18 and charge −2. Neutrons change isotope and electrons change charge, while proton count retains element identity. A drawing is a counting model, not a scaled photograph or a literal electron orbit.

What should we investigate next?

Which condition could be changed to test a remaining uncertainty in this explanation? State the predicted observation and what result would challenge your model.

Investigate, explain and revise

Use these tasks after the three teaching sessions as needed. Discuss an attempted explanation before opening response guidance; a matching sentence is less important than a supported mechanism.

Activity 1 · Explain the new evidence case

Assigned reading and preparation

Use the visible scenario above, Parts 1–4 and your first notebook model. No outside reading or equipment is required.

Your task

Two supplied particle records contain A: 8 protons, 8 neutrons, 8 electrons; B: 8 protons, 10 neutrons, 10 electrons. Neither record is a microscope image. Compare element identity, mass number and charge. Explain what changed and what stayed the same; name one limitation of a particle drawing. Write 100–150 words or give a labeled drawing with an equivalent oral explanation.

Success criteria

Use two relevant stated records or conditions.
Explain the mechanism rather than only repeat a definition.
Label observation and inference.
Name a changed model feature or a remaining uncertainty.

Hint

Read the stated boundary and conditions before selecting a relationship. The unit’s worked examples show intermediate steps.

Response guidance — discuss after your attempt

Activity 2 · Read and extend the figure

Assigned reading and preparation

Study the new labeled figure and its caption, then compare it with the associated teaching part.

Your task

If only electrons rise to 10, what entries change? Explain what each important label represents. Add one missing feature that would be needed for a different question, and explain why the original figure leaves it out.

Success criteria

Answer the figure question with its stated labels.
Connect the visual representation to the scientific idea.
Identify a specific limitation from the caption.
Propose a useful change without misrepresenting supplied evidence.

Hint

Response guidance — discuss after your attempt

The charge becomes −2 while atomic number 8, mass number 16 and oxygen identity remain unchanged. This oxygen-16 counting model locates protons and neutrons in a nucleus and assigns electrons to a cloud region. It is not to scale: the relative sizes and distances, as well as electron behavior, are omitted. The numbers describe one neutral isotope, not every oxygen particle.

Words to know

atom: The smallest unit of an element that still has the properties of that element.
proton: A positively charged particle found in the nucleus.
neutron: A particle with no charge (neutral) found in the nucleus, alongside the protons.
electron: A negatively charged particle that moves around the nucleus in an electron cloud.
nucleus: The dense, positively charged center of an atom, containing the protons and neutrons.
atomic number: The number of protons in an atom; it identifies which element the atom is.
mass number: The total number of protons plus neutrons in an atom's nucleus.
isotope: An atom of an element distinguished by neutron count; isotopes share proton count.
ion: An atom or group of atoms with net electric charge because positive and negative charges do not balance.
model: A representation that explains selected features and makes predictions; it leaves other details out.
evidence: Relevant observations or records used to support or challenge a claim.
system boundary: The chosen limit separating the objects being studied from their surroundings.
assumption: A condition taken as true for a model or calculation and needing to be checked against the case.

Why atoms are mostly empty space

If an atom's nucleus were the size of a marble, the nearest electrons would be about two football fields away, with nothing in between. Atoms are roughly 99.9999999% empty space. The reason solid objects still feel solid is not that atoms are packed tight — it's that the electron clouds of neighboring atoms repel each other, so your hand never actually touches the desk; the electrons push back before contact.

Isotopes: same element, different mass

Every atom of an element has the same number of protons, but the number of neutrons can vary. These variations are called isotopes. Carbon-12 and carbon-14 are both carbon (6 protons), but carbon-14 has two extra neutrons. Carbon-14 is slightly radioactive and decays at a known rate — which is exactly how scientists carbon-date fossils and ancient artifacts.

Where the mass really is

Protons and neutrons each have about the same mass, and each is roughly 1,836 times heavier than an electron. So nearly all of an atom's mass sits in the tiny nucleus, while electrons — which take up almost all the atom's volume — contribute almost none of its weight. That's why the atomic mass number counts protons + neutrons and ignores electrons.

In the real world

Doctors use isotopes — atoms of the same element with different numbers of neutrons — in medical imaging and in treating certain cancers.

Exit check

If only electrons rise to 10, what entries change? Explain the relevant mechanism and one model limitation.

Check your explanation after answering

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Which subatomic particle has a negative charge?

proton
neutron
electron
nucleus

Answer and explanation

electron

Electrons carry negative charge and are described by an electron cloud, not literal planet-like orbits.

Problem 2

Where are protons and neutrons located in an atom?

In the nucleus
Orbiting the nucleus
Outside the atom entirely
Only in charged atoms

Answer and explanation

In the nucleus

Protons and neutrons are packed together in the central nucleus.

Problem 3

A sodium (Na) atom has atomic number 11 and mass number 23. How many neutrons does it have?

Answer and explanation

Neutrons = mass number − atomic number = 23 − 11 = 12.

Problem 4

A magnesium atom has 12 protons and 12 neutrons. What is its mass number?

Answer and explanation

Mass number = protons + neutrons = 12 + 12 = 24.

Problem 5

Explain why an atom is electrically neutral overall.

Answer and explanation

A neutral atom has the same number of protons (positive) as electrons (negative), so the charges cancel out and the atom has no overall charge.

Problem 6

An atom of chlorine has 17 protons and a mass number of 37. How many neutrons does it have?

Answer and explanation

Neutrons = mass number − atomic number = 37 − 17 = 20.

Problem 7

An atom has 8 protons, 9 neutrons and 10 electrons. Identify the element, mass number and charge. Explain the reasoning in your own words.

Answer and explanation

An oxygen-17 ion with charge −2.

Atomic number 8 identifies oxygen.

Mass number is 8+9=17; 8 positive and 10 negative charges give a net charge of −2.

Response criteria

Use the stated conditions.
Show the evidence or calculation.
Explain the conclusion and any stated limitation.

Problem 8

A neutral atom has 13 protons and 14 neutrons. State its mass number and electron count and explain both. Show the reasoning, not just the conclusion.

Hints

Check that changing neutron count leaves both proton count and charge unchanged.

Answer and explanation

Mass number=13+14=27; there are 13 electrons because neutrality requires equal positive and negative charges.

Problem 9

Atoms A and B have 9 and 10 protons but both have 10 neutrons. Are they isotopes? Explain.

Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

No. Isotopes share proton count. These have different atomic numbers and are different elements.

Problem 10

Draw two water molecules and a larger repeating salt patch. Compare total atom counts with a formula ratio, and explain why a two-ball salt molecule is an incomplete crystal model.

Hints

Use the illustrated study titled Molecules and extended structures. Keep the observations, model and conditions separate.

Answer and explanation

Response criteria

Distinguish molecule from extended arrangement.
Count water atoms correctly.
Explain formula ratio and a model limit.

Problem 11

For this unit’s illustrated case, a particle has 8 protons, 8 neutrons and 10 electrons. Is it carbon, oxygen or a new element? Explain its charge.

Answer and explanation

It is oxygen, because it still has 8 protons. It has two more electrons than protons and therefore charge −2 in elementary-charge units.

Response criteria

Use the named labels, directions or supplied quantities accurately.
Connect the visual evidence to the scientific mechanism or calculation.
State the relevant model conditions or limitation.

Problem 12

An atom has 12 protons, 13 neutrons and 10 electrons. State atomic number, mass number and net charge, and explain whether losing electrons changed its element.

Answer and explanation

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Read proton count for atomic number.

Add 12+13 for mass number; compare 12 positive with 10 negative charges.

Atomic number 12; mass number 25; charge +2 in elementary-charge units. Electron loss changes charge, while the 12 protons still determine the element.

Response criteria

Use the named labels, directions or supplied quantities accurately.
Connect the visual evidence to the scientific mechanism or calculation.
State the relevant model conditions or limitation.

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