Atoms, Electrons & Periodicity — Problem set, Grade 11

Connect atomic evidence, isotopes, electron arrangements, spectra, and periodic patterns.

Review the key ideas

Atomic Models & Subatomic Particles

Atomic models connect evidence with nuclear and electronic structure. Proton count fixes element identity; neutron count distinguishes isotopes, electron imbalance gives ion charge, and orbitals describe quantum states rather than classical tracks.

Isotopes & Average Atomic Mass

Isotopes are same-element atoms with different neutron counts; the periodic table's decimal atomic mass is a weighted average of those isotopes, found by summing each isotope's mass times its fractional abundance.

Electron Configuration

Count electrons for the specified species and state, then apply orbital occupancy and ground-state filling models with known exceptions. Main-group valence patterns and transition-ion electron removal require their appropriate context.

The Periodic Table

The periodic table orders elements by atomic number. Groups connect recurring configurations and periods organize successive elements; regional and block patterns support conditional predictions with exceptions.

Periodic Trends

Periodic tendencies reflect nuclear attraction, shielding, shell structure and electron interactions. Compare the defined property and species; broad radius, ionization-energy and electronegativity trends have local exceptions.

Electron Transitions & Emission Spectra

A spectral line corresponds to an allowed energy change, with ΔE = hf.

Problem 1

An atom has 26 protons and 30 neutrons. What is its mass number, and how many electrons does it have if it is neutral?

Answer and explanation

Mass number 56; 26 electrons

Mass number = protons + neutrons = 26 + 30 = 56. A neutral atom has electrons equal to protons, so 26 electrons (this atom is iron).

Problem 2

Aluminum-27 has atomic number 13 and mass number 27. Find its numbers of protons, neutrons, and electrons.

Answer and explanation

13 protons, 14 neutrons, 13 electrons

Protons = Z = 13. Neutrons = A − Z = 27 − 13 = 14. Electrons = protons (neutral atom) = 13.

Problem 3

An ion has 20 protons, 20 neutrons, and 18 electrons. What are its mass number and its charge?

Answer and explanation

Mass number 40; charge 2+

Mass number = protons + neutrons = 20 + 20 = 40. Charge = protons − electrons = 20 − 18 = +2, so it is a 2+ ion (this is Ca²⁺).

Problem 4

Define isotopes. Do two isotopes of the same element react the very similarly chemically in the introductory electron-structure model? Explain.

Answer and explanation

Same protons, different neutrons; their electron structures usually give similar chemistry, with possible isotope-dependent kinetic effects.

Isotopes are atoms of one element (same proton count) with different numbers of neutrons. Chemistry is controlled by electrons, and isotopes have the same number of electrons, so they behave almost identically in reactions — only their mass and nuclear stability differ.

Problem 5

Lithium has two isotopes: lithium-6 (mass 6.0 amu, 7.5% abundance) and lithium-7 (mass 7.0 amu, 92.5% abundance). Find lithium's average atomic mass.

Answer and explanation

About 6.93 amu

Convert percents to fractions: 0.075 and 0.925. Average = 6.0 × 0.075 + 7.0 × 0.925 = 0.45 + 6.475 = 6.925 ≈ 6.93 amu, close to the periodic-table value of 6.94.

Problem 6

An element has only two isotopes, of mass 10.0 amu and 11.0 amu. Its average atomic mass is 10.8 amu. Find the percent abundance of each isotope.

Answer and explanation

20% mass-10 and 80% mass-11

Let x be the fraction of the mass-10 isotope. Then 10x + 11(1 − x) = 10.8 → 11 − x = 10.8 → x = 0.20. So 20% is mass-10 and 80% is mass-11 (this is boron).

Problem 7

Write the ground-state electron configuration for nitrogen (Z = 7) and state its number of valence electrons.

Answer and explanation

1 s² 2 s² 2p³; 5 valence electrons

Fill in Aufbau order until 7 electrons are placed: 1 s² 2 s² 2p³ (2 + 2 + 3 = 7). The valence electrons are in the highest level (n = 2): 2 s² 2p³ = 5.

Problem 8

Write the noble-gas shorthand electron configuration for calcium (Z = 20).

Answer and explanation

[Ar] 4 s²

Argon accounts for the first 18 electrons, so start from [Ar]. The remaining 20 − 18 = 2 electrons go into 4 s, giving [Ar] 4 s².

Problem 9

Chlorine (Z = 17) has the configuration 1 s² 2 s² 2p⁶ 3 s² 3p⁵. How many valence electrons does it have, and what group does that place it in?

Answer and explanation

7 valence electrons; Group 17

Valence electrons are those in the highest energy level (n = 3): 3 s² 3p⁵ = 7. Seven valence electrons places chlorine in Group 17, the halogens.

Problem 10

An element has the configuration [Ne] 3 s² 3p¹. Identify its period, group, block, and whether it is a metal or nonmetal.

Answer and explanation

Period 3, Group 13, p-block, metal (aluminum)

Highest level is n = 3 → Period 3. Valence electrons 3 s² 3p¹ = 3 → Group 13. The outer electrons fill a p sublevel → p-block. It is aluminum, a metal.

Problem 11

Arrange magnesium (Mg), silicon (Si), and sulfur (S) — all in Period 3 — in order of increasing atomic radius.

Answer and explanation

S < Si < Mg

Atomic radius decreases left to right across a period as effective nuclear charge rises. Mg is farthest left (largest), then Si, then S farthest right (smallest). Increasing radius: S, Si, Mg.

Problem 12

Potassium (K) and calcium (Ca) are both in Period 4. Which has the higher first ionization energy, and why?

Answer and explanation

Calcium

Across a period, effective nuclear charge rises, holding valence electrons more tightly, so ionization energy increases left to right. Calcium is just to the right of potassium and has one more proton pulling on its valence electrons, so calcium has the higher first ionization energy.

Problem 13

An ion has 17 protons and 18 electrons. Identify charge and element.

Answer and explanation

Charge=17−18=−1; atomic number 17 identifies chlorine, so Cl⁻.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 14

Chapter review: A species has 17 protons, 18 neutrons and 18 electrons. Give Z, mass number, element and charge. Explain which change would make an isotope rather than an ion.

Answer and explanation

Z=17, A=35, chlorine with charge −1: ³⁵Cl⁻. Changing neutron number while keeping 17 protons makes another chlorine isotope. Changing only electron number changes its ionization state.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

Problem 15

Isotopes 10u at 20% and 11u at 80% give what average?

Answer and explanation

10(.20)+11(.80)=10.8u. The result lies between the two masses and closer to the more abundant one.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 16

Chapter review: Two modeled isotopes have masses 24.0 and 26.0 u. A sample mean is 24.6 u. Find the fraction of each and check the answer.

Answer and explanation

24.6=24.0x+26.0(1−x)=26.0−2.0x, so x=0.70. Fractions are 70% lighter and 30% heavier; 0.70(24.0)+0.30(26.0)=24.6 u.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

Problem 17

Write sodium ground-state configuration and valence count.

Answer and explanation

1 s²2 s²2p⁶3s¹; eleven electrons total and one outer-shell electron.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 18

Chapter review: A neutral atom has Z=16. Write its ground-state configuration and electron total for its common 2− ion. Explain why gaining electrons does not change the element.

Answer and explanation

Neutral sulfur: 1s²2s²2p⁶3s²3p⁴, totaling 16 electrons. S²⁻ has 18 electrons, with 3p⁶. Its 16 protons retain sulfur identity.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

Problem 19

Locate [Ne]3 s²3p³.

Answer and explanation

Outer n=3 gives period 3; five valence electrons give main-group 15; this is phosphorus.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 20

Chapter review: Element A has outer configuration 3s²3p⁵ and B has 4s²4p⁵. Identify their main-group relationship, explain one likely similarity and one reason properties need not match.

Answer and explanation

Both fit Group 17. Similar valence patterns support related bonding tendencies, while B has an additional occupied shell and different nuclear charge/shielding. Exact radius, energy and reaction behavior need the specified species and conditions.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

Problem 21

Explain why K is usually larger than Na.

Answer and explanation

K has an occupied outer shell farther from the nucleus with more inner shielding, outweighing its greater proton count in this comparison.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 22

Chapter review: Rank Ne, F⁻ and Mg²⁺ by the usual radius trend for an isoelectronic series. Give electron/proton counts and the model limitation.

Answer and explanation

All have ten electrons; proton counts are F=9, Ne=10, Mg=12. The usual order is F⁻ > Ne > Mg²⁺ under a compatible radius comparison. Defined atomic/ionic radii and environments must be matched before treating reference numbers as directly interchangeable.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

Problem 23

Compare 400 nm and 600 nm photons without calculating constants.

Answer and explanation

400 nm has higher frequency and energy because E is inversely proportional to wavelength.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 24

Two unknown lines match one reference. What would strengthen identification?

Answer and explanation

Match additional lines, check resolution and calibration, and evaluate whether a mixture could give the same subset.

Response criteria

State a scientifically accurate conclusion.
Show the calculation or mechanism that supports it.
Include units, conditions, or limits where relevant.

Problem 25

Chapter review: A selected atom has levels at 0, 1.5 and 4.5 eV. Find the energy and approximate vacuum wavelength for a 4.5-to-1.5 eV emission. Could a further drop emit another photon?

Answer and explanation

The first photon has 3.0 eV, or about 4.81×10⁻¹⁹ J, giving λ≈4.14×10⁻⁷ m =414 nm. A later allowed 1.5-to-0 transition could emit a 1.5 eV photon; the selected model does not supply transition probabilities.

Response criteria

Use the stated measurements or conditions, with correct units.
Connect the result to the chemical mechanism or evidence relationship.
Identify the assumption, model limit or further evidence requested.

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