Atom — Science definition
Defined in 3 courses:
- The smallest unit of an element that still has the properties of that element. (Physical Science: Matter, Forces & Energy)
- The smallest unit of an element that still has that element's properties; built from protons, neutrons, and electrons. (Life Science: Biology)
- The smallest particle of an element that retains the chemical identity of that element. (Physical Science: Chemistry)
Where it is taught
- Atomic Structure — Properties of Matter, Physical Science: Matter, Forces & Energy, Grade 8
- Atoms, Bonds & Water — Investigating the Chemistry of Life, Life Science: Biology, Grade 10
- Atomic Models & Subatomic Particles — Atoms, Electrons & Periodicity, Physical Science: Chemistry, Grade 11
In use
The nucleus contains nearly all the atom’s mass and is much smaller than the atom’s overall extent.
Atomic Structure
Everything around you — this page, the air in your lungs, the water in a glass, and every cell in your body — is built from atoms, the tiny particles that make up all matter.
Atoms, Bonds & Water
His model pictured the atom as a solid, featureless sphere — a tiny billiard ball.
Atomic Models & Subatomic Particles
Questions using atom
Problem 1
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 2
A sodium (Na) atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer and explanation
12
Neutrons = mass number − atomic number = 23 − 11 = 12.
Problem 3
A magnesium atom has 12 protons and 12 neutrons. What is its mass number?
Answer and explanation
24
Mass number = protons + neutrons = 12 + 12 = 24.
Problem 4
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.
Taught alongside
- proton
- neutron
- electron
- nucleus
- atomic number
- mass number
- isotope
- ion
- model
- evidence
- system boundary
- assumption
- element
- covalent bond