Everything in the universe — the chair you are sitting on, the water you drink, even your own body — is made of atoms. Atoms are incredibly tiny (about 0.0000001 mm across), yet understanding their structure explains almost all of chemistry. Here is how they work.
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Book free demoWhat Is an Atom?
Definition: Atom
| Particle | Location | Charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around nucleus | −1 | ≈ 0 (negligible) |
The Nucleus and Electron Shells
Protons and neutrons are packed tightly in the nucleus at the centre of the atom. Electrons move around the nucleus in regions called shells (or energy levels). Each shell can hold a set maximum number of electrons.
Shell Capacity Rules
- • Shell 1 (closest to nucleus): maximum 2 electrons
- • Shell 2: maximum 8 electrons
- • Shell 3: maximum 8 electrons (for the first 20 elements)
Draw the electron arrangement of Sodium (Na, atomic number 11)
Sodium has 11 protons, so 11 electrons (neutral atom)
Fill shell 1 first (max 2)
Fill shell 2 (max 8)
Remaining electrons go to shell 3
Write the electron configuration
Atomic Number, Mass Number, and Isotopes
Atomic Number
Mass Number
Carbon has atomic number 6 and mass number 12. How many neutrons does it have?
Recall the relationship
Substitute
Isotopes
The Periodic Table — Why It Is Organised That Way
The periodic table lists every known element in order of atomic number. Elements in the same column (group) have the same number of electrons in their outer shell — which is why they have similar chemical properties.
Hydrogen
No. 1 | 1
1 electron, 1 shell
Helium
No. 2 | 2
Full outer shell — very stable
Lithium
No. 3 | 2, 1
1 outer electron — reactive
Neon
No. 10 | 2, 8
Full outer shell — noble gas
Sodium
No. 11 | 2, 8, 1
1 outer electron — reactive
Chlorine
No. 17 | 2, 8, 7
7 outer electrons — reactive
- Atomic number = number of protons (unique per element).
- Mass number = protons + neutrons.
- Neutrons = mass number − atomic number.
- A neutral atom has equal protons and electrons.
- Electrons fill shells in order: 2, 8, 8 (for first 20 elements).
- Isotopes: same element, different neutron count.
Practice Problems
- 1
Oxygen has atomic number 8 and mass number 16. How many protons, neutrons, and electrons does a neutral oxygen atom have?
Hint: Neutrons = 16 − 8. Electrons = protons (neutral atom).
- 2
Write the electron arrangement of Magnesium (atomic number 12).
Hint: Fill shells in order: 2, 8, then the rest in shell 3.
- 3
An atom has 17 protons and 18 neutrons. What is its mass number? What element is it?
Hint: Mass number = protons + neutrons. Atomic number 17 = Chlorine.
- 4
Chlorine-35 and Chlorine-37 are isotopes. What do they have in common? What is different?
Hint: Same: protons (17). Different: neutrons (18 vs 20).
- 5
Why do elements in the same group of the periodic table react similarly?
Hint: Think about outer electrons.
Next: Bonding and Reactions
Once you understand atoms, the next step is how they bond — ionic bonds (electron transfer) and covalent bonds (electron sharing). These explain why water is a liquid and table salt is a crystal. If you would like help making sense of chemistry, book a free demo session and we will work through it together.
