Chemistry · Both
Atomic Structure & the Periodic Table
Structure of the atom, atomic number, mass number, electron shells and groups/periods in the periodic table.
Everything is made of atoms. You need atomic number, mass number, electron configuration and how the periodic table is organised.
Particles in an atom
• **Proton** – charge +1, mass 1, in nucleus
• **Neutron** – charge 0, mass 1, in nucleus
• **Electron** – charge −1, mass ≈ 0, in shells around nucleus
**Atomic number (Z)** = number of protons
**Mass number (A)** = protons + neutrons
Number of electrons in a neutral atom = number of protons
Neutrons = mass number − atomic number
Worked example
An atom has atomic number 11 and mass number 23. How many protons, neutrons and electrons?
Solution
Protons = 11 Electrons = 11 (neutral) Neutrons = 23 − 11 = **12**
Electronic structure
Electrons fill shells:
Shell 1 holds up to 2
Shell 2 holds up to 8
Shell 3 holds up to 8 (at GCSE level)
Example: Sodium (Z = 11) → 2,8,1
Chlorine (Z = 17) → 2,8,7
Oxygen (Z = 8) → 2,6
The periodic table
• **Group** = column → number of electrons in outer shell (Group 1 = 1 outer electron)
• **Period** = row → number of shells
Group 1 (alkali metals): reactive, form +1 ions
Group 7 (halogens): reactive non-metals, form −1 ions
Group 0 (noble gases): unreactive, full outer shell
Practice questions
Try these without looking at the answers first.
Q1.How many neutrons in an atom of ¹⁴C (atomic number 6)?
Show answer
8
Q2.Electronic structure of aluminium (Z = 13)?
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2,8,3
Q3.Why are Group 0 elements unreactive?
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Full outer electron shell