Atomic Structure & Periodic Table

Atomic Structure — Carbon-12 (2,4 electronic configuration)

Sub-atomic particles
Electronic configuration
The periodic table
Isotopes
Common exam traps

Topic 1 of 11

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1. Sub-Atomic Particles

Particle	Location	Relative charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Shells (orbitals)	−1	≈ 0 (negligible)

Atomic number (Z) = number of protons. This defines the element.
Mass number (A) = protons + neutrons.
Number of neutrons = A − Z.
In a neutral atom: number of protons = number of electrons.
Ions: cation (lost electrons, positive charge); anion (gained electrons, negative charge).

Worked example

An ion is written ²⁷₁₃Al³⁺. Find the number of protons, neutrons and electrons.

Protons = 13 (atomic number). Neutrons = 27 − 13 = 14. Electrons = 13 − 3 = 10 (lost 3 to form 3+ ion).

2. Electronic Configuration

Electrons occupy shells (energy levels) around the nucleus. Each shell holds a maximum number of electrons:

Shell 1: maximum 2 electrons
Shell 2: maximum 8 electrons
Shell 3: maximum 8 electrons (at O-Level)

Electrons fill the lowest energy shells first (closest to nucleus).

Element	Symbol	Atomic no.	Configuration
Hydrogen	H	1	1
Carbon	C	6	2, 4
Sodium	Na	11	2, 8, 1
Chlorine	Cl	17	2, 8, 7
Calcium	Ca	20	2, 8, 8, 2

Valence electrons

The number of electrons in the outermost shell = valence electrons. This determines the chemical properties and the group number in the periodic table. Na has 1 valence electron → Group I.

3. The Periodic Table

Elements are arranged in order of increasing atomic number. Elements in the same group have the same number of valence electrons and similar chemical properties.

Period / Group	What it tells you
Period (row)	Number of electron shells
Group (column)	Number of valence electrons (Group I = 1, Group VII = 7)
Group 0 (Noble gases)	Full outer shell; chemically unreactive

Periodic trends (across a period, left to right)

Atomic number increases.
Metallic character decreases; non-metallic character increases.
Reactivity of metals decreases; reactivity of non-metals increases.

Periodic trends (down a group)

Number of shells increases → atomic radius increases.
Group I metals: reactivity increases (easier to lose outer electron).
Group VII non-metals: reactivity decreases (harder to gain electron as shells increase).

4. Isotopes

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons (and therefore different mass numbers).

Examples: ¹²C and ¹⁴C (both carbon, Z=6, but mass numbers 12 and 14). ³⁵Cl and ³⁷Cl (both chlorine, Z=17).

Isotopes have identical chemical properties (same electron configuration) but different physical properties (different mass → different density, rate of diffusion, melting point in some cases).

Relative atomic mass (Ar)

Chlorine exists as 75% ³⁵Cl and 25% ³⁷Cl. Find the relative atomic mass.

Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5

Must-Know for Exam

Atomic number = protons. In neutral atom: protons = electrons.
Mass number = protons + neutrons. Neutrons = mass number - atomic number.
Isotopes: same element (same protons), different neutrons, different mass number. Same chemical properties.
Electronic config examples: Na (11) = 2,8,1. Cl (17) = 2,8,7. Ca (20) = 2,8,8,2.
Group I: 1 outer electron. Group VII: 7 outer electrons. Group 0: full outer shell.
Same GROUP = same outer electrons = similar chemical properties.

5. Common Exam Traps

Trap 1 — Ion electron count

A Na⁺ ion has lost one electron: 11 protons, 10 electrons. A Cl⁻ ion has gained one electron: 17 protons, 18 electrons. Always adjust electron count by the ion charge.

Trap 2 — Isotopes have identical chemistry

"Isotopes of the same element have different chemical properties" — FALSE. Chemical properties depend on electron configuration, which is the same for all isotopes of an element.

Trap 3 — Period ≠ Group

Period = row (number of shells). Group = column (number of valence electrons). Na is in Period 3, Group I — three shells, one valence electron.

Key Terms — Flashcard Review

Tap each card to reveal the definition.

Proton

Relative mass: 1. Relative charge: +1. Found in nucleus. Number of protons = atomic number.

Neutron

Relative mass: 1. Relative charge: 0. Found in nucleus. Protons + neutrons = mass number.

Electron

Relative mass: negligible. Relative charge: -1. Arranged in shells around nucleus.

Isotopes

Atoms of same element with same proton number but different neutron number and mass number.

Electronic configuration

Electrons fill shells in order: Shell 1 max 2, Shell 2 max 8, Shell 3 max 8 (O-Level).

Group and Period

Group = number of outer electrons. Period = number of occupied shells. e.g. Na: Group I, Period 3.

🎯 Practice Quiz — Test Yourself

8 O Level-style questions on this topic. Select an answer to see instant feedback.

Question 1 of 8

The atomic number equals:

Explanation: Atomic number = number of protons. In neutral atom: protons = electrons.

Question 2 of 8

Isotopes are atoms with:

Explanation: Isotopes: same element (same protons/atomic number), different mass numbers (different neutrons).

Question 3 of 8

An ion X²⁻ means the atom has:

Explanation: Negative ion = gained electrons. X²⁻ gained 2 electrons.

Question 4 of 8

Electron configuration of Na (Z=11):

Explanation: Na has 11 electrons: 2 in shell 1, 8 in shell 2, 1 in shell 3.

Question 5 of 8

Elements in the same group have:

Explanation: Group = same valence electrons → similar chemical properties. Period = same number of shells.

Question 6 of 8

An atom has atomic number 17 and mass number 35. How many neutrons does it have?

Explanation: Neutrons = mass number - atomic number = 35 - 17 = 18. Atomic number = protons = 17. The remaining nuclear particles are neutrons.

Question 7 of 8

Isotopes of the same element have:

Explanation: Isotopes have the same proton number (same element) and same electron number. They differ only in neutron number - giving different mass numbers. Chemical properties depend on electrons, so isotopes have identical chemical properties.

Question 8 of 8

Chlorine (atomic number 17) has the electronic configuration:

Explanation: Fill shells: Shell 1 = 2, Shell 2 = 8, Shell 3 = 17-2-8 = 7. Cl = 2,8,7. Seven outer electrons = Group VII. Gains 1 electron to form Cl- (stable noble gas config 2,8,8).

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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.

Atomic Structure & the Periodic Table

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