A3 — Atomic Structure | CSEC Chemistry Hub

Saves the complete Notes tab as a clean, print-ready PDF

1. Subatomic Particles

Every atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons in energy shells. Nearly all mass is in the nucleus; electrons define the atom's size and all its chemical behaviour.

Particle	Relative Charge	Relative Mass	Location
Proton	+1	1	Nucleus
Neutron	0 (neutral)	1	Nucleus
Electron	−1	1/1836 (≈ 0)	Energy shells

🧠 Memory trick — PPP NNN EEE: Protons = Positive. Neutrons = Neutral. Electrons = nEgative.

In a neutral atom: number of protons = number of electrons. The charges balance exactly.

2. Atomic Number, Mass Number & Nuclear Notation

The atomic number (Z) = number of protons — unique to every element. The mass number (A) = protons + neutrons.

Nuclear notation: mass number top-left, atomic number bottom-left. Example:

²³₁₁Na → A=23, Z=11 → protons=11, neutrons=12, electrons=11

Quantity	Formula	⁴⁰₂₀Ca example
Protons	= Z	20
Neutrons	= A − Z	40 − 20 = 20
Electrons (neutral)	= Z	20

3. Electronic Configuration

Electrons fill the lowest available energy shells first (innermost first). Shell 1 holds max 2; Shell 2 max 8; Shell 3 max 8 (CSEC level).

Element	Z	Configuration	Valence e⁻	Group / Period
Hydrogen	1	(1)	1	I / 1
Carbon	6	(2,4)	4	IV / 2
Sodium	11	(2,8,1)	1	I / 3
Chlorine	17	(2,8,7)	7	VII / 3
Argon	18	(2,8,8)	8 (full)	0 / 3
Calcium	20	(2,8,8,2)	2	II / 4

🔑 Group = valence electrons. Period = number of occupied shells.

Valence electrons determine reactivity — elements in the same group react similarly because they have the same number of outer electrons.

4. Isotopes & Isotopy

Isotopes are atoms of the same element with the same protons and electrons but different numbers of neutrons (different mass numbers). Isotopy is the occurrence of isotopes for a given element.

Isotope	Notation	Protons	Neutrons	Abundance
Chlorine-35	³⁵₁₇Cl	17	18	75%
Chlorine-37	³⁷₁₇Cl	17	20	25%

Isotopes have identical chemical properties — same electron configuration
Isotopes have different physical properties — different masses, densities

5. Relative Atomic Mass (Aᵣ)

Because natural elements are mixtures of isotopes, Aᵣ is a weighted average based on isotope abundances. It has no units (it's a ratio vs 1/12 of carbon-12).

Aᵣ = Σ (% abundance / 100) × mass number

Chlorine example: Aᵣ = (75/100 × 35) + (25/100 × 37) = 26.25 + 9.25 = 35.5

6. Radioactivity

Radioactive isotopes have unstable nuclei that spontaneously emit radiation to become more stable.

Type	Composition	Charge	Stopped by
Alpha α	2p + 2n (helium nucleus)	+2	Paper / skin
Beta β	High-speed electron	−1	Aluminium sheet
Gamma γ	Electromagnetic wave	0	Thick lead/concrete (reduced)

Uses of Radioactive Isotopes

Carbon-14 — radiocarbon dating of organic material (half-life 5,700 years)
Cobalt-60 / Iodine-131 — radiotherapy for cancer treatment
Technetium-99m — medical tracer for organ imaging (gamma camera)
Uranium-235 — nuclear fission to generate electricity
Plutonium-238 — long-life pacemaker power source (87-year half-life)

⏱️ Half-life: time for half the radioactive nuclei in a sample to decay.

After n half-lives, amount remaining = initial × (1/2)ⁿ

⚡ Interactive Atom Builder

Select an element to see its animated Bohr model. Electrons orbit in colour-coded shells with the configuration shown live.

📈 Isotope Abundance & Aᵣ Calculator

Select an element to see its natural isotope abundances. The Aᵣ recalculates live as you adjust the sliders.

Element:

🃏 Flashcards — Atomic Structure

Click to flip!

Answer

👆 Click card to flip

❓ Quiz — Atomic Structure

🔢 Worked Example — Relative Atomic Mass

Problem: Boron has two isotopes: boron-10 (20% abundant) and boron-11 (80% abundant). Calculate the relative atomic mass of boron.

Step 1: How many isotopes are being considered?

Count the isotopes listed: boron-10 and boron-11. Type the number.

Step 2: Contribution from boron-10

(20/100) × 10 = ?

Step 3: Contribution from boron-11

(80/100) × 11 = ?

Step 4: Final Aᵣ = sum of contributions

2.0 + 8.8 = ?

🔗 Matching — Atomic Structure

Click a term, then its matching description. Green = correct!

Term

Definition / Description

📝 CSEC-Style Questions

Q1. An atom of iron is ⁵⁶₂₆Fe. (a) State the number of protons, neutrons and electrons. (b) Write the electronic configuration. (c) ⁵⁴₂₆Fe is another isotope — state what is the same and what is different. [6 marks]+

Mark Scheme

1 (a) Protons = 26 ✓ | Neutrons = 56−26 = 30 ✓ | Electrons = 26 ✓

2 (b) Configuration = (2, 8, 14, 2) ✓ [Accept (2,8,8,8,2) or simplified CSEC versions]

3 (c) Same: atomic number (26), protons (26), electrons (26), chemical properties ✓ | Different: mass number (56 vs 54), neutrons (30 vs 28), physical properties ✓

Q2. Chlorine-35 is 75% abundant and chlorine-37 is 25% abundant. (a) Calculate Aᵣ(Cl). (b) Explain why Aᵣ is not a whole number. [4 marks]+

Mark Scheme

1 (a) Aᵣ = (75/100×35) + (25/100×37) = 26.25 + 9.25 ✓

2 Aᵣ = 35.5 ✓

3 (b) Aᵣ is a weighted average of the masses of all naturally occurring isotopes ✓ — no individual chlorine atom has mass 35.5; actual atoms are always mass 35 or 37. ✓

Q3. (a) Define isotopes. (b) Explain why C-12 and C-14 have (i) identical chemical properties but (ii) different physical properties. [5 marks]+

Mark Scheme

1 (a) Atoms of the same element with the same number of protons but different numbers of neutrons (different mass numbers). ✓

2 (b)(i) Both have 6 protons → 6 electrons in identical configuration (2,4). ✓ Chemical reactions depend on electron arrangement — same config = same reactivity. ✓

3 (b)(ii) C-12 has 6 neutrons; C-14 has 8 neutrons → different masses. ✓ Different density; C-14 is radioactive (unstable nucleus). ✓

Q4. State THREE uses of radioactive isotopes and for each name the specific isotope used. [6 marks]+

Mark Scheme — Award 2 marks per use (application + isotope)

1 Carbon dating → Carbon-14 ✓✓

2 Radiotherapy / cancer treatment → Cobalt-60 or Iodine-131 ✓✓

3 Medical imaging / tracer → Technetium-99m ✓✓

4 Nuclear power / electricity generation → Uranium-235 ✓✓

5 Pacemaker power → Plutonium-238 ✓✓

⭐ Key Formulas & Concepts

Subatomic Particles

Proton: +1, mass 1, nucleus Neutron: 0, mass 1, nucleus Electron: −1, tiny, shells

Nuclear Notation

A (top) = p + n Z (bottom) = protons Neutrons = A − Z

Electronic Config

Shells fill: 2, 8, 8... Group = valence electrons Period = occupied shells

Isotopes

Same Z, different A & neutrons Same chemical properties Different physical properties

Relative Atomic Mass

Aᵣ = Σ(% / 100 × A) Weighted average — no units

Radiation & Half-Life

α: +2, stopped by paper β: −1, stopped by Al γ: 0, reduced by lead

📚 Resources

🔬 PhET: Build an Atom 🔬 PhET: Alpha Decay 🔬 ptable.com ▶ YouTube: Atomic Structure ▶ YouTube: Isotopes & Aᵣ