Module 02
AtomicStructure
Everything around you β your phone screen, the air, your own body β is made of atoms. An atom is more than 25 million times smaller than 1 mm, and yet it has a complex internal structure that determines every chemical property you'll ever study. Let's go inside! βοΈ
1. The Structure of an Atom
An atom is the smallest particle of an element that can exist on its own and still have the same chemical properties as that element. Imagine breaking a gold ring into smaller and smaller pieces β the last piece you could still call "gold" is a gold atom. Break it further and it stops being gold!
Mind-Blowing SizeA typical atom is about 2.5 Γ 10β»β· mm across. That's more than 25 million times smaller than 1 mm. If an atom were the size of a football pitch, the nucleus would be a tiny marble in the centre β most of an atom is empty space!
The atom has a central nucleus containing protons and neutrons. Electrons orbit the nucleus in regions called energy shells. Nearly all the mass of an atom is in the nucleus, but the electrons define the atom's size.
2. Subatomic Particles
Proton
Charge+1
Mass1
LocationNucleus
SymbolpβΊ
Neutron
Charge0 (neutral)
Mass1
LocationNucleus
Symbolnβ°
Electron
Chargeβ1
Mass1/1836
LocationEnergy shells
Symboleβ»
Memory Trick β PPE!
Protons = Positive (+1)
Neutrons = Neutral (0)
Electrons = nEgative (β1)
In any neutral atom: number of protons = number of electrons always!
Protons = Positive (+1)
Neutrons = Neutral (0)
Electrons = nEgative (β1)
In any neutral atom: number of protons = number of electrons always!
3. Atomic Number, Mass Number & Nuclear Notation
Each element has a unique atomic number (Z) β the number of protons. No two elements share the same atomic number. The mass number (A) is the total number of protons + neutrons.
| Term | What it Counts | Symbol |
|---|---|---|
| Atomic number | Number of protons | Z |
| Mass number | Protons + neutrons | A |
| Neutrons | Mass number β atomic number | A β Z |
| Electrons | Same as protons (neutral atom) | = Z |
Nuclear Notation
An atom is written with the mass number (A) top-left and atomic number (Z) bottom-left:
Worked Example β Sodium Β²Β³ββNa
| What | Calculation | Answer |
|---|---|---|
| Mass number | Given directly | A = 23 |
| Atomic number | Given directly | Z = 11 |
| Protons | = atomic number | 11 |
| Neutrons | 23 β 11 | 12 |
| Electrons | = protons (neutral) | 11 |
4. Electronic Configuration
The electronic configuration describes how electrons are arranged in the energy shells. Fill from the inside out β never skip a shell!
| Shell Number | Max Electrons | Notes |
|---|---|---|
| Shell 1 (inner) | 2 | Always fills first |
| Shell 2 | 8 | Fills second |
| Shell 3 (CSEC) | 8 | Treated as max 8 at CSEC level |
Key Examples
H (Z=1): H (1) β 1 valence electron
C (Z=6): C (2,4) β 4 valence electrons
Na (Z=11): Na (2,8,1) β 1 valence electron
Cl (Z=17): Cl (2,8,7) β 7 valence electrons
Ar (Z=18): Ar (2,8,8) β 8 valence electrons (full outer shell!)
Ca (Z=20): Ca (2,8,8,2) β 2 valence electrons
C (Z=6): C (2,4) β 4 valence electrons
Na (Z=11): Na (2,8,1) β 1 valence electron
Cl (Z=17): Cl (2,8,7) β 7 valence electrons
Ar (Z=18): Ar (2,8,8) β 8 valence electrons (full outer shell!)
Ca (Z=20): Ca (2,8,8,2) β 2 valence electrons
Valence Electrons = Chemical Personality!Valence electrons are the electrons in the outermost occupied shell. They determine how an atom reacts chemically. Elements in the same GROUP have the same number of valence electrons β that's why they have similar reactions. Na has 1 valence eβ» β explodes in water. Ne has 8 (full outer shell) β completely unreactive (used safely in neon signs!).
Try This!Visit phet.colorado.edu β Build an Atom. Drag protons, neutrons, and electrons to build any atom and watch the electron shells fill in real time! Also try ptable.com β click any element to see its shell structure.
5. Isotopes & Isotopy
Key Definitions
Isotopes β Different atoms of the SAME element with the same number of protons and electrons, but DIFFERENT numbers of neutrons.
Isotopy β The occurrence (existence) of isotopes for a given element.
Memory trick: Isotopes are like twins β same family (same atomic number), but not identical (different mass numbers)!
Isotopes β Different atoms of the SAME element with the same number of protons and electrons, but DIFFERENT numbers of neutrons.
Isotopy β The occurrence (existence) of isotopes for a given element.
Memory trick: Isotopes are like twins β same family (same atomic number), but not identical (different mass numbers)!
Example β Chlorine Isotopes
| Isotope | Notation | Protons | Neutrons | Electrons | Abundance |
|---|---|---|---|---|---|
| Chlorine-35 | Β³β΅ββCl | 17 | 18 | 17 | 75% |
| Chlorine-37 | Β³β·ββCl | 17 | 20 | 17 | 25% |
Properties of Isotopes
| Property | Same or Different? | Reason |
|---|---|---|
| Chemical behaviour | SAME β | Identical electron arrangement β reactions depend on electrons |
| Atomic number (Z) | SAME β | Same number of protons |
| Mass number (A) | DIFFERENT β | Different number of neutrons |
| Density & mass | DIFFERENT β | Different neutron count β different mass |
6. Relative Atomic Mass
Because real atomic masses are inconveniently tiny (a hydrogen atom weighs about 1.67 Γ 10β»Β²β΄ g), chemists use a relative scale based on carbon-12.
DefinitionRelative atomic mass (Aα΅£) = the average mass of one atom of an element compared to one-twelfth the mass of one carbon-12 atom. It has NO UNITS (it's a ratio).
Formula
Aα΅£ = Ξ£ (percentage abundance/100 Γ mass number) for each isotope
Worked Example β Chlorine
Chlorine is 75% Cl-35 and 25% Cl-37:
Aα΅£ = (75/100 Γ 35) + (25/100 Γ 37)
= 26.25 + 9.25
= 35.5
= 26.25 + 9.25
= 35.5
Notice no real chlorine atom has 35.5 nucleons β this is a weighted average that accounts for the natural mix of isotopes. That's why Aα΅£ is usually NOT a whole number!
7. Radioactivity
Key Definitions
Radioactive isotope β an isotope with an unstable nucleus that spontaneously decays by emitting particles/energy (radiation) to become more stable.
Half-life β the time for half the nuclei in a sample to decay.
Radioactive isotope β an isotope with an unstable nucleus that spontaneously decays by emitting particles/energy (radiation) to become more stable.
Half-life β the time for half the nuclei in a sample to decay.
Types of Radiation
| Radiation | Symbol | Composition | Charge |
|---|---|---|---|
| Alpha | Ξ± | 2 protons + 2 neutrons | +2 |
| Beta | Ξ² | High-speed electron | β1 |
| Gamma | Ξ³ | Electromagnetic wave | 0 (neutral) |
Uses of Radioactive Isotopes
| Application | Isotope Used | How It Works |
|---|---|---|
| Carbon dating | Carbon-14 (half-life: 5700 years) | Living things absorb C-14. After death, it decays. Comparing C-14/C-12 ratio gives the age (up to ~60,000 years). |
| Radiotherapy (cancer) | Cobalt-60, Iodine-131 | Radiation kills rapidly-dividing cancer cells. I-131 targets thyroid cancer specifically. |
| Medical tracers | Technetium-99, Iodine-131 | Injected in small doses; gamma cameras image organs (brain, heart, kidneys). Short half-lives = not radioactive for long. |
| Nuclear energy | Uranium-235 | Neutron splits the nucleus (fission) β enormous energy + more neutrons β chain reaction β electricity. |
| Pacemakers | Plutonium-238 (half-life: ~87 years) | Half-life long enough to power a pacemaker for a patient's lifetime β no replacement surgery needed! |
SimulationsVisit PhET: Alpha Decay to watch atoms decay one by one. Also try PhET: Nuclear Fission to trigger your own chain reaction!
8. Simulations & Videos
9. CSEC Practice Questions
Try each question yourself first, then click to reveal the full step-by-step answer. π―
If the mass number of an atom is 59 and the number of neutrons is 32, the atomic number is: A) 27 B) 32 C) 59 D) 91
Multiple Choice
βΆ
Step-by-Step Answer
1Use the formula: Atomic number (Z) = Mass number (A) β Neutrons
2Z = 59 β 32 = 27
3Check: This element is Cobalt (Co), which has Z = 27. β
β
Answer: A) 27
A potassium atom is written as Β³βΉββK. Which is true? A) It has 19 neutrons B) It has 20 protons C) It has 19 protons and 20 neutrons D) Its atomic number is 39
Multiple Choice
βΆ
Step-by-Step Answer
1Read nuclear notation: bottom number (19) = atomic number = number of protons. Top number (39) = mass number.
2Protons = Z = 19 β
3Neutrons = A β Z = 39 β 19 = 20 β
4Check options: A says 19 neutrons β | B says 20 protons β | C says 19 protons AND 20 neutrons β | D says atomic number is 39 β
β
Answer: C) 19 protons and 20 neutrons
Naturally occurring element X consists of 85% Β²Β³ββX and 15% Β²β΅ββX. (a) What element is X? (b) Calculate the relative atomic mass of X. (c) Explain why the two isotopes have the same chemical properties but slightly different physical properties. [5 marks]
5 Marks
βΆ
Step-by-Step Answer
1Part (a): Atomic number Z = 11. Look up the periodic table: Z = 11 β Sodium (Na). [1 mark]
2Part (b): Use Aα΅£ = (abundanceβ/100 Γ Aβ) + (abundanceβ/100 Γ Aβ)
3Aα΅£ = (85/100 Γ 23) + (15/100 Γ 25) = 19.55 + 3.75 = 23.3 [2 marks]
4Part (c) β Same chemical properties: Both isotopes have identical electron arrangements (both have 11 electrons in configuration 2,8,1). Chemical reactions depend on electron arrangement, so they react identically. [1 mark]
5Part (c) β Different physical properties: The isotopes have different numbers of neutrons (23β11=12 vs 25β11=14), giving different masses and slightly different densities. [1 mark]
β
Answers: (a) Sodium (Na) | (b) Aα΅£ = 23.3 | (c) Same electrons = same reactions; different neutrons = different mass/density
Write the electronic configurations of: Mg (Z=12), P (Z=15), and Ne (Z=10). For each, state the number of valence electrons. [3 marks]
3 Marks
βΆ
Step-by-Step Answer
1Rule: Fill shells in order: max 2 in shell 1, then max 8 in shell 2, then max 8 in shell 3.
2Mg (Z=12): Shell 1 β 2, Shell 2 β 8, Shell 3 β 2 β΄ Mg(2,8,2) β 2 valence electrons
3P (Z=15): Shell 1 β 2, Shell 2 β 8, Shell 3 β 5 β΄ P(2,8,5) β 5 valence electrons
4Ne (Z=10): Shell 1 β 2, Shell 2 β 8 β΄ Ne(2,8) β 8 valence electrons (full outer shell β completely unreactive noble gas!)
β
Mg(2,8,2) = 2 val. eβ» | P(2,8,5) = 5 val. eβ» | Ne(2,8) = 8 val. eβ»
Boron consists of 20% B-10 and 80% B-11. Calculate the relative atomic mass of boron. Why is the answer not a whole number? [3 marks]
3 Marks
βΆ
Step-by-Step Answer
1Aα΅£ = (20/100 Γ 10) + (80/100 Γ 11) = 2.0 + 8.8 = 10.8 [2 marks]
2Aα΅£ is not a whole number because it is a weighted average of the masses of both isotopes, taking into account their relative abundance. No single boron atom has a mass of 10.8 β it is either B-10 or B-11. [1 mark]
β
Aα΅£ = 10.8. Not a whole number because it is a weighted average of isotope masses.