This mini-lesson walks you through the whole of CCEA Unit 1.1 — Atomic Structure: the sub-atomic particles, atomic & mass number, isotopes, calculating relative atomic mass, and writing electronic configurations for the first 20 elements.
A lithium atom: a tiny dense nucleus (protons + neutrons) surrounded by electrons in shells.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
The three sub-atomic particles
What an atom is made of
Every atom has a tiny central nucleus made of protons and neutrons, with electrons moving around it in shells. CCEA wants you to know each particle's relative charge and relative mass:
Protons and neutrons sit in the nucleus; electrons are almost massless and orbit in shells.
Protons — charge +1, mass 1, found in the nucleus.
Neutrons — charge 0 (neutral), mass 1, found in the nucleus.
Electrons — charge −1, mass ≈ 1/1836 (so light it counts as zero), in shells.
Why an atom is neutral: a neutral atom has equal numbers of protons and electrons, so the +1 and −1 charges cancel exactly.
Match it
Match each particle to its property
Tap a particle on the left, then its correct charge & mass on the right.
Atomic number & mass number
The two numbers on every symbol
Each element symbol carries two numbers. The bottom one is the atomic number; the top one is the mass number.
For sodium: atomic number 11 (= protons = electrons), mass number 23.
number of neutrons = mass number − atomic numbere.g. sodium: 23 − 11 = 12 neutrons
Atomic number = number of protons (and, in a neutral atom, the number of electrons).
Mass number = number of protons + neutrons (the heavy particles in the nucleus).
Ions: an ion forms when an atom gains or loses electrons, so its electrons no longer equal its protons. A Na⁺ ion still has 11 protons but only 10 electrons → charge +1. Protons and neutrons never change when an ion forms.
Calculate
Your turn — count the neutrons
1An atom of aluminium has atomic number 13 and mass number 27. How many neutrons does it contain?
neutrons
Hint: neutrons = mass number − atomic number = 27 − 13.
Calculate
Your turn — electrons in an ion
2A magnesium ion is written Mg²⁺. Magnesium has atomic number 12. How many electrons does the Mg²⁺ ion have?
electrons
Hint: a 2+ ion has lost 2 electrons → 12 − 2.
Isotopes
Same element, different neutrons
Isotopes are atoms of the same element with the same atomic number but a different mass number. In other words, they have the same number of protons (and electrons) but a different number of neutrons.
All three carbon isotopes have 6 protons and 6 electrons — only the neutron count (6, 7, 8) changes.
Common mistake: isotopes do not differ in protons. If the proton number changed it would be a different element. Isotopes also behave identically in chemical reactions, because chemistry depends on electrons — and the electron arrangement is unchanged.
Quick check
Which pair are isotopes?
?Which pair of atoms are isotopes of the same element?
Relative atomic mass (Ar)
A weighted average of the isotopes
Because most elements are a mixture of isotopes, the relative atomic mass (Ar) is the weighted mean mass of the atoms, taking into account how much of each isotope is present (its abundance).
Ar = Σ(mass number × % abundance) ÷ 100add up (mass number × abundance) for every isotope, then divide by 100
Worked example — chlorine
Chlorine is 75% chlorine-35 and 25% chlorine-37.
Ar = (35 × 75) + (37 × 25) ÷ 100
= (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5
Why it isn't a whole number: Ar is an average over the isotope mixture, so it sits between the isotope mass numbers and is usually not a whole number — even though every individual atom has a whole-number mass number.
Calculate
Your turn — relative atomic mass
3Boron exists as 20% boron-10 and 80% boron-11. Calculate the relative atomic mass of boron.
Hint: Ar = (10 × 20 + 11 × 80) ÷ 100.
Calculate
Your turn — copper's Ar
4Copper is 70% copper-63 and 30% copper-65. Calculate the relative atomic mass of copper.
Hint: (63 × 70 + 65 × 30) ÷ 100.
Electronic configuration
Filling the electron shells
Electrons sit in shells around the nucleus. CCEA expects you to write the electronic configuration of atoms (and ions) for the first 20 elements. The rules are simple:
The first shell holds a maximum of 2 electrons.
The second and third shells hold a maximum of 8 each.
Fill the inner shells first, then start the next one.
Sodium's 11 electrons fill as 2, 8, 1 — two inner shells full, one electron left over in the outer shell.
Link to the Periodic Table: the number of shells used = the period (row), and the number of electrons in the outer shell = the group. Sodium (2,8,1) → 3 shells = Period 3, 1 outer electron = Group 1.
Quick check
Write the configuration
?Chlorine has atomic number 17. What is its electronic configuration?
Calculate
Your turn — group from configuration
5Calcium has atomic number 20, configuration 2, 8, 8, 2. Using the outer shell, which group of the Periodic Table is calcium in?
Group
Hint: group number = number of electrons in the outer shell.
Calculate
Your turn — period from configuration
6Argon has the configuration 2, 8, 8. How many shells contain electrons — i.e. which period is argon in?
Period
Hint: count how many numbers are in the configuration.
Recap
Lock it in
Particles: proton (+1, mass 1), neutron (0, mass 1) in the nucleus; electron (−1, mass ≈ 0) in shells.
Atomic number = protons; mass number = protons + neutrons; neutrons = mass − atomic number.
Ions form by gaining/losing electrons; protons and neutrons stay the same.
Isotopes = same protons, different neutrons (different mass number).
Ar = weighted mean of isotope masses = Σ(mass × abundance) ÷ 100.
Electronic configuration (1–20): fill 2, then 8, then 8. Outer electrons = group; number of shells = period.
Spec-exact: this covers CCEA Unit 1.1 only. The Periodic Table's group trends, bonding and chemical formulae are separate CCEA topics.
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