UNIT 2 CHEMISTRY · PEARSON AAQ · A1.1.1
Atomic Structure
FREE CHEMISTRY REVISION · TOPIC 1
Small particles. Clear rules.
Read atomic symbols, count subatomic particles and explain the difference between atoms, isotopes and ions.
New Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · A1.1.1
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What you will learn
- Identify protons, neutrons and electrons and compare their charges and relative masses.
- Read element symbols with atomic number, mass number and ionic charge.
- Calculate particle numbers in neutral atoms, isotopes, positive ions and negative ions.
- Explain why changing electrons makes an ion, whereas changing neutrons makes a different isotope.
Mapped to Unit 2 A1.1.1 in Pearson’s AAQ Applied Science Extended Certificate specification, Issue 7 (May 2026), printed page 36. Relative atomic mass (A1.1.2), detailed electron configurations (A1.2) and ionisation energies (A1.3) are separate topics.
1. The particles inside an atom
An atom has a small central nucleus containing protons and, in most atoms, neutrons. Electrons occupy regions around the nucleus. The nucleus contains almost all the atom’s mass, while most of the atom’s volume lies outside it. A proton and an electron have equal-sized but opposite charges.
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | Approximately 1 |
| Neutron | Nucleus | 0 | Approximately 1 |
| Electron | Outside the nucleus | −1 | Approximately 1/1836 |
A neutral atom has equal numbers of protons and electrons, so its total charge is zero. Neutrons add mass but carry no electric charge. Hydrogen-1 is an important exception to the usual picture of a nucleus containing neutrons: its nucleus contains one proton and no neutrons.
2. Read the atomic symbol
Atomic number, Z: the number of protons in a nucleus. Every atom or ion of the same element has the same Z. Changing the proton number changes the element.
Mass number, A: the total number of protons and neutrons in one nucleus. These particles are collectively called nucleons. Mass number is a whole-number count; it is not the decimal relative atomic mass commonly displayed on a periodic table.
Upper left: A = protons + neutrons.
Lower left: Z = protons.
X = element symbol, for example Al for aluminium.
Upper right: ionic charge, for example 3+ or 2−. No charge shown normally indicates a neutral atom.
Element symbols are case-sensitive: Co is cobalt, whereas CO represents carbon monoxide. In nuclide notation, the mass number identifies a particular isotope, for example ²³₁₁Na (sodium-23).
Protons = Z
Neutrons = A − Z
Electrons in a neutral atom = Z
3. Isotopes: same element, different neutron number
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Their atomic numbers match, but their mass numbers differ. Neutral isotopes also have equal electron numbers.
Chlorine-35: ³⁵₁₇Cl
Protons = 17
Neutrons = 35 − 17 = 18
Electrons = 17 (neutral)
Chlorine-37: ³⁷₁₇Cl
Protons = 17
Neutrons = 37 − 17 = 20
Electrons = 17 (neutral)
Both are chlorine because both have 17 protons. At this level, isotopes of an element are treated as having the same chemical behaviour because their electron arrangements are the same, while their different masses can affect physical properties.
4. Ions: count the electrons carefully
An ion is an atom or group of atoms with an overall electric charge. Here we focus on single-atom (monatomic) ions. They form when atoms lose or gain electrons. Ordinary ion formation does not change the number of protons or neutrons.
Positive ions: cations
Electrons have been lost. There are fewer electrons than protons. For an n+ ion:
Electrons = Z − n
Mg²⁺ has lost two electrons: 12 − 2 = 10.
Negative ions: anions
Electrons have been gained. There are more electrons than protons. For an n− ion:
Electrons = Z + n
Cl⁻ has gained one electron: 17 + 1 = 18.
Charge check: ionic charge (in units of the elementary charge) = protons − electrons. Ten protons and eight electrons give 2+, whereas eight protons and ten electrons give 2−.
5. Worked examples: a method you can reuse
For every symbol, identify Z, A and the charge first. Count protons, subtract to find neutrons, then adjust electrons for the charge. Finish by checking protons − electrons.
Example 1 · Neutral sodium, ²³₁₁Na
- Z = 11, so there are 11 protons.
- A = 23, so neutrons = 23 − 11 = 12.
- No charge is shown, so electrons = 11.
- Charge check: 11 − 11 = 0.
Example 2 · Positive aluminium ion, ²⁷₁₃Al³⁺
- Z = 13, so there are 13 protons.
- Neutrons = 27 − 13 = 14.
- 3+ means three electrons have been lost: 13 − 3 = 10 electrons.
- Charge check: 13 − 10 = +3. The mass number stays 27.
Example 3 · Negative oxide ion, ¹⁶₈O²⁻
- Z = 8, so there are 8 protons.
- Neutrons = 16 − 8 = 8.
- 2− means two electrons have been gained: 8 + 2 = 10 electrons.
- Charge check: 8 − 10 = −2.
Example 4 · Identify an unfamiliar species
A species contains 20 protons, 22 neutrons and 18 electrons. Z = 20 identifies calcium, Ca. A = 20 + 22 = 42. Charge = 20 − 18 = +2, so the symbol is ⁴²₂₀Ca²⁺. You can reconstruct the symbol from particle counts as well as read counts from a symbol.
6. Common misconceptions
- “A positive ion gained positive particles.” A monatomic cation normally forms by losing negative electrons; its proton number stays the same.
- “The minus sign means subtract electrons.” A negative ion has gained electrons. Subtract for a positive charge; add for a negative charge.
- “Mass number includes electrons.” It counts protons and neutrons only. Electron mass is small, not literally zero.
- “Isotopes have different proton numbers.” That would make them different elements. Isotopes differ in neutron number.
- “The periodic-table decimal is the mass number.” Relative atomic mass is an average; mass number describes one nucleus. Do not round an average to invent an isotope.
7. Check your understanding
Choose one answer, then select Check Answer. Read the explanation even if you were correct. Use Try Again to clear your choice and feedback. These are free practice checks; no account or payment is needed. Answers are not saved.
Your revision summary
- Z identifies the element: protons = Z.
- A counts nucleons: neutrons = A − Z.
- Neutral: electrons = protons.
- Positive ion: lose electrons. Negative ion: gain electrons.
- Isotopes: same protons, different neutrons.
- Check the charge: protons − electrons.
Before moving on, explain without notes why ²⁷₁₃Al³⁺ has 13 protons, 14 neutrons and 10 electrons.