Relative charge, mass and atomic numbers for iGCSE Chemistry
Every atom is made of three subatomic particles: Protons, neutrons and electrons. Each has a relative charge and a relative mass that you must recall from memory for Cambridge iGCSE Chemistry. Protons carry a relative charge of +1 and a relative mass of 1, neutrons carry no charge and a relative mass of 1, and electrons carry a relative charge of –1 and a relative mass that is so small it is treated as negligible (about 1/1836).
This guide covers the three particles in detail, how atomic number and mass number are defined, how to work out the number of each particle in any atom or ion, and the wording examiners reward in the Cambridge mark scheme.
Three particles, two locations
Protons and neutrons sit in the nucleus. Electrons orbit in shells. The nucleus holds almost all the mass of an atom.
Two numbers identify any atom
Atomic number (Z) is the number of protons. Mass number (A) is the total number of protons plus neutrons.
Atoms are neutral overall
In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges cancel.
Relative charge and relative mass of each particle
The Cambridge iGCSE specification expects you to recall the relative charge and relative mass of protons, neutrons and electrons. These are relative values, meaning they are given on a scale where a proton is defined as having a mass of 1, so you do not need to know the actual masses in kilograms.
The electron mass is so small compared to a proton that it is treated as effectively zero in mark schemes. Writing "negligible" or "1/1836" both score the mark.
| Particle | Relative charge | Relative mass | Location in the atom |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | –1 | 1/1836 (negligible) | Shells around the nucleus |
Watch the sign on the electron The electron carries a relative charge of –1, not 1. Forgetting the minus sign is one of the most common slips in the question that simply asks for the relative charge of an electron. Always write the sign explicitly.
Atomic number and mass number
Every element has an atomic number and every isotope of that element has a mass number. The atomic number (symbol Z) is the number of protons in the nucleus, and it defines what element the atom is. Change the atomic number and you change the element.
The mass number (symbol A) is the total number of protons plus neutrons in the nucleus. Electrons are not counted in the mass number because their mass is negligible. You will see atoms written in the form ²³₁₁Na, where 23 is the mass number and 11 is the atomic number.
| Symbol | What it counts | How to read it from the periodic table |
|---|---|---|
| Z (atomic number) | Number of protons | Smaller number, usually written below the element symbol |
| A (mass number) | Number of protons plus neutrons | Larger number, usually written above the element symbol |
| A – Z | Number of neutrons | Subtract atomic number from mass number |
Working out the number of each particle
To find the number of protons, neutrons and electrons in any neutral atom, you only need the atomic number and mass number. Number of protons equals Z. Number of electrons equals Z (because the atom is neutral). Number of neutrons equals A minus Z.
This is one of the highest-frequency question types in Cambridge iGCSE Chemistry Paper 1. Practise it until it is automatic, because it is essentially free marks once you have the method.
Worked example: Sodium-23 Sodium has atomic number 11 and mass number 23. Number of protons = 11. Number of electrons = 11 (neutral atom). Number of neutrons = 23 – 11 = 12. Always lay out your answer with these three lines so the examiner can see the working.
What changes when an atom becomes an ion
An ion forms when an atom gains or loses electrons. The number of protons never changes, because that would change the element. Only the electron count changes, which is why ions carry a net charge.
A positive ion (cation) has lost electrons, so it has fewer electrons than protons. A negative ion (anion) has gained electrons, so it has more electrons than protons. The mass number is essentially unchanged because electron mass is negligible.
| Ion | Protons | Electrons | Neutrons |
|---|---|---|---|
| Na⁺ (sodium ion) | 11 | 10 | 12 |
| Cl⁻ (chloride ion) | 17 | 18 | 18 |
| O²⁻ (oxide ion) | 8 | 10 | 8 |
| Mg²⁺ (magnesium ion) | 12 | 10 | 12 |
Isotopes and why mass number can change
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because protons are unchanged, isotopes have the same atomic number and the same chemical properties. Because neutrons differ, isotopes have different mass numbers.
For example, chlorine has two common isotopes: Chlorine-35 has 17 protons and 18 neutrons, chlorine-37 has 17 protons and 20 neutrons. Both still react as chlorine, because chemical behaviour depends on electron count, which is determined by the proton count.
Why relative atomic mass is rarely a whole number The relative atomic mass on the periodic table is a weighted average of the masses of all naturally occurring isotopes. Chlorine sits at 35.5 because natural chlorine is roughly 75 percent chlorine-35 and 25 percent chlorine-37. This is why exam questions often ask you to calculate a weighted average from isotope abundances.
Where students lose marks
Cambridge examiner reports flag the same handful of slips on this topic every year. Most are about precision with wording and signs rather than missing knowledge.
Common mistakes that cost easy marks Writing electron charge as 1 instead of –1. Saying electrons have no mass instead of negligible mass. Confusing atomic number with mass number. Calculating neutrons as Z minus A instead of A minus Z. Forgetting that ions have a different electron count to neutral atoms. Saying isotopes have different chemical properties when they do not.
Key facts to memorise for the exam
- Proton: Relative charge +1, relative mass 1, found in the nucleus
- Neutron: Relative charge 0, relative mass 1, found in the nucleus
- Electron: Relative charge –1, relative mass negligible (1/1836), found in shells
- Atomic number Z = number of protons (defines the element)
- Mass number A = number of protons plus neutrons
- Number of neutrons = A – Z
- Neutral atom: Number of electrons equals number of protons
- Isotopes: Same Z, different A, same chemical properties
