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GCSE Chemistry Revision

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GCSE Chemistry revision

Relative electrical charges of subatomic particles

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

AQA 4.1.1.4 Foundation & Higher
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Key knowledge

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What is an Atom?

  • Everything, living or non-living, is made up of incredibly tiny particles called atoms.
  • Atoms are so small that a single cell contains more than 100 trillion of them.
  • Atoms have a radius of around 0.1 nanometres, making them far too small to see with the naked eye.

Basic Structure of an Atom

  • An atom consists of a central nucleus surrounded by electrons that orbit in regions called shells.
  • The nucleus is made up of two types of particles: protons and neutrons, packed tightly together.
  • Most of an atom is empty space, as the distance is measured from the nucleus all the way to the outermost electron shell.
  • The nucleus is approximately 10,000 times smaller than the overall width of the atom.

Relative Mass of Subatomic Particles

  • Protons and neutrons both have a relative mass of 1, as they are similar in size.
  • Electrons are approximately 2,000 times smaller in mass than protons or neutrons, so their relative mass is considered negligible (effectively 0).

Relative Charge of Subatomic Particles

  • Protons carry a relative charge of +1 — both 'proton' and 'positive' begin with the letter P, which helps you remember this.
  • Neutrons carry no charge and are electrically neutral — 'neutron' and 'neutral' share the same beginning.
  • Electrons carry a relative charge of −1, equal in magnitude but opposite in sign to the charge of a proton.
  • Because protons and neutrons are in the nucleus and electrons are outside it, the nucleus is positively charged overall.

Overall Charge of an Atom

  • In a neutral atom, the number of protons always equals the number of electrons, so the positive and negative charges cancel out.
  • Because the charges balance, an atom as a whole has no overall electrical charge.

Ions, Charged Atoms

  • When an atom gains or loses electrons, the charges no longer balance and the atom becomes a charged particle called an ion.
  • If an atom gains electrons, there are more negative electrons than positive protons, forming a negative ion (e.g. a 1− or 2− ion).
  • If an atom loses electrons, there are more positive protons than negative electrons, forming a positive ion (e.g. a 1+ ion).

Elements and the Periodic Table

  • Each different type of atom is called an element, and every element has its own box in the periodic table.
  • Each box in the periodic table is known as a nuclear symbol and contains key information about that element.
  • The elemental symbol is a one- or two-letter abbreviation representing the element, such as O for oxygen and Li for lithium.

Atomic Number

  • The atomic number, found in the bottom left of the nuclear symbol, tells you the number of protons in an atom of that element.
  • It is the number of protons that determines which element an atom is — no two elements have the same atomic number.
  • Because a neutral atom has equal numbers of protons and electrons, the atomic number also tells you the number of electrons.

Mass Number and Calculating Neutrons

  • The mass number, found in the top left of the nuclear symbol, gives the total number of protons and neutrons in the nucleus.
  • The number of neutrons can be calculated using the formula: neutrons = mass number - atomic number.
  • For example, oxygen has a mass number of 16 and an atomic number of 8, so it has 16 - 8 = 8 neutrons.
  • The number of neutrons does not always equal the number of protons — for instance, lithium has 3 protons but 4 neutrons.

Quick Reference: Subatomic Particles Summary

  • Protons: located in the nucleus, relative mass = 1, relative charge = +1.
  • Neutrons: located in the nucleus, relative mass = 1, relative charge = 0.
  • Electrons: orbit the nucleus in shells, relative mass ≈ 0, relative charge = −1.