The Structure of the Atom (Oxford AQA IGCSE Chemistry)

The Atomic Model

How the atomic model has changed over time

• In 1803, the atomic model proposed by John Dalton was a simple sphere of matter

• At the time, the theory was correct but new experimental evidence, from scientific advances, led to the atomic model developing over time

  • This is a fundamental feature of science: new experimental evidence leads to scientific models being changed or replaced

 The evolution of atomic models

Atomic Structure

• Atoms are tiny particles of matter, which are the building blocks of all matter

• Each atom is made of subatomic particles called:

  1. Protons

  2. Neutrons

  3. Electrons

The structure of a carbon atom

• Most of an atom is empty space

• The centre of an atom is called the nucleus

  • The nucleus consists of protons and neutrons

  • The nucleus is positively charged 

• The electrons surround the nucleus in shells

  • Electrons have almost no mass, so most of the mass of an atom is located in the nucleus

Relative Charges of the Proton, Electron & Neutron

• Protons, neutrons and electrons are so small that it is not practical to measure their charges using conventional units, such as coulombs

• Instead, their charges are compared to each other

  • This is why they are called relative electrical charges

• The relative electrical charges of the subatomic particles are:

Table of relative electrical charges

• Atoms have no overall charge

• This is because they have the same number of positive protons as negative electrons

  • The negative charge of one electron exactly cancels out the positive charge of one proton

• Ions are formed when an atom loses or gains electrons to achieve a full outer shell 

  • If an atom loses one negative electron, it forms a positively charged 1+ ion 

  • If an atom gains one negative electron, it forms a negatively charged 1- ion