Later experiments led to the idea that the positive charge of any nucleus…

The idea

This statement explains how evidence from later experiments changed the nuclear model. After the nucleus was discovered, its positive charge was not explained. Experiments showed that the positive charge of any nucleus is not continuous but is made of a whole number of identical positive particles. For example, a helium nucleus has a positive charge of 2⁺, so it contains two of these particles; a hydrogen nucleus has a charge of 1⁺, so it contains one. Each particle carries the same amount of positive charge. These particles were named protons. The whole-number rule means you never get half a proton's charge in a nucleus. This idea is assessed by asking you to describe the evidence and explain how it led to the proton model, or to use nuclear charges to work out proton numbers.

What earns the marks

• Later experiments showed that the positive charge of a nucleus is not a single continuous amount but can be subdivided into a whole number of equal positive particles.
• Each of these positive particles carries the same amount of positive charge, now called the charge on a proton.
• The whole-number result means the positive charge on a nucleus is an integer multiple of the proton charge, for example 1⁺, 2⁺ or 3⁺, not a fraction such as 0.5⁺.
• The name proton was given to these particles after the evidence supported their existence.
• The number of protons in a nucleus equals the number of whole positive charges, so a nucleus with charge 3⁺ contains three protons.

Common mistakes and how to put them right

• Thinking the positive charge is spread evenly through the nucleus.

  Put it right by stating that the charge is made of separate, identical protons, each with the same positive charge.

• Confusing protons with electrons.

  Put it right by noting that protons are positive and are found in the nucleus, while electrons are negative and occupy energy levels around the nucleus.

• Believing that different nuclei contain protons of different positive charge.

  Put it right by stating that every proton has the same amount of positive charge, so a 2⁺ nucleus has two protons, not one proton with twice the charge.

Exam tips

• Link the evidence to the conclusion: say that the whole-number positive charges observed in experiments led to the idea of identical positive particles called protons.
• Use nuclear charge to count protons: for example, a nucleus with charge 2⁺ contains two protons, and a nucleus with charge 1⁺ contains one proton.
• Avoid saying the experiments 'proved' the proton in a single step; describe how later experiments led to the idea and the name.