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

The idea

After the nucleus was discovered, further experiments showed that the positive charge of a nucleus is not a single continuous quantity. Instead, it can be divided into a whole number of smaller particles, each carrying the same amount of positive charge. These particles are protons. The charge on a nucleus is therefore an integer multiple of the charge on one proton, which is why atomic number is a whole number. For example, a helium nucleus has a charge of 2⁺ because it contains two protons, while a hydrogen nucleus has a charge of 1⁺ because it contains one proton. This idea helped establish the proton as a fundamental particle.

What earns the marks

• State that later experiments showed nuclear positive charge is made of smaller particles.
• Identify these particles as protons, each carrying the same amount of positive charge.
• Explain that the total positive charge of a nucleus is a whole-number multiple of the charge on one proton.
• Relate the number of protons to the atomic number and to the charge on the nucleus.
• Use examples such as hydrogen 1⁺ and helium 2⁺ to show whole-number charge.

Common mistakes and how to put them right

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

  Put it right by stating that it is concentrated in a whole number of protons.

• Confusing protons with electrons.

  Put it right by stating that protons carry positive charge and electrons carry negative charge.

• Writing charges without superscript signs.

  Put it right by using Unicode superscript notation such as 1⁺, 2⁺, Na⁺ and O²⁻.

Exam tips

• Use the term proton for the particle carrying a single unit of positive charge.
• Link the number of protons to the atomic number and to the overall nuclear charge.
• Write ionic charges with superscript signs, for example 1⁺ and 2⁺.