Describe an atom as a positively charged nucleus, consisting of protons…

The idea

An atom is built from three subatomic particles. At its centre sits a nucleus that carries positive charge because it contains protons, which are positive, and neutrons, which carry no charge. Around this nucleus move electrons, each carrying negative charge. The nucleus is tiny compared with the whole atom: its radius is far smaller than the atomic radius, so the atom is mostly empty space occupied by the electron cloud. Despite its small size, the nucleus holds almost all of the atom's mass, because protons and neutrons are far more massive than electrons. A useful picture is a marble at the centre of a large sports hall, with the electrons moving near the hall's outer walls. In a multiple-choice question, match each option against all four ideas: positive nucleus, protons plus neutrons, negative electrons around it, tiny nucleus holding nearly all the mass.

What earns the marks

• Identifies the nucleus as positively charged because it contains protons.
• States that the nucleus contains both protons and neutrons.
• Places negatively charged electrons around the nucleus.
• Recognises that the nuclear radius is much smaller than the radius of the atom.
• States that almost all of the mass of the atom is in the nucleus.

Common mistakes and how to put them right

• Claims that electrons orbit inside the nucleus.

  Put it right: Place the electrons around the outside of the nucleus.

• Describes the nucleus as negatively charged.

  Put it right: State that the nucleus is positive because of its protons.

• Says the mass is spread evenly through the atom.

  Put it right: State that almost all the mass is in the nucleus.

• Treats the nucleus as about half the atom's size.

  Put it right: State that the nuclear radius is much smaller than the atomic radius.

Exam tips

• Read every option and test it against all four features: charge, particles present, electron position and mass location.
• Reject any option that puts electrons inside the nucleus or gives the nucleus a negative charge.
• Remember the size contrast: the nucleus is far smaller than the atom, yet holds almost all the mass.
• If two options seem similar, choose the one that correctly combines positive nucleus, surrounding electrons and concentrated mass.