Describe the structure of the atom, limited to the position, mass and…

The idea

An atom has a small central nucleus surrounded by electrons. The nucleus contains protons and neutrons. A proton has a relative charge of +1 and a relative mass of 1. A neutron has a relative charge of 0 and a relative mass of 1. An electron has a relative charge of −1 and a relative mass of about 1/1836, which is negligible compared with a proton or neutron. Electrons occupy shells or energy levels around the nucleus. The nucleus therefore carries the positive charge and nearly all the mass, while the electrons balance the charge in a neutral atom. A correct answer places each particle correctly and gives its charge and relative mass.

What earns the marks

• Places protons and neutrons in the nucleus and electrons in shells around the nucleus.
• Gives the proton a relative charge of +1 and a relative mass of 1.
• Gives the neutron a relative charge of 0 and a relative mass of 1.
• Gives the electron a relative charge of −1 and a relative mass that is negligible compared with a proton.

Common mistakes and how to put them right

• Places electrons inside the nucleus.

  Put it right by stating that electrons occupy shells around the nucleus.

• Gives the neutron a charge of +1 or −1.

  Put it right by stating that a neutron has a relative charge of 0.

• Gives the electron a relative mass of 1.

  Put it right by stating that its relative mass is negligible, about 1/1836 of a proton.

Exam tips

• Match each particle to its position, charge and relative mass in a table before choosing an option.
• Remember that only protons and neutrons sit in the nucleus.
• Check the sign of each charge carefully, since a single sign error changes the answer.