Review of the Atom

11SCI - Chemical Reactions

Finn LeSueur


About The Unit

  • Chemical Reactions
  • Internal
  • 4 Credits
  • Assessment Date: Term 3, Week 3 (August 3rd)

Ngā Whāinga Ako

  • Review the structure of the atom

Copy this diagram into your book.


An atom is made up of two distinct parts. The nucleus and electrons.

  • The nucleus is the centre of the atom (B).
  • The electrons (C) move around the nucleus in a shell (A)
  • If one orbit is a shell, all the shells together are called the electron shells (F)

The Nucleus

  • The nucleus holds most of the atoms mass and is made of two particles
  • The proton (D) and the neutron (E)

The Three Particles

The three particles have different electrical charges. Copy this table into your book and fill it out if you know the answers!

Location Electric Charge Mass
Location Electric Charge Mass
Proton Nucleus +1 1
Electron Orbits -1 0
Neutron Nucleus 0 1

Atomic Number

The number of protons inside the nucleus.

Mass Number

The number of protons + neutrons inside the nucleus.

NB: In a neutral atom, the number of protons is equal to the number of electrons. Why?


This is sodium. How many electrons does it have? How many protons does it have? What is its atomic number?


Draw this table in your book and fill it out to describe the following atom.

Question Answer
Number of Protons
Number of Neutrons
Number of Electrons
Atomic Number
Mass Number


An isotope is when the element is the same but the number of neutrons has changed.