Atomic Structure and the Periodic Table

Inside the Atom

An atom is the smallest particle of an element. It is made of three sub-atomic particles:

• Protons: positive charge, found in the nucleus.
• Neutrons: no charge, found in the nucleus.
• Electrons: negative charge, orbit the nucleus in shells.

Atomic Number and Mass Number

• Atomic number = number of protons.
• Mass number = protons + neutrons.

The Periodic Table

Elements are arranged in order of increasing atomic number. Vertical columns are called groups and horizontal rows are called periods. Elements in the same group have similar chemical properties.

Key Points

Atoms are neutral because the number of protons equals the number of electrons. The group number often tells you the number of outer-shell electrons.