Atomic Structure

• Atomic number (Z): the number of protons in the nucleus of an atom
  • Each element has its own unique atomic number — no two elements share one
  • In a neutral atom, the number of electrons also equals Z
• Mass number (A): the total number of protons + neutrons in the nucleus

Counting subatomic particles

• Number of protons = Z

• Number of electrons = Z (for a neutral atom)

• Number of neutrons = A − Z

• Example. An atom of beryllium has atomic number 4 and mass number 9, so it has:

  • 4 protons
  • 4 electrons
  • 9 − 4 = 5 neutrons

Reading the periodic table

• Both Z and A are given alongside every element on the periodic table
• The larger of the two numbers is the mass number
• The smaller is the atomic number