unit 3.2

• Proton (atomic) number (Z) of an element is the number of protons in its atom (equal to the number of electrons)
• Nucleon (mass) number (A) of an element is the total number of protons and neutrons in its nucleus
• Subtracting the proton number from the nucleon number gives the number of neutrons present in an atom
• In atoms, the number of electrons is always equal to the number of protons
• The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom
• An ion is an atom or group of atoms with a positive or negative charge
• Ions form when atoms lose or gain electrons to obtain a fully-filled valence shell
• Metal atoms lose electrons from their valence shell when they form ions
• Non-metal atoms gain electrons when they form ions
• The ions formed are negative because they have more electrons than protons
• The ions formed are positive, with more protons than electrons
• Isotopes are atoms of the same element with the same number of protons and electrons, but different numbers of neutrons
• Isotopes of an element have:

  • The same atomic / proton number
  • Different nucleon / mass numbers
• Isotopes have identical chemical properties because they have the same number of electrons involved in chemical reactions
• Isotopes have different physical properties because they have different masses
• Isotopes have identical chemical properties but their physical properties (e.g. melting & boiling points, solubility, density, etc) are different