1.2A The Subatomic Particles

Atoms contain a positively-charged dense nucleus composed of protons and neutrons (together called nucleons). Negatively-charged electrons occupy the space outside of the nucleus

All elements are made of the same subatomic particles; the only difference is how many of these subatomic particles are present in the atoms of the different elements.

The atomic number (Z) is the number of protons in the atom

The mass number (A) is the number of protons + the number of neutrons in an atom

Ions

• Neutral atoms have equal numbers of protons and electrons

• Ions are formed when atoms gain or lose electrons

• When an atom loses electrons, positively-charged cations are formed

• When an atom gains electrons, negatively-charged anions are formed

• The charge of ions depends on the number of electrons lost/gained