Atomic Structure

Atoms and molecules

• An atom is the smallest particle of an element that still has that element's chemical properties
• A molecule is two or more atoms chemically bonded together to form one identifiable unit

Inside an atom

• Every atom contains three types of subatomic particle:
  • Protons — positive charge, found in the nucleus
  • Neutrons — no charge, found in the nucleus
  • Electrons — negative charge, found in shells around the nucleus
• The nucleus is the dense centre of the atom holding the protons and neutrons together
  • It is positively charged (because of the protons)
• Electrons move around the nucleus in shells (also called energy levels)
• Almost all of the atom's mass sits in the nucleus; the mass of an electron is negligible

Relative masses and charges

• Subatomic particles are far too small for their masses to be measured in grams or their charges in coulombs
• Their masses and charges are quoted relative to each other instead

• A proton and a neutron have almost identical masses; both are given a relative mass of 1
• An electron is roughly 1840 times lighter than a proton, so its mass is treated as negligible in exam answers