Atomic Structure (OCR Gateway GCSE Physics: Combined Science)

• Atoms are the building blocks of all matter
• Atoms have a tiny, dense, positively charged nucleus at their centre, with negatively charged electrons orbiting around the nucleus
• The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom

Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!

Parts of the Atom

• The nucleus contains:
  • Protons - positively charged particles with a relative atomic mass of one unit
  • Neutrons – no charge, and also with a relative atomic mass of one unit

• Almost all of the atom is empty space, but moving around the nucleus there are:
  • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)

• Atoms are incredibly small, with a radius of only 1 × 10^-10 m
• This is 0.0000000001 m when written without standard form
  • This means that about one hundred million atoms could fit side by side across your thumbnail

• A group of atoms can bond together to make molecules, such as water
  • A water molecule is comprised of two hydrogen atoms and one oxygen atom

A 200 ml glass of water will contain approximately 7,000,000,000,000,000,000,000,000 water molecules!