- Extremely small
- Mostly empty space
- They have a dense core called a nucleus
- There are 3 sub-atomic particles in an atom: protons{positive}, neutrons{no charge}, electrons{negative}
- The periodic table is a list of all the known elements.
- Each element has a chemical symbol and an atomic number and mass number.
- The atomic number tells us he number of positive protons in the nucleus of one atom.
- The mass number tells us the number of protons and neutrons in the nucleus of one atom of the element.
- An atom is neutral{it has no charge}. The number of positive protons must be equal to the number of negative electrons.