Science: Chemistry

Atom comes from the greek word "atomos" which means uncuttable and indivisible.

Protons and Neutrons.

Protons are positively charged, neutrons have no charge (are neutral) and electrons are negatively charged.

The atomic number is the number of protons in an atom (also means number of electrons). This determines where the element sits on the periodic table.

The mass number is the number of protons and neutrons added together. Basically, the mass of the atoms nucleus.

Hydrogen is the lightest element as it only has one proton, one neutron and one electron, meaning that the elements overall mass is smaller than the other atoms as they contain more.

The opposite charges of the protons (positive) and electrons (negative) are attracted to each other. This keeps the atoms stuck together.

In an atom the first valence shell contains up to 2 electrons, the second contains up to 8, the third up to 8 and the fourth up to 8. You must fill up each level of the valence shells.