Chemistry (school) · Part 02
The periodic table
How the elements are arranged and why groups behave alike.
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- Electron shellA level around the nucleus that can hold a fixed number of electrons. The first shell holds 2, the second holds 8 and the third holds 8 (for the first 20 elements).
- PeriodA horizontal row of the periodic table. All elements in a period have the same number of electron shells.
- GroupA vertical column of the periodic table. Elements in a group have the same number of outer electrons, so they react in similar ways.
- MetalAn element that is usually shiny, hard, strong and a good conductor of heat and electricity. Metals sit on the left and centre of the periodic table.
- Non-metalAn element that is usually dull, brittle when solid and a poor conductor. Non-metals sit on the right of the periodic table.
- Alkali metalsThe soft, reactive metals of group 1: lithium, sodium and potassium. They react with water to make an alkaline solution and hydrogen.
- HalogensThe reactive non-metals of group 7, such as fluorine, chlorine, bromine and iodine. They have 7 outer electrons and form molecules of two atoms.
- Noble gasesThe unreactive gases of group 0, such as helium, neon and argon. Their outer shells are full, so they hardly ever form bonds.
- Transition metalsThe block of metals in the middle of the periodic table, including iron, copper and zinc. They are harder and denser than group 1 metals and make coloured compounds.