The periodic table · Year 9
The periodic table lists every element in order of atomic number (the number of protons). It is arranged so that elements with similar properties line up in columns called groups; the rows are called periods. Metals are on the left and in the middle, non-metals on the right. Dmitri Mendeleev built the first table like this in 1869 and left gaps for elements nobody had found yet.
Key facts
- Elements are in order of atomic number. Columns are groups; rows are periods.
- Elements in the same group have the same number of outer electrons, so they react in similar ways.
- The period number is the number of electron shells. Going across a period, each element has one more proton and one more electron.
- Metals are to the left of the staircase line and in the middle; non-metals are to the right. About three quarters of the elements are metals.
- Metals are shiny, conduct heat and electricity, and are malleable. Most non-metals are dull, brittle when solid and do not conduct (graphite is an exception).
- On a tile, the atomic number (bottom) is the number of protons, and of electrons in an atom. Neutrons = mass number (top) − atomic number.
- Mendeleev put elements in order of atomic weight but swapped some and left gaps so that groups matched. His predictions for the missing elements came true.