The periodic table

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.

Worked examples

Example 1

Find sodium in the periodic table. Which group and period is it in, and is it a metal?

  1. Sodium, Na, is atomic number 11.
  2. It is in the first column (group 1) and the third row (period 3).
  3. It is on the far left, so it is a metal: an alkali metal.

Answer: group 1, period 3; a metal

Example 2

Here is part of the table. Which element is marked ? and what group is it in?

  1. The ? is in row 3, so it is in period 3.
  2. It is in the column under oxygen: group 6 (counting the main groups 1–7 and 0), or group 16 when all 18 columns are numbered.
  3. Period 3, group 6 is sulfur, S. Like oxygen, it is a non-metal.

Answer: sulfur; group 6 (group 16 in the 1–18 numbering)

Example 3

How many protons, electrons and neutrons are in this atom of fluorine?

  1. The bottom number is the atomic number: 9 protons.
  2. An atom has no overall charge, so it has as many electrons as protons: 9.
  3. The top number is the mass number (protons + neutrons). 19 − 9 = 10 neutrons

Answer: 9 protons, 9 electrons, 10 neutrons

Example 4

An element has the electron arrangement 2,8,2. Which group and period is it in?

  1. There are 3 shells with electrons, so it is in period 3.
  2. There are 2 electrons in the outer shell, so it is in group 2.
  3. It has 2 + 8 + 2 = 12 electrons, so its atomic number is 12: magnesium.

Answer: group 2, period 3 (magnesium)