Periodic table organisation

The periodic table arranges elements in order of increasing atomic number. An element’s atomic number is the number of protons in the nucleus of each of its atoms. In a neutral atom, it is also the number of electrons.

Electrons are arranged in shells around the nucleus. As atomic number increases, the arrangement of electrons changes, but patterns in the outer shell repeat. Elements with similar outer-shell arrangements tend to have similar chemical properties, so these properties appear again at intervals. This repeating pattern is why the table is called periodic.

Groups and outer-shell electrons

A vertical column of the periodic table is called a group. In the main groups, elements in the same group generally have the same number of outer-shell electrons. Chemical reactions mainly involve these electrons, so elements in the same group tend to react in similar ways.

For example, lithium has atomic number 3 and its electrons are arranged as 2 in the first shell and 1 in the second. Sodium has atomic number 11 and its electrons are arranged as 2, 8, 1. Both have one outer-shell electron and are in the same group.

Helium is an exception to the usual outer-electron count within its group: it has 2 electrons in its full outer shell, while the other elements in that group have 8. An element’s position in the table therefore reflects both its atomic number and the arrangement of its electrons.

Predicting properties from periodic table position

An element’s position in the periodic table gives clues about its chemical behaviour. Elements in the same vertical column, or group, often react in similar ways because their atoms have the same number of electrons in their outer shell. These outer-shell electrons are involved in forming compounds.

A pattern observed for one element can therefore help predict the behaviour of another element in the same group. The prediction might concern the type of reaction it undergoes or the kind of compound it forms. The similarity does not mean that the elements react at the same rate or behave identically.

Using group patterns to make predictions

Lithium, sodium and potassium are in Group 1. Each has one outer-shell electron, which its atoms can lose when they react. Sodium and potassium both react with chlorine to form compounds with similar formulae: sodium chloride, $$\ce{NaCl}$$, and potassium chloride, $$\ce{KCl}$$. From this pattern, it is sensible to predict that lithium will also react with chlorine to form a similar compound.

Position can also help predict relative reactivity. In Group 1, reactivity increases down the group: potassium is more reactive than sodium, which is more reactive than lithium. Further down the group, the outer electron is farther from the nucleus and is shielded by more inner electrons, so it is more easily lost.

Reactivity does not increase down every group. A prediction about reactivity must use the pattern for the particular group, rather than assuming that all groups follow the Group 1 trend. Periodic patterns give a reasoned expectation about an element’s behaviour, even when its individual reactions are not already known.

Periodic table periods and electron shells

A period is a horizontal row of the periodic table. The period number tells us how many electron shells are occupied in an atom. For example, atoms in period 3 have electrons in three shells.

An element’s atomic number is the number of protons in each atom. A neutral atom has the same number of electrons as protons, so its atomic number also tells us its number of electrons.

Across a period, atomic number increases from one element to the next. Neutral atoms therefore have successively more electrons, but the number of occupied shells stays the same. A new period begins when electrons start to occupy a new outer shell.

Shell arrangements in the first 20 elements

For the first 20 elements, the first shell holds up to two electrons and the second holds up to eight. Up to eight electrons occupy the third shell before electrons begin to enter the fourth.

Sodium, with atomic number 11, has 11 electrons arranged as $$2,8,1$$. Three shells are occupied, so sodium is in period 3. Magnesium has one more electron, giving it the arrangement $$2,8,2$$. It is also in period 3 because it still has three occupied shells.

Potassium has atomic number 19 and the arrangement $$2,8,8,1$$. Its outermost electron occupies a fourth shell, so potassium begins period 4.