The periodic table
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.
Chemical similarity within a group
A group is a vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell.
Outer-shell electrons determine how an atom takes part in chemical reactions: they are the electrons that can be lost, gained or shared. Because elements in a group have the same number of outer-shell electrons, they tend to react in similar ways and have similar chemical properties.
For example, lithium and sodium are both in Group 1 and each has one outer-shell electron. Each can lose that electron to form an ion with a charge of $$+1$$. Knowing that another element is in Group 1 therefore helps predict that it can behave similarly.
Elements in the same group are similar, but their reactions are not necessarily identical.
Electron shells and periodic-table position
Electrons in an atom occupy shells around the nucleus. The arrangement of these electrons helps locate the element in the periodic table: its group is a vertical column, and its period is a horizontal row.
Group and outer-shell electrons
For elements in Groups I to VII, the group number equals the number of electrons in the outer shell. For example, an element in Group III has three outer-shell electrons.
Group VIII contains the noble gases, which have full outer electron shells. Helium’s only shell is full with 2 electrons. The other noble gases among the first 20 elements—neon and argon—each have 8 electrons in their outer shell.
Period and occupied shells
The period number equals the number of occupied electron shells: shells that contain electrons. An element with electrons in three shells is in Period 3.
For example, chlorine has the electron arrangement 2, 8, 7. It has 7 outer-shell electrons, so it is in Group VII. It has 3 occupied shells, so it is in Period 3.