The periodic table

The periodic table shows all the different elements that are in existence. In the periodic table, elements with similar properties form columns. The vertical columns are known as groups. Each row in the periodic table is called a period.

Metals and non-metals in the periodic table

The periodic table is made up of metallic and non-metallic elements. Approximately 75% of the elements on the periodic table are metals and 25% non-metals. The chemical and physical properties of metallic and non-metallic elements differ, as shown in the table below.

Electrical conductivity can be used to classify an element as a metal or non-metal. With the exception of carbon and silicon, all non-metals will not conduct electricity, whereas all metals do conduct electricity. The metals are found in the middle and on the left side of the periodic table and the non-metals on the right side of the periodic table, as shown in the diagram on the next page.

All of the elements in groups 1 and 2 are metals. In group 3 the only non-metal is boron, with all elements below boron being metals. In group 4 carbon and silicon are the only non-metals, with all elements below silicon being metals. In group 5 only antimony and bismuth are metals and in group 6 polonium is the only metal. The elements in groups 7 and 0 are all non-metals.

Why elements in the same group of the periodic table have similar properties

The group number of the element gives you the number of electrons in the outer shell. This means that all elements in the same group have the same number of electrons in their outer energy shell.

Atoms always react to gain or lose electrons in order to have a full outer shell. Their chemical reactivity therefore depends on the number of electrons they need to gain or lose to achieve this full outer shell.

As elements in the same group will want to gain or lose the same number of electrons to gain a full outer shell, this explains why elements in the same group have similar chemical properties. This also allows you to predict properties of other elements in that group.

Atomic number and mass number

Each element on the periodic table is represented using a chemical symbol, mass number (or relative atomic mass) and atomic number. The element’s individual chemical symbol and individual atomic number never change. For example, as shown below, carbon has the chemical symbol C, atomic number 6 and mass number 12.

The higher of the two numbers is the mass number. Mass number tells you the total number of protons and neutrons in the nucleus of the atom. The lower of the two numbers is the atomic number. Atomic number (proton number) tells you the number of protons present in the nucleus of the atom. The chemical elements in the periodic table are arranged in order of increasing atomic number.

The number of neutrons present in the nucleus of the atom can be deduced by subtracting the number of protons from the total number of protons and neutrons as shown in the equation:

The number of protons and electrons in an atom is always equal, as a proton has a charge of +1 and an electron has a charge of -1 and the atom must have an overall neutral charge.

Therefore, if we know the mass number and atomic number of an element, we can work out the numbers of protons, neutrons and electrons it has. Using carbon as an example, we known that it has an atomic number of 6 so has 6 protons. This also means that carbon must have 6 electrons. Carbon has a mass number of 12 (so there are 12 protons and neutrons) and an atomic number of 6 (so there are 6 protons). To determine the number of neutrons we use the equation:

Therefore, carbon has 6 neutrons.

The periodic table in terms of electronic structure

The group number of an element tells you how many electrons there are in the outer shell and the period number tells you how many electron-containing energy shells the element has. All elements in the same group have the same number of electrons in their outer energy shell. For example, group 7 elements have 7 electrons in their outer shell.

The nucleus is positively charged due to the presence of the protons. As electrons are negatively charged, they are attracted strongly to the positive charge of the nucleus. This strong force of attractions between the electrons and the nucleus holds them in place.

Key terms

Atomic number – the number of protons in an atom.

Mass number – the number of protons and neutrons in the nucleus of an atom.

As the outer shell electrons get further away from the nucleus, this force of attraction becomes weaker. As you go down a group of the periodic table, there is an increase in the number of energy shells the elements have. This means that there are more energy shells between the outer electrons and the nucleus and the force of attraction between them weakens. This is known as the shielding effect. As you descend any group in the periodic table, the increased shielding effect means that the force of attraction between the outer electrons and nucleus becomes weaker. Some elements want to lose electrons to gain a full outer shell whereas some want to gain them. If the force of attraction between the nucleus and outer electrons is weak, it is easier to lose electrons but more difficult to gain electrons. This trend will be explored and explained in more detail later in the unit using the group 1 and group 7 elements.

Deducing electronic configurations of the first 20 elements from their positions in the perioidic table