Atomic Structure and the Periodic Table

Matter, atoms and elements

Everything material is matter: it has mass and occupies space. Matter is made from tiny particles, and its different forms can ultimately be understood in terms of atoms. There are about 100 different naturally occurring types of atom, called elements.

An element contains only one type of atom. What makes all atoms of an element the same is their number of protons. Every carbon atom has 6 protons, for example, whereas every oxygen atom has 8. Matter may contain atoms of one element, or atoms of different elements combined in compounds. A molecule containing two atoms is not automatically a compound: hydrogen, $\mathrm{H_2}$, still contains only hydrogen atoms and is therefore an element.

Inside an atom

At the centre of an atom is a tiny nucleus containing positively charged protons and uncharged neutrons. Negatively charged electrons occupy regions around the nucleus called or . Shell diagrams draw these levels as rings, but the rings are a model rather than fixed physical tracks.

The atomic number is the number of protons in the nucleus. This number defines the element. An atom is electrically neutral, so it contains equal numbers of positive protons and negative electrons. An oxygen atom has atomic number 8, for instance, so it has 8 protons and 8 electrons. The number of neutrons can vary between atoms of the same element without changing which element they are.

Building electronic structures

Electrons fill the available energy levels in a particular order. In the simple GCSE model for the first 20 elements, the first shell holds up to 2 electrons. The second holds up to 8, and the third fills to 8 before the fourth begins to fill.

Electronic structures are written as comma-separated numbers, working outwards from the nucleus. Oxygen's 8 electrons give $2,6$: 2 electrons fill the first shell and the remaining 6 occupy the second. Sodium has 11 electrons, giving $2,8,1$, while calcium has 20, giving . Each structure must contain the same total number of electrons as the of the neutral .

This simplified filling pattern is appropriate for the first 20 elements. It must not be extended mechanically to elements such as gold. From atomic number 21, the transition elements also make the relationship between shell filling and group position more complicated.

How atomic structure organises the periodic table

The periodic table lists elements in increasing atomic number. A period is a horizontal row, while a group is a vertical column. Their arrangement reflects electronic structure rather than being an arbitrary grid.

For the first 20 elements, the period number tells you the number of occupied electron shells. Magnesium has the structure $2,8,2$, so it has three occupied shells and belongs to Period 3. Lithium, with $2,1$, begins Period 2 because its third electron starts a second shell.

For elements in Groups 1–7, the group number tells you the number of electrons in the outer shell. Magnesium has two outer electrons, so it is in Group 2. Sulfur has the structure $2,8,6$, so it is in Period 3 and Group 6. Group 0 contains the noble gases, whose outer shells are full; helium's full first shell contains 2 electrons, whereas the later noble gases considered here have 8 outer electrons.

You can also work backwards. An atom with structure $2,8,7$ has 17 electrons in total, so its atomic number is 17 and the element is chlorine. Its three occupied shells place it in Period 3, and its seven outer electrons place it in Group 7.

Why properties repeat periodically

An element's chemical behaviour depends strongly on its outer-shell electrons because these are involved when atoms react. Moving across a period adds electrons to the same outer shell. On reaching a full outer shell, the next element begins a new period with an electron in a new shell. Similar outer structures therefore recur at regular intervals as increases.

This produces periodic relationships: patterns in physical and chemical properties repeat across the table. Elements in the same group have the same number of outer-shell electrons, so they often react in similar ways. Lithium, sodium and potassium all have one outer electron and are reactive Group 1 metals. Fluorine and chlorine have structures $2,7$ and $2,8,7$ respectively; both have seven outer electrons, belong to Group 7 and show related halogen chemistry. The Group 0 elements have full outer shells and are very unreactive gases.

The transition elements occupy the block between Groups 2 and 3 in the longer periods. Their electron arrangements are more complicated than the first-20 filling pattern, but their position and related properties are still consequences of atomic structure. Thus the periodic table connects a microscopic feature—how electrons are arranged—to observable patterns in elements' chemical and physical properties.