SQA Higher Chemistry

The periodic table

The elements are arranged in order of increasing atomic number, which is the number of protons in the nucleus. Because the arrangement follows the electron arrangement of the atoms, an element's position lets a chemist predict its physical properties and its chemical behaviour, even for an element that has never been handled.

A group is a vertical column. The elements in a group have similar chemical properties because their atoms have the same number of electrons in the outer shell. The alkali metals (group 1) all have one outer electron, and all form a $1+$ ion by losing it; the halogens (group 7) all have seven outer electrons, and all tend to gain one more.

A period is a horizontal row. Moving from left to right across a period the atomic number rises, so the number of outer electrons rises by one at each step, and the elements change from metallic to non-metallic. Sodium to argon, for example, runs from a reactive metal with one outer electron, through a semi-metal (silicon), to reactive non-metals (phosphorus, sulfur, chlorine) and finally an unreactive monatomic gas with a full outer shell.

Bonding and structure in the first 20 elements

Each of the first 20 elements falls into one of four categories, and the category decides its physical properties.

Metallic elements

The outer electrons of a metal are not held by one atom. They are delocalised, which means they are free to move through the whole lattice, and the positive ions are held together by their attraction to this mobile negative charge. The delocalised electrons carry current, so metals conduct electricity as solids and as liquids. The metallic bond is strong in most metals, so melting points are generally high, although the group 1 metals are soft and melt at low temperatures compared with most.

Covalent molecular elements

The atoms inside a molecule are held by covalent bonds, which are strong, but when a molecular element melts or boils it is only the weak forces between the molecules that are overcome, not the covalent bonds. Melting and boiling points are therefore low, and these elements are gases, liquids or low-melting solids at room temperature. They do not conduct electricity, because the electrons are held within the molecules and there are no charged particles free to move.

Fullerenes are molecules made only of carbon atoms, with the atoms arranged in a hollow cage. The best-known, $\ce{C60}$, is a sphere of 60 carbon atoms. Because it is made of discrete molecules it is classed with the covalent molecular substances and not with diamond and graphite.

Covalent network elements

In a covalent network there are no molecules: every atom is bonded to its neighbours throughout the whole lattice, so the crystal is one giant structure. To melt it, covalent bonds themselves must be broken, which takes a great deal of energy, and the melting points are very high. Network elements are hard and insoluble in all common solvents.

In diamond each carbon atom is bonded to four others in a tetrahedral arrangement, and there are no free electrons, so diamond does not conduct electricity. In graphite each carbon atom bonds to three others in flat layers of hexagons. The fourth outer electron of each atom is delocalised along the layer, so graphite does conduct electricity, and the layers are held together only by weak forces, so they slide over one another and graphite is soft. Silicon has the same structure as diamond but is a semiconductor.

Monatomic elements

The noble gases exist as separate atoms because each already has a full outer shell and has no tendency to form bonds. Only very weak forces act between the atoms, so their melting and boiling points are extremely low.

Covalent radius

The covalent radius measures the size of an atom. It is taken as half the distance between the nuclei of two atoms of the same element joined by a single covalent bond, and is quoted in picometres ($\pu{1 pm} = 10^{-12}\,\mathrm{m}$).

Trend across a period

The covalent radius decreases from left to right. Every atom in a period has its outer electrons in the same shell, so the amount of screening by inner electrons is almost unchanged, but the nuclear charge rises by one with each element. The stronger attraction from the nucleus pulls the outer shell in closer.

Trend down a group

The covalent radius increases down a group. Each step down adds an occupied shell, so the outer electrons are further from the nucleus and there are more inner shells of electrons between them and the nucleus. The nuclear charge also rises, but the extra shell outweighs it.

These two trends, and the words used to explain them (number of occupied shells, nuclear charge and screening by inner electrons), are the basis for explaining the trends in ionisation energy and electronegativity.

Melting points across period 3

The structure of each element explains how its melting point changes across a period, and period 3 shows all the types in one row.

• From sodium to aluminium the melting point rises, because each atom contributes more delocalised electrons (one, two and three) and the ions get smaller and more highly charged, so the metallic bond is stronger.
• Silicon has the highest value in the period, since its melting requires the breaking of covalent bonds throughout a giant network.
• Phosphorus, sulfur and chlorine are molecular, so only the weak forces between molecules are overcome on melting, and the values are low. Sulfur ($\ce{S8}$) melts higher than phosphorus ($\ce{P4}$), and both melt higher than chlorine ($\ce{Cl2}$), because the larger the molecule and the more electrons it has, the stronger the London dispersion forces.
• Argon has the lowest melting point in the row, since single atoms with a small number of electrons have the weakest forces of all.

The same reasoning applies to any element in the first 20: identify the category from its position, and then link the property to the particles and the forces that must be overcome.

Explaining a trend

A full explanation of a trend in covalent radius, ionisation energy or electronegativity names three things, in this order: the change in the number of occupied shells (or whether it is constant), the change in nuclear charge, and the change in screening by inner electrons. It then states what this does to the attraction on the outer electrons and so to the property. For a trend across a period, the shells and screening are the same and the nuclear charge is the factor that changes. For a trend down a group, the extra shell and extra screening outweigh the larger nuclear charge.