Periodicity: Trends Across Period 3 (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about periodicity: the trends in atomic radius, first ionisation energy and melting point across Period 3.

These trends sit together because they repeat across every period, but they do not all have the same explanation: atomic radius and first ionisation energy come from the nuclear charge and the shielding of the outer electrons, while melting point comes from structure and bonding.

Elements are arranged in order of increasing atomic number, and across each period their properties show repeating patterns. That is what periodicity means. Across Period 3 the atomic radius decreases and the first ionisation energy generally increases. Both of those follow from two separate things about the outer electrons: the nuclear charge pulling on them, and the shielding between them and the nucleus. Melting point is not due to those. It follows from the structure and bonding of each element.

We start with the table itself and how elements are classified. Then atomic radius. Then first ionisation energy. And last melting point, which has a different explanation behind it.

The periodic table lists all the known elements in order of increasing atomic number. The rows are called periods, and the period number is the outer energy level occupied by electrons. The columns are called groups, and elements in the same group share the same outer-shell electronic configuration, which is why they show similar properties. Every element belongs to one of four blocks, s, p, d or f, according to which kind of electron is in its outer shell. As atomic number increases, the properties of the elements show trends that repeat themselves in each period, and that repetition is what periodicity means.

The atomic radius is the distance between the nucleus and the outermost electron of an atom. It is measured by taking two atoms of the same element, measuring the distance between their nuclei, and halving it. In metals it is also called the metallic radius, and in non-metals the covalent radius. Across Period 3, the atomic radii decrease. Two separate things are going on here. First, the number of protons increases by one at each step, so the nuclear charge increases. Second, the elements in a period all have the same number of shells, so the shielding effect is the same right across the period. Taken together, those two mean the nucleus attracts the outer electrons more strongly and pulls them closer in, so the atoms get smaller.

The first ionisation energy is the energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state, to form one mole of gaseous ions. Across Period 3 it generally increases. The explanation is the same pair of factors as for atomic radius: the effective nuclear charge increases because the number of protons increases, and the shielding stays approximately constant across the period. The stronger attraction between the nucleus and the outer electron makes that electron harder to remove. There are two dips in the pattern, between magnesium and aluminium and between phosphorus and sulfur, and those come from which sub-shell the electron is removed from, and from repulsion between a pair of electrons sharing an orbital.

Melting point is the one trend here that does not come from nuclear charge and shielding. It comes from the structure and bonding of each element. Sodium, magnesium and aluminium are metals with giant metallic structures: positive ions in a giant lattice, held together by a sea of delocalised electrons. Sodium donates one electron into that sea, magnesium two and aluminium three, so the metallic bonding is stronger in aluminium than in sodium, and the melting points increase from sodium to aluminium.

Silicon has the highest melting point of all, because of its giant molecular structure, in which each silicon atom is held to its neighbours by four strong covalent bonds. Phosphorus, sulfur, chlorine and argon are simple molecules. The covalent bonds inside those molecules are strong, but between the molecules there are only weak instantaneous dipole-induced dipole forces, and it does not take much energy to break those. Among those molecular elements, sulfur has the highest melting point because S8 is the largest molecule, then phosphorus as P4, with chlorine and argon much lower.

For a melting point answer, state the structure, state the type of bonding or the forces being broken, and state their relative strength.

Elements are ordered by atomic number, and their properties repeat across each period. Across Period 3, atomic radius decreases and first ionisation energy generally increases, and both of those are due to the nuclear charge increasing and the shielding staying the same. Melting point rises as far as silicon and then falls away, and that comes from structure and bonding instead. Three trends across the same eight elements, with two different explanations behind them.