Periodic trendsIB MYP Chemistry: Revision notes

The periodic table is arranged in periods (rows) and groups (columns). Elements in a period have the same number of electron shells, and the period number tells you how many. For the main groups, the group number tells you how many outer electrons an atom has (Group 1 has one, Group 7 has seven).

An electron in the outer shell is attracted to the positive nucleus. The strength of this attraction decides most of the trends in this topic. It depends on two things: how many protons are in the nucleus, and how far the outer electron is from it. Inner shells of electrons also shield the outer electron from the nucleus.

Atomic radius is a measure of the size of an atom. Across a period, atomic radius decreases. For period 3 it falls from about 186 pm for sodium to about 99 pm for chlorine.

The reason: moving across, each atom has one more proton and one more electron, but the electrons are added to the same shell. Shielding by the inner shells stays about the same, so the outer electrons feel a stronger pull from the nucleus. The shell is drawn in closer and the atom becomes smaller.

Down a group, atomic radius increases. Each element is in the next period down, so it has one more electron shell. The outer electrons are further from the nucleus and there are more inner shells shielding them. The attraction between the nucleus and the outer electrons is weaker, even though the number of protons has gone up.

For Group 1, the radius rises from about 152 pm for lithium to about 227 pm for potassium.

Metals tend to lose electrons to form positive ions. The easier an atom loses its outer electrons, the more metallic it is.

• Across a period, metallic character decreases. In period 3, sodium, magnesium and aluminium are metals, silicon is a metalloid, and phosphorus, sulfur and chlorine are non-metals. The outer electrons are held more strongly, so they are harder to lose.
• Down a group, metallic character increases. In Group 4, carbon is a non-metal, silicon and germanium are metalloids, and tin and lead are metals. The outer electrons are further away and more shielded, so they are lost more easily.

Group 1 (alkali metals): reactivity increases down the group. A Group 1 atom reacts by losing its one outer electron. Going down, the outer electron is further from the nucleus and more shielded, so it is lost more easily. Lithium fizzes in water, sodium melts into a ball, and potassium burns with a lilac flame.

Group 7 (halogens): reactivity decreases down the group. A halogen atom reacts by gaining one electron. Going down, the atom is larger and the incoming electron is further from the nucleus and more shielded, so it is attracted less strongly. A more reactive halogen displaces a less reactive one from a solution of its salt, so chlorine displaces bromine and iodine, but iodine displaces neither.

Across a period: the reactivity of the metals decreases (sodium > magnesium > aluminium), and the reactivity of the non-metals increases towards Group 7.

Trend questions give numbers or observations in words. Describe the pattern first, using values from the data, then explain it with shells and nuclear attraction.

Worked example: the radii of the period 3 elements fall from 186 pm (sodium) to 99 pm (chlorine), a decrease of 87 pm. Describe: 'the radius decreases across the period'. Explain: 'protons increase but the shells stay the same, so attraction increases'.

In a fair investigation, change only one independent variable, measure one dependent variable and keep the control variables the same. In halogen displacement, the independent variable is the halogen added, the dependent variable is whether a colour change occurs, and the controls are the volumes, concentrations and temperature.