Periodicity is to do with the patterns of behaviour and properties that are seen in the periodic table.
The periodic table can be divided into various blocks based on the more advanced electron configuration introduced at this level.
There are the s, p and d blocks. An element is in the s-block, for example, if its outermost electron is in the s sub energy level. Elements in the same block have similar properties. For example, the transition metals are all in d-block.
Period 3: Atomic Radius
Under this heading we will look at the properties of the elements in period 3 (Na, Mg, Al, Si, P, S, Cl, Ar) in atomic radius.
The atomic radius of an atom is defined as half the distance between the nuclei of two covalently bonded atoms, (called the covalent radius) as shown in the diagram below. However, it is not possible to get this measurement for noble gases since they do not bond. So instead we use the Van der Waals radius.
As you go along a period, the atomic radius decreases, this is because the increasing number of protons exerts more pull on the electrons and so moves them closer to the nucleus. Hence they take up less volume.
Period 3: Electronegativity
Electronegativity is the ability of an atom or molecule to attract electrons. Click here to see how this affects bonding.
It is measured using the Pauling scale where fluorine is given a value of 4 (the highest) and Francium 0.7 the lowest.
As you move along the period, the electronegativity increases as you can see in the below diagram. This trend is because as more protons are added and te nucleus' charge becomes higher, the atom has more attractive power. Noble gases do not have electronegativity.
Period 3: Conductivity
Across period 3, conductivity of both electricity and heat is a story of two halves...
Na, Mg, and Al all form metallic structures where the delocalised electrons can move freely and carry electronic charge or heat energy.
Si, O, S, Cl and Ar however are insulators, this is because they bond covalently where the electrons are in a fixed position so cannot transfer electric charge or heat.