The link with electronic configuration

The atomic number gives the number
of protons and number of electrons in an atom of the element. So a carbon atom with an atomic number of 6 configuration contains six protons and six electrons. Protons are found in the nucleus and electrons move around the nucleus. How the electrons are arranged in
shells is called electronic configuration.

[Source-Pixabay](The link with electronic configuration

electronic
configuration for sodium

Now if you look at the electronic configurations of elements in the first period (hydrogen and helium), you will notice that these elements only have
electrons in the first shell. If you look at the electronic configurations of elements in the second period (lithium to neon), you will notice that these elements have electrons in both the first and the second shells. If you look at the electronic configurations of elements in the third period (sodium to argon), you will notice that these elements have electrons in the first three shells. Lithium, sodium and potassium are three elements in the alkali metal family. They are in the same group
(group IA). Looking at the electronic configurations of atoms of these three elements, you will notice that they
all have a single electron in the outer shell. It is this fact that makes their properties similar and puts them in the same family. The other alkali metals (rubidium,
caesium and francium) also have one electron in the outer shell of the atom.

Elements in group IIA are called the alkaline-earth metals. The first three members are beryllium, magnesium and calcium. You will notice that atoms of
these three elements all have two electrons in the outer shell. Strontium, barium and radium similarly have two
electrons in their outer shell. This similarity gives the elements similar properties and makes them a chemical
family in the same group of the periodic table. Now look at fluorine and chlorine. They both have seven electrons in the outer shell of their atoms. They
belong to the non-metal halogen family and group VIIA of the periodic table. The other elements in group VIIA (bromine, iodine and astatine) are also in the
same family and have seven electrons in the outer shell. The elements helium, neon, argon, krypton, xenon and radon in group 0 comprise the family of noble gases.

To summarise, for elements in the main block (i.e. with a group number ending in A or 0), the number of electrons in the outer shell of an atom is the same as the
group number, and elements in the same family with similar properties are in the same group.

Atomic and ionic radii

Going down any group in the main block of the periodic table, there is an increase in atomic and ionic radii (and atomic size). This would be expected, as going from one period to the next there is an additional energy shell. So lithium has electrons in two shells and sodium in three. More surprisingly, moving from left to right in any period, the atomic (and ionic) radii decrease. Each step to the right adds an extra proton and an extra electron
but the atom (or ion) gets smaller. So from sodium to chlorine in period 3, the atomic radius falls from 0.157 nm to 0.099 nm. This can be explained when you
remember that the extra protons go into the nucleus and the extra electrons go into the same electron shell, an outer and larger one. Also, because there are
re protons, the electronic attraction betwe the cleus and the electrons is greater. This pulls the shell electrons slightly closer to the nucleus.

Ionisation energy

Ionisation energy is the energy absorbed when one mole of electrons is removed from one mole of atoms of an element in the gaseous state to form one mole of
positively charged ions:
M(g) → M-(g) + e-
This is the first ionisation energy. The energy is usually quoted in units of kilojoules per mole (kJ mol-?).
Energy is required to remove an electron from any atom because there is an attractive force between the
nucleus and the electron that is being removed and this has to be overcome.
The value of the first ionisation energy depends on:

• the attraction between the nucleus and the outermost
electron - this decreases (reducing the ionisation
energy) as the distance between them increases
• the size of the positive nuclear charge - a more
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