The fundamental principles of chemistry is the periodic law,what states that:
The chemical and physical properties of elements are a periodic functaion of atomic number.
We will consider how the periodic table can be used to correlate properties on an atomic scale.In perticular,we will see how atomic radius vary horizontally and vertically in the periodic table.
Trend in atomic radius
An atom does not have a fixed size.Ordinarily, the atomic radius is taken to be one half the distance of closest approach between atoms in an elemental substance.
In general , atomic radii-
- decreases across the period from left to right in the periodic table.
- increases down a group in the periodic table.
It is possible to explain these trends this way:
Consider first increase in radius observed when we move down the table,let's say amoung alkali metal elements ,all elements have have a single s electron outside the filled p sublevel.Electrons in these inner levels are much closer to the nucleus than the outer s electron and hence effectively shield it from the positive charge of the nucleus, this is called shielding effect.To a first approximation ,each inner electrons cancels the charge of one proton in the nucleus,so outer s electrons is attracted by a net positive charge of +1 .In this sense,it has the properties of an electron in the hydrogen atom.Because the average distance of the electron from the hydrogen nucleus increases with the principal quantum number, n,the radius increases moving down the group. For example: in Li atom,the nuclear charge is 3e but the effect of this charge on the 2s electron is reduced by the distribution of two 1s electrons between nucleus and 2 s electrons ,roughly we can say that each core electron reduced the nuclear charge by 1e.
Now consider a given period of elements:decrease in atomic radius moving across a periodic table can be explained in a similar manner that the principal quantum number of outer orbitals remains constant .However the effective nuclear charge increases(which is the positive charge that an electron experiences from the nucleus,equals to the nuclear charge but reduced by any shielding or screening from any intervening electron distribution) because the nuclear charge increases and the number of core electrons remains constant as a result of this the size of outermost electron and therefore,radius of atom decreases with increasing Z in any period as electrons are pulled in more tightly and thereby decreasing the size of atom.
Lets see a solved example:
Sample problem: Using the periodic table and trend noted for the size of atomic radii to arrange the following in the order of increasing atomic radii Al,C,Si .
Step 1: First consider C atom which belongs to group IV A . Look at the periodic table and notice that C is above the Si in group VI A .Therefore,the radius of C is smaller than that of Si as the radius increases going down a group
Step 2 : Now , consider Si and Al , note that Al and Si are in the same period.Therefore, the radius of Si smaller than that of Al because radius decreases with the Z in the period.So,order of elements are in increasing order of atomic radii: