What is Valency and Atomic Radius

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What is Valency and Atomic Radius 

Valence: Valence (or) valency of an element was defined as the combining power of an element with respect to hydrogen, oxygen or indirectly any other element through hydrogen and oxygen.
Valence of an element with respect to hydrogen is the number of hydrogen atoms with which one atom of that element chemically combines. Valence of an element with respect to oxygen is twice the number of oxygen atoms with which one atom of that element combines.
For example one atom of ‘Na’ chemically combines with one atom of ‘H’ to give NaH. Therefore, the valence of Na is 1. One atom of ‘Ca’ combines with one atom of ‘O’ to give CaO. So the valency of Ca is 2.
In general, the valence of an element with respect to hydrogen is its traditional group number. If the element is in the group V or above, its valence is 8– group number. For example, chlorine valence is 8-7 = 1.
In general, each period starts with valency 1 for 1 st group elements, increases upto 4 with respect to the group number and then decreases from 4 to 3 to 2 to 1 to zero in the following groups (this is applicable
only for main group elements i.e., ‘s’ and ‘p’ block elements.
Now a days the valence of an elment is generally taken as the number of valence shell (outer most shell) electrons in its atom. Oxidation number concept almost is the latest subsititute to the valence concept in the modern literatura.

Atomic radius
Atomic radius of an element may be difined as the distance from the centre of the nucleus of the atom to its outermost shell.
Atomic radius of an element is not possible to measure in its isolated state. This is because it is not possible to determine the location of the electron that surrounds the nucleus. However, we can measure the distance between the nuclei of adjacent atoms in a solid. From this we can estimate the size of the atom by assigning half of this distance to the radius of each atom. This method is best suited to elements such as the metals that exist in the solid state. More than 75 percent of the elements are metals and
atomic radii of metals are called metallic radii. Another way of estimating the size of an atom is to measure the distance between the two atoms in covalent molecules. The size of a chlorine atom is estimated by measuring the length of the covalent bond between two chlorine atoms in a Cl2
molecule. Half of this distance is taken as atomic radius which is called as the covalent radius of chlorine atom.
Atomic radius is measured in ‘pm’ (pico meter) units.
1 pm = 10-12m

Variation of atomic radii in group
Atomic radii increase from top to bottom in a group (column) of the periodic table. As we go down in a group, the atomic number of the element increases. Therefore to accommodate more number of electrons, more shells are required. As a result the distance between the nucleus and the outer shell of the atom increases as we go down the group in spite of increase in nuclear change.

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Variation of atomic radii in period
Atomic radii of elements decrease across a period from left to right. As we go to right, electrons enter into the same main shell or even inner shell in case of ‘d’ block and ‘f’ block elements. Therefore, there should be no change in distance between nucleus and outer shell but nuclear charge increases because of the increase in the atomic number of elements in period. Hence, the nuclear attraction on the outer shell electrons increases. As a result the size of the atom decreases.

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