ATOMIC SIZES

ATOMIC SIZES:-

    Size of an atom is very important in determining its properties whether physical or chemical properties. Atoms are assumed to be spherical so radii is used to determine there sizes. There are various radii to determine size of an atom depending upon type of compound.

• ATOMIC RADII:-

                   Atomic radii is defined as the average distance between nucleus of the atom and outermost electronic shell. Atomic radii is usually used for metallic compounds or elements in atomic form for example metals as shown in the fig.
The atomic radii can not be measured precisely directly because of following reasons:

1. There is no sharp boundary of an atom. Probability of finding electron is not exactly zero even at a very large distance from the nucleus.
2. Electronic probability of an atom is disturbed by by the neighbouring atom that is electron in the outermost shell are repelled by outermost electron of its neighbouring atom.   

So atomic radii is determined by measuring the distance between the centres of adjacent atoms by X-rays or spectroscopic measurement and then by taking its half.

VARIATION OF ATOMIC RADII IN PERIODIC TABLE:-

  Atomic radii decreases from left to right in a period and increases from top to bottom in a group of periodic table. The decreasing trend in a period is due increase in nuclear charge which increases the pull on electrons resulting in decrease in atomic size. Moreover shielding effect remains the same from left to right in a period. 

Increase in atomic radii from top to bottom in a group is due to increase in the number of shell and shielding effect. In a period decrease of atomic radii is very prominent in second period but less in higher period. Moreover decrease is small when we move from left to right in transition elements this is due to intervening electrons. These are responsible for the decrease in force of attraction between nucleus and the valence electron or increase in shielding effect. Some of the transition element of fourth period are shown in the fig below   

as we see that electrons instead of increasing in fourth shell which is valence shell electrons are increasing in third shell thus these electrons are responsible for the decrease in the force of attraction between nucleus and valence shell electron and the decrease in the atomic radius is not very prominent. Similar is the reason for not prominent decrease in the transition elements of fifth period that are Y(39)-Cd(48).

• IONIC RADIUS:-

          The ionic radius of an ion is the radius of the ion while considering it to be spherical in shape.
Radius of cation is always less than the atomic radius of the element from which it is derived.this is because as the cation is formed by losing electron and we know that if electron is removed than nuclear pull on the remaining electrons would be greater which results in decrease in decrease in radius. Contrary to it radius of anion is always greater than the atomic radius of the element from which it is derived, this is because as we know anion is formed by gaining electron so as an atom gain electron repulsion between the electrons in the valence shell increases due to which they push each other away as a result of which radius increases. for example radius of Na atom reduces from 186 pm to 95 pm after conversion into Na⁺ ion and ionic radii of Cl- atom increases from 99pm to 181pm after conversion in Cl^- ion. The cationic radius decreases with increase in effective nuclear charge tha  is decrease in radius Is larger fro divalent ion eg for (Mg⁺²) and even greater fro trivalent ion eg for (Al⁺³). This is due to the fact that with the successive lose of electron the nuclear charge attract the remaining electron with greater force and similarly greater the amount of negative charge on an atom greater the size of ion order of increasing the size of ions is

A^-1<A^-2<A^-3

VARIATION OF IONIC RADII IN PERIODIC TABLE:-

   One thing we should keep in mind that ionic radii of metals is for positive ions and for elements of group V-A to VII-B are for negative ions. As you move across a row of period of the periodic table, the ionic radius decreases for metals forming cations, as the metals lose their outer electron orbitals. The ionic radius increases for non-metals as the effective nuclear charge decreases due to the number of electrons exceeding the number of protons. As you move down a group in the periodic table, additional layers of electrons are being added, which naturally causes the ionic radius to increase as you move down the periodic table.

    Let us consider positive and negative ions which are held together by electrostatic force of attraction in a crystal lattice. r+ and r- are the radii of cation and anion respectively.

Inter ionic distance "R" in a crystal lattice is equal to sum of cation radii and anion radii i.e r+ and r- we can write it mathematically as  R=r+ + r- 
Pauling was able to determine the distance between K⁺ and Cl^- ions in potassium chloride (KCl) and found that it was sum of radii of the two ions

R= 133pm + 181pm = 314pm

Pauling extends this concept to other potassium (K⁺) salt and calculate radii of other ions from relation

  r- =R – r+

similarly radii of different cations can also be determined.

• COVALENT RADII:-

                  Covalent radii of an element is defined as half of the single bond length between two similar atoms covalently bonded in a molecule. eg bond length of H-H molecule is 75.4 pm so covalent radii of of H- atom is half of 75.4pm that is 37.7 pm.          Covalent radii of an element can be used to determine the covalent radius of another element. For example the experimentally determined bond length of C-Cl in CH₃Cl is 176.7 pm. The covalent radius of Cl-atom being known as 99.4 pm and that C-atom can be calculated by subtracting this value from C-Cl bond length so covalent radii of C-atom is 176.7-99.4= 77.3 pm  

VARIATION OF COVALENT RADII IN PERIODIC TABLE:-

      Atomic radii decreases from left to right in a period and increases from top to bottom in a group of periodic table.