As we know, the atomic number is the fundamental property of an atom. Most of the physical and chemical properties of the elements change periodically with the atomic number as described below.
The elements in any group have similar electronic configuration. For example, the atoms of all the elements of group 1 (alkali metals) have one electron in their valence shell while those of group 2 (alkaline earth metals) have two electrons in their outermost shell. Thus, alkali metals have ns1 configuration while alkaline earth metals have ns2 configuration. In the long form of the periodic table, the elements are arranged in order of increasing atomic number.
Thus, an additional electron enters the principal shell at each step. Therefore, as we move across a period, each element acquires a new electronic configuration as shown below in the case of second period elements.
||$1s^2 2s^2\ 2p^1$
||$1s^2\ 2s^ 2p^2$
||$1s^2\ 2s^2 2p^3$
||$1s^2\ 2s^2\ 2p^4$
||$1s^2\ 2s^2 2p^5$
Valency of an element is defined as the number of hydrogen atoms or the number of chlorine atoms or double the number of oxygen atoms that combine with one atom of the element.
Elements having the valency of 1,2,3 and 4 are called monovalent, divalent, trivalent and tetravalent respectively.
The valency of representative elements is usually equal to the number of electrons in the outermost orbit or group number 10. For example, valency of S in SO3 = 2 x 3 = 6 (16 -10 = 6). However, transition elements exhibit variable valency .
Variation in a Group
Due to the similarities of electronic configuration, the elements of a group have the same valency. Thus, all the elements of group1 have valency one while of group 2 have valency two.
Variation in a Period
As we move across a period, the number of valence electrons increases. For example, sodium has one valence electron (3s1), Magnesium has two valence electrons (3s2) while aluminum has three valence electrons (3s23p1). As a consequence, the valency of the elements changes in a period as we move from the left towards the right. The valency with respect to hydrogen increases from 1 to 4 and then decreases to zero. However, the valency with respect to oxygen increases from 1 to 7 and is 0 for inert gas.
|| PH3 || H2S
Atomic volume of an element is the volume in cm3, occupied by one gram atom of the element in the solid state and hence it is also called gram atomic volume. It can be stated as the ratio between gram atomic weight of the element and its density, i.e., Atomic volume = Atomic weight/density.
Variation of atomic volume in a group: Atomic volume increases almost regularly in going down a group due to the successive addition of new shells.
Variation of atomic volume in a period: Atomic volume decreases at first for some elements, becomes minimum in the middle and then increases.
Atomic radius is the most probable distance from the centre of the nucleus to the outermost shell of electrons.
The electron density around an atom is affected by the presence of neighboring atoms. Hence, the size of the atom changes somewhat in going from one condition to another.
Covalent radius : Inter-nuclear distance between two covalently bonded atoms/2
Van der Waal's radius : Inter-nuclear distance between two non bonded nearest neighboring atoms/2
Metallic radius : Inter-nuclear distance between two nearest metal atoms in metallic lattice/2
Variation of atomic radius in a period: As we move from left to right in a period, the atomic radii goes on decreasing. Across a period, the atomic number, i.e., nuclear charge is increasing. The electrons get added in the same shell. The added electrons do not screen the nucleus appreciably. The attraction of the nucleus for the outermost electrons goes on increasing. As a result atomic radius goes on decreasing.
It may be noted that in the case of noble gases, we measure van der Waal's radii instead of covalent radii since these elements do not form covalent bonds. Therefore, at the end of each period, the radii show an abrupt increase because van der Waal's radii give higher values of atomic radii than covalent radii.
Variation of atomic radii in a group: As we move, down a group in the periodic table, the atomic radii go on increasing due to the addition of new shells.
Ionization enthalpy or Ionization energy or Ionization potential
It is defined as the amount of energy required to remove one valence electron from an isolated neutral gaseous atom resulting in the formation of a monovalent positive ion. For example, the ionization energy of Na is 496 KJ/mole. This means that 496 KJ of energy would remove an electron from each Na atom in one mole of Na atoms.
Na(g) + I.E. $\to$ Na+(g) + 1e-
Variation of Ionization energy in a group:
As we move down a group the Ionization energy goes on decreasing.Variation of Ionization energy along a period:
Ionization energy values go on increasing across a period.
Electron Affinity or Electron Gain Enthalpy
Electron affinity is the amount of energy released when one electron is added to a neutral gaseous atom to form a monovalent negative ion.
For example, electron affinity of chlorine is -349 KJ/mole. This means that 349 KJ of energy is released when one mole of chlorine atoms change into Cl- ions.
Cl(g) + 1e- $\to $ Cl- (g) + 349 KJ
Variation of electron affinity down a group: The electron affinity of elements decreases down the group.
Variation of electron affinity along a period: The electron affinity increases across a period due to increase in nuclear charge.
The relative tendency or power of an element in a molecule to attract the shared pair of electrons towards itself is known as its electronegativity.
Variation of electronegativity down a group: Down the group electronegativity decreases as atomic size increases.
Variation of electronegativity along a period: While going from left to right in a period electronegativity increases as the atomic size decreases.
Fluorine is the most electronegative element in periodic table.
The d block elements are called transitional elements. This block contains elements lying between the s and p blocks, i.e., between groups 2 and 13, starting from 4th period and onwards.
In the mentioned elements, the outermost shell contains one or may be two electrons in their s orbital (ns) but the remaining electron enters the last but one d sub shell, i.e., (n-1)d.
The elements of this block have the general characteristic properties which are intermediate between the elements of s block and p block.
The study of transition metals is important because the precious metals silver, gold and platinum and industrially important metals like iron, zinc, palladium, copper, titanium, nickel, chromium, etc., are transition elements.
- Nearly all transition elements have typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic luster.
- They have high melting and boiling points and have higher heats of vaporization than non transitional elements.
- Transition elements have very high densities as compared to s block metals.
- Most of them have colored compounds.
- They have good tendencies to form complexes.
- They exhibit several oxidation states.
- They form alloys with other metals.
- They form interstitial compounds with elements such as hydrogen, boron, carbon etc.
- Most of the transition metals such as Mn, Ni, Cr, V, Pt, etc., and their compounds have been used as good catalyst.