* In Groups, On moving from top to bottom, the valency remains same because the number of valence electrons remains the same.
(ii) Atomic size- Atomic size refers to radius of an atom. It is a distance between the center of the nucleus and the outermost shell of an isolated atom.
* In Period, On moving from left to right, the atomic size decreases because nuclear charge increases.
e.g. Size of 2nd period elements are:-
Li > Be > B > C > N > O > F
* In Groups, On moving from top to bottom, Atomic size increases because of increase of number of orbits.
e.g. Atomic size of 1st group elements are:-
Li < Na < K < Rb < Cs < Fr
(iii) Metallic and Non-metallic Properties:- The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element. The tendency to gain electrons from the outermost shell of an atom, is called
non-metallic character of an element.
* In Period, On moving from left to right, metallic characters decreases because a tendency to lose electron decreases due to the increase in nuclear charge.
e.g. Metallic character of 2nd period elements are:-
Li > Be > B > C >> N > O > F
* In Group, when moving from top to bottom, metallic character increases because the atomic size and tendency to lose electrons increases.
e.g. 1st group elements are:-
Li < Na < K < Rb < Cs
* In Period, On moving from left to right, non-metallic character increases because tendency to gain electrons increases due to increase in nucleus charge.
e.g. Non-metallic character of 2nd period elements are:-
Li < Be < B < C < N < O < F
* In Groups, On moving from top to bottom, non-metallic character decreases because atomic size increases and tendency to gain electrons decreases.
e.g. Non-metallic character of 17th period elements are:-
F > Cl > Br > I
e.g.
Explanation:- In the first element of 3rd period, sodium has 1 valence electron which it can lose easily to react with other substances. So, it is very reactive. The second element magnesium has 2 Valence electrons. It is not so easy for an atom to lose 2 electrons. So, magnesium is less reactive than sodium.
Aluminium and silicon are still more unreactive because they have 3 and 4 Valence electrons respectively. Sodium react with water vigorously to form sodium hydroxide and hydrogen, magnesium react with water only on heating, aluminium react with water very slowly, and silicon does not react with water at all.
So, the chemical reactivity is:-
Na> Mg> Al> Si
Phosphorus has 5 Valence electrons. So, it needs 3 more electrons to complete the octet, sulphur has 6 Valence electrons and need 2 more electrons to complete the octet, chlorine has 7 Valence electrons and needs 1 more electrons to complete the octet. Now, it is quite difficult for an atom to gain 3 electrons, It is easier to gain 2 electrons and it is very easy to gain 1 electrons.
So, the chemical reactivity increases from phosphorus to chlorine. Thus, the chemical reactivity is:-
P< S< Cl
* In Groups, (a) In metals:- Chemical reactivity of metals increases down the group because of tendency to lose electrons increases.
e.g. Li < Na < K < Rb < Cs < Fr (1st group)
(b) In non-metals:- Chemical reactivity of non-metals decreases down the group because tendency to gain electrons decreases.
e.g. F > Cl > Br > I (17th group)
(v) Nature of Oxides:- As we know that Metal oxides are basic in nature.
e.g. Na2O, MgO, etc. And Non-metal oxides are acidic in nature.
e.g. Cl2O7, SO3, P2O5 etc. So,
* On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases.
* On going down in a group of the periodic table, there is no change in the nature of oxides of elements because all elements have similar properties.
It increases with increase of nuclear charge or decrease in atomic size. So,
-> Along the period, electronegativity increases.
e.g. Li < Be < B < C < N < O < F
-> Down the group, electronegativity decreases.
e.g. Li > Na > K > Rb > Cs F > Cl > Br > I
Merits of Modern Periodic Table
1. All elements are fundamentally arranged according to their atomic number. So it becomes easy to understand and compare their properties, thereby making it easier to remember the position of the elements.
2. The position of the elements in the periodic table makes it easy to predict and compare their properties, and explains the reason for their specific position in the periodic table.
3. It gives explanation for the periodicity of elements and tells the reason why all elements in a group having similar properties, which differ between the groups and periods.
4. Positions of isotopes are taken care of within the elements itself.
5. In the Modern periodic table, the metals, non-metals, transition metals, gases are separately placed in a specific location with a specific identity.
6. The modern periodic table tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements. Electronic Configuration (EC).
Note:- Maximum electrons are present in orbitals are-
| Orbitals | Maximum No. of electrons |
| s-Orbital | 2 |
| p-Orbital | 6 |
| d-Orbital | 10 |
| f-Orbital | 14 |
* Orbitals- It is a 3-D region around the nucleus where the probability of finding an electron is maximum.
An orbital can accommodate maximum 2 electrons.
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