Periodic classification of elements class 10 short notes | cbse24

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1. Periodic classification of elements class 10 notes→Part 1

Characteristic of Periods

In the characteristics of periods, we will discuss the variation of some of the important properties of elements like the number of valence electrons, valency, size of atoms, metallic character, chemical reactivity and nature of oxides, on moving from left to right in a period of the periodic table that is on moving from left to right in the horizontal row of the periodic table.

[1] Valence Electrons(or Outermost Electrons)

On moving from left to right in a period, the number of valence electrons in elements increases from 1 to 8.

[2] Valency

On moving from left to right in each short period, the valency of elements increases from 1 to 4 and then decreases to 0 (zero).

Question:-What is Valency?

Answer:-The number of electrons lost or gained(or shared) by one atom of an element to achieve the nearest inert gas electron configuration, gives us the valency of an element. 

Example:-(i) The atomic number of magnesium is 12 so its electronic configuration is 2,8,2(having 2 valence electrons). A magnesium atom can lose its 2 valence electrons to achieve the inert gas electron configuration 2,8(of neon), so the valency of magnesium is 2.

(ii) The atomic number of sulphur is 16, so its electronic configuration is 2,8,6 (having 6 valence electrons). A sulphur atom can't lose  6 electrons to achieve the inert gas electron configuration due to energy considerations. A sulphur atoms gains(or accepts) 2 electrons from some other atoms to achieve the nearest inert gas electron configuration,2,2,8 (of argon), So the valency of sulphur is 2 

Question:-What is Inert Gas?

Answer: Noble gases(inert gases) belong to the 18th group of the periodic table. They include helium, neon, argon, krypton, xenon and radon. They are called inert gases because they are stable and non-reactive

[3] Size of Atoms (or Atomic size)

On moving from left to right in a period of the periodic table, the size of atoms decreases (or atomic size decreases). The atomic radius is expressed in a 'picometer' whose symbol is 'pm'.

[4] Metallic Character

On moving from left to right in a period, the metallic character of the element decreases (but the non-metallic character increases)

Metals lose electrons from positive ions, so metals are called electropositive elements. On the other hand,non-metals accept electrons and form positive ions, so non-metals are called electronegative elements. So on moving from left to right in a period, the electropositive character of elements decreases, but the electronegative character increases.

[5] Chemical Reactivity

On moving from left to right in a period, the chemical reactivity of the element first decreases and then increases.

The variation in chemical reactivity of elements in a periodic can be explained as follows. In the first element of the third period, sodium. there is 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.

[6] Nature of Oxides

On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases.

Characteristic of Groups

[1] Valence Electrons (or Outermost Electrons)

All the elements of a group of the periodic table have the same number of valence electrons.

Example:-All the elements of group 1 of the periodic table like lithium, sodium and potassium have 1 valence electron each in their atoms.

[2] Valency

Since the number of valence electrons(which determine the valency) in a group is the same, all the elements in a group have the same valency.

Example:-Group 1 elements like lithium, sodium and potassium etc all have 1 valence electron each, so all the elements of group 1 have the same valency of 1.

Note:-(i) Group 1 to 14 valency is increasing and Group 15 to 18 start decreasing.

          (ii) Group 1 to 14 Valency= No of valence electron.

         (iii) Group 15 to 18 Valency=(8-Valence electron)

1. The valency of group 1 is 1
2. The valency of group 2 is 2
3. The valency of group 13 is 3
4. The valency of group 14 is 4
5. The valency of group 15 is 3
6. The valency of group 16 is 2
7. The valency of group 17 is 1
8. The valency of group 18 is 0

[3] Size of Atoms (Atomic Size)

 Ongoing in a group of the periodic table, the size of atoms increases(or atomic size increases)

The increase in the size of the atoms moving from top to bottom in a group of the periodic table can be explained as follows:-When we move from top to bottom in a group, a new shell of electrons is added to the atoms at every step. In this way number of electron shells in the atoms increases gradually due to which the size of atoms also increases.

Example:-Lithium atom(Li) has only two electron shells K and L in it whereas Sodium atom(Na) has three electrons shells K, L and M in it. 

[4] Metallic Character

On moving down in a group of the periodic table, the metallic character of elements increases

Ongoing down in a group of the periodic table, the electropositive character of elements increases. In group 1 of the periodic table, As we go down in a group of the periodic table, one more electron shell is added at every stage and the size of the atom increases. The valence electrons become more and more away from the nucleus and the hold of the nucleus on valence electrons decreases. Due to this, the atom can lose valence electrons more easily form positive ions and hence the electropositive character increases.

Ongoing down in group 17 electronegative character of elements decreases

Example:-When we go down in group 17 of the halogen elements, the electronegative character decrease from fluorine to iodine.

[5] Chemical Reactivity

• The chemical reactivity of metals increases on going down in a group of the periodic table
• The chemical reactivity of non-metals decreases on going down in a group of the periodic table.

[6] Nature of Oxides

Ongoing down in a group of the periodic table, there is no change in the nature of oxides of elements.

Example:-All the elements of oxides of all the elements of a group is the same. For example, all the elements of group 1(like Li, Na, K) are from basic oxides, whereas all the elements of group 17(like F, Cl, Br) form acidic oxides.