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Characteristics of periods

1. Valence electrons: The number of valence electrons in a period increase from 1 to 8 on going from left to right in a period. It is so because the atomic number of the elements increase by 1 in comparison to the preceding element on going from left to right.
2. Valency: Valency also varies on going from left to right in a period. The valencies of group IA, II A and III A are +1, +2 and +3 respectively but the valencies of group VA, VIA and VII A are -3, -2 and -1 respectively. The valency of group IVA is =4 or -4. And group Zero has no valency.
3. Atomic size: On going from left to right in a period, atomic size decreases. It is so because along with electron, the number of proton in nucleus also increases on going from left to right which holds the outer electron so forcibly which decreases atomic size.
4. Ionization energy:On going from left to right in a period,theionization energy increases as the outer electrons are held tightly by nucleus on going from left to right. 5. Metallic characters: The metallic characters decreases on going from left to right. 6. Electropositivity: The capacity of losing electrons decreases on going from left to right in a period. So, the electropositivity also decreases on going from left to right.
7. Electronegativity: The capacity of gaining electrons increases as we go from left to right in a period. So, the electronegativity also increases as we go from left to right. ### Electronic configuration

The systematic distribution of electrons in different shells of an atom is known as electronic configuration. • 2n2 rule explains the distribution of electrons in different shells of an atom.
• The shells of an atom are named as K, L, M and N.
• According to 2n2 rule, for K shelln=1, so, 2×12 = 2So, K-shell can hold 2 electrons
• For L shell, n=2, so 2×22=8, so, it can hold 8 electrons.
• For M shell, n=3, so 2×32=18, so, it can hold 18 electrons.
• FOR N shell, n=4, so,2×42=32, so it can hold 32 electrons.

But, according to duplet and octet rule, K-shell can hold maximum 2 electrons and the other shells can hold 8 electrons. So, to overcome the contradiction, between 2n2rule and duplet and octet rule, the concept of sub shell is introduced.

### Concept of sub shell

According to this concept, different shell has their sub shell named as s, p, d and f sub shell and they can hold 2,6,10 and 14 electrons respectively.

 Shell Sub Shell No of electron K s 2 L sp 2+6=8 M spd 2+6+10=18 N spdf 2+6+10+14=32 • Valence electrons, valency, atomic size, ionization of energy, metallic characters, electropositivity, etc. are the characteristics of periods.
• The number of valence electrons in a period increase from 1 to 8 on going from left to right in a period.
• The systematic distribution of electrons in different shells of an atom is known as electronic configuration.
• The capacity of losing electrons decreases on going from left to right in a period. So, the electropositivity also decreases on going from left to right.
• The capacity of gaining electrons increases as we go from left to right in a period. So, the electronegativity also increases as we go from left to right.
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#### Click on the questions below to reveal the answers

These metals are soluble in water and form metallic hydroxides and evolve hydrogen gas. These hydroxide are called alkalis. Therefore the elements of group IA are called alkali metals.

Sodium has one valence electron. Therefore, it has tendency to lose its valence electron during chemical reaction to be octet. So, it is called active metal.

1. Fluorine (F) and Chlorine (Cl)
2. Sodium (Na)
3. The Neon (Ne) and Argon (Ar) are the inert elements.

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Fluorine (the most electronegative element) is assigned a value of 4.0

The horizontal rows of periodic table are called periods.

On moving left to right along the period the metallic character goes on decreasing and non-metallic character goes on increasing.

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• ### On moving from left to right in periodic table the electronegativity ______.

increases

decreases

increases for certain time

remains constant

• ### On moving from left to right in periodic table the atomic size ______.

increases

both increases and decreases

decreases

remains constant

• ### D shell of electronic configuration can hold _______.

14 elecrons

2 electrons

10 electrons

none of above

• ### Which element has the highest first ionization energy?

sodium
aluminum
phosphorus
none of above

2
1
4
3
• ### Which one of the following statement about s orbitals is correct ?

the maximum number of s orbitals in any principal level is 1
they can only hold one electron
they are found in all principal energy levels
they are spherical in shape

1s2 2s6
2s2 2p6
1s2 2s2 2p6
1s2 2s2 2p4
• ### The order of filling orbitals is ______.

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p

1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p

none of above

4p, 4s, 3d, 3p, 3s, 2p, 2s, 1s

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