(a) Define the following terms :
(i) Valency (ii) Atomic size
b) How do the valency and the atomic size of the elements vary while going from left to right along a period in the modern periodic table?
a) i) Valency is the combining power or the combining capacity of an atom. The number of electrons gained, lost or shared by an atom so as to achieve the nearest noble gas configuration gives us valency.
ii) Atomic size: The distance between the centre of the nucleus and the outermost shell of an isolated atom gives us the atomic size or atomic radius.
b) Variation in valency:
On moving from left to right in the periodic table, valency increases up to 4 and then decreases. Valency of a particular element is determined by the electrons present in the last shell.
If the number of valence electrons is less than or equal to 4, valency is equal to the number of valence electrons.
If the number of valence electrons is more than 4, valency is equal to 8 minus number of valence electrons.
Variation in atomic size:
On moving across a period atomic size decreases because of the electron increases by 1 whereas, the number of valence shell remains the same. Therefore as a result of an increase in nuclear charge, atomic radius decreases.
Write the number of vertical columns in the modern periodic table. What are these columns called?
There are 18 vertical columns in the modern periodic table and these are known as groups.
From the following elements:
4Be; 9F; 19K; 20Ca
(i) Select the element having one electron is the outermost shell.
(ii) two elements of the same group.
Write the formula and mention the nature of the compound formed by the union of 19K and element X(2, 8, 7).
Electronic configuration of 19K is 2,8,8,1
Electronic configuration of 4Be is 2,2
Electronic configuration of 20Ca 2,8,8,2
Electronic configuration of 9F is 2,7
i) Among the given elements, 19K has one electron in the outermost shell.
ii) Elements 4Be and 20Ca both have two electrons each in their outermost shells.
Therefore, both these elements belong to the same group.
Atomic number of element X = 17
Electronic configuration of element X is 2, 8, 7.
Number of valence electrons in X = 7
Therefore, valency of X = 8-7 = 1
Atomic number of K = 19
Electronic configuration of K is 2, 8, 8, 1.
Number of valence electrons in K = 1
Valency of K = 1
So, the compound formed is KX and is ionic in nature.+
How many groups and periods are there in the modern periodic table? How do the atomic size and metallic character of elements vary as we move:
(a) Down a group and
(b) From left to right in a period
The modern periodic table consists of 18 groups and 7 periods.
i) Down the group, the atomic size and metallic character of elements increase.
ii) On moving from left to right in a period, the atomic size and metallic character of elements decreases.
(i) How do you calculate the possible valency of an element from the electronic configuration of its atoms?
(ii) Calculate the valency of element X whose atomic number is 9.
(i) The valency of an element is determined by the number of electrons present in the last shell.
If a number of valence electrons are less than or equal to 4 then, valency = a number of valence electrons.
If, the number of valence electrons greater than 4 then, valency = 8 – number of valence electrons.
(ii) The electronic configuration of element X with atomic number 9 is 2,7
Valency of X = 8−7 =1
Hence, the valency of the element with atomic number 9 is 1.
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