SCIENCE-10TH CBSE ANSWER KEY

PERIODIC CLASSIFICATION OF ELEMENTS(27-32)

Question-27Name four alkaline earth metals. To which group do they belong?

Solution:The four alkaline earth metals are beryllium, magnesium, calcium and strontium. They belong to II A group.

Question-28What is called diagonal relationship?

Solution:The similarity in properties between certain elements of different periods placed diagonally to one another in a periodic table is called diagonal relationship.

Question-29Define electron affinity.

Solution:It is defined as the amount of energy released in adding an extra electron to an isolated neutral gaseous atom in its lowest energy state, to convert it into gaseous ion. Electron affinity increases as one moves from left to right in periods or top to bottom in groups.

Question-30What is called electronegativity?

Solution:The property of atoms to attract electrons in a covalent bond is called electronegativity.
(i) In periods, electronegativity increases with the increase in atomic number.
(ii) In groups, electronegativity decreases with the increase in atomic number.

Question-31(a) Name an alkali metal other than lithium, sodium and potassium.

(b) Name an alkaline earth metals other than calcium, strontium and barium.
(c) Name one halogen other than chlorine, bromine and iodine.
(d) Name a non-metal having properties similar to carbon.
(e) Name a rare gas other than neon, krypton and xenon.

Solution:(a) Rubidium
(b) Magnesium
(c) Fluorine
(d) Silicon
(e) Argon.

Question-32Explain why the following statements are not correct?

i)  All groups contain both metals and non-metals
ii) In group I, reactivity decreases with increases in atomic number
iii) Elements in a period become more metallic with increasing atomic number
iv) Atoms of elements in the same group have the same number of electrons.

 Solution:i) All groups do not contain both metals and non-metals. Group I and II contain only metals.

ii) In group I, reactivity increases with increase in atomic number. This is because as the size of the atoms increases, the valence shell electrons can be easily removed.

iii) The metallic character decreases in a period with increase in atomic number. This is because across the period, the size of the atom decreases and the valence shell electrons are held more tightly.

iv) In a neutral atom the number of electrons is equal to the number of protons. In a group the atomic number increases as we move down.

Question-33Why are the elements sodium and chlorine in the same period of the Periodic table?

Solution:This is because the atoms of both the elements have three shells containing the electrons.

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