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(a) If the valency of carbon is 4 and that of hydrogen is 1 then the molecular formula of methane is CH₄

Explanation:

The valency of carbon is 4

The valency of hydrogen is 1

Thus, the molecular formula will be:

�Molecular formula: CH₄

(b) There are two electrons in the outermost orbit of the magnesium atom. Hence, the valency of magnesium is 2.

Explanation:

Valency is the combining capacity of the element.

We can calculate the valency of an element by counting the no. of electrons in the outermost shell.

As magnesium has two electrons in the outermost shell, thus

its valency is 2.

(c) ₃₅¹⁷Cl and ₃₇¹⁷Cl are isotopes of chlorine.

Explanation:

Chlorine has two isotopes (having same atomic number and different atomic mass number).

In ₃₅¹⁷Cl, the atomic mass number of chlorine is 35.

In ₃₇¹⁷Cl, the atomic mass number of chlorine is 37.

(d) Isotopes of uranium are used as fuel in atomic reactors.

Explanation:

Isotopes of uranium (₉₂²³⁵U, ₉₂²³⁸U and ₉₂²³⁴U) are used as fuels in atomic reactors due to their significant features in the nuclear reactors.