CBSE Class 9 Answered
(a) Valency is the combining capacity of an atom. Valency is calculated by using number of valence electrons present in an atom. The number of electrons present in the last shell of an atom and take part in chemical bond formation are known as valence electrons.The number of electrons which an atom has in its last orbit can be found out by writing the electronic configuration of an atom. So, electronic configuration expresses the valence electrons which are related to the valency of an atom.
(b) Mass number = Number of protons + Number of neutrons
Hence, mass number of A = 6 + 6 = 12
mass number of B = 6 + 8 = 14
The two atomic species have the same number of protons but different number of neutrons.
In other words, the two species have the same atomic number but different mass number.
Such atoms are called isotopes.
The atoms A and B represent the same element carbon with atomic number 6.
OR
(a) The atomic number of helium is two. It has two electrons in first shell of K shell. It is its outermost shell and has maximum capacity of two electrons. So, the outermost shell of an atom is completely filled. Hence, helium atom is inert.
(b) Atomic number of Na is 11, so one atom of Na contains 11 protons and 11 electrons which means 11 units positive charge and 11 units negative charge. Hence, it does not have any net charge.
When one electron is lost, negative charge becomes 10 units and positive charge is 11 units. So, Na gets one positive charge.
(c) As we know, when charged bodies move in circular motion, they emit radiations. This means that the electrons revolving round the nucleus (as suggested by the Rutherford) would lose energy and come closer and closer to nucleus.
A stage will come when they will finally merge into the nucleus. This makes the atom unstable, which is clearly not the case.
(d) The mass on atom is determined by the number of protons and neutrons.
Atomic mass = number of neutrons + Number of protons