In the following table the mass number and the atomic number of certain elements are given

Making use of these data find. 1. Cation 2. Anion 3.a pair of isotopes 4. an atom of noble gas

For A atom,

Atomic no. = 1, Electronic configuration - 1
So no. of electrons = no. of protons = atomic no. = 1
Mass no. = no. of proton + no. of neutron
1 = 1 + no. of neutron
no. of neutron = 0
So, A is an H atom that contains 1 proton,1 electron, and 0 neutrons.
For the B atom,
Atomic no. = 3, Electronic configuration - 2,1
So no. of electrons = no. of protons = atomic no. = 3
Mass no. = no. of proton + no. of neutron
7 = 3 + no. of neutron
no. of neutron = 4
So, B is a Li atom that contains 3 protons,3 electrons, and 4 neutrons.
For the C atom,
Atomic no. = 7, Electronic configuration - 2,5
So no. of electrons = no. of protons = atomic no. = 7
Mass no. = no. of proton + no. of neutron
14 =7 + no. of neutron
no. of neutron = 7
So, C is an N atom that contains 7 protons, 7 electrons, and 7 neutrons.
For the D atom,
Atomic no. = 18, Electronic configuration - 2,8,8
So no. of electrons = no. of protons = atomic no. = 18
Mass no. = no. of proton + no. of neutron
40 =18 + no. of neutron
no. of neutron = 22
So, D is an Ar atom that contains 18 protons, 18 electrons, and 22 neutrons.
For the E atom,
Atomic no. = 20, Electronic configuration - 2,8,8,2
So no. of electrons = no. of protons = atomic no. = 20
Mass no. = no. of proton + no. of neutron
40 =20 + no. of neutron
no. of neutron = 20
So, E is a Ca atom that contains 20 protons, 20 electrons, and 20 neutrons.
1. Atom B (Li) atomic no. = 3, Electronic configuration - 2,1
Hence atom B loses one electron to form a stable octet and forms cation i.e $B^{+}$.
Atom E (Ca) atomic no. = 20, Electronic configuration - 2,8,8,2
Hence atom E loses two electron to form a stable octet and forms cation i.e $E^{+}$

2. Atom C (N) atomic no. = 7, Electronic configuration - 2,5
Hence atom C gains one electron to form a stable octet and forms anion i.e $C^{-}$
3. Isotopes are atoms of the same element which have the same atomic no. but a different mass numbers.
Isobars are atoms of the same element which have different atomic no. but same mass numbers.
Elements D and E are isobars as they have the same mass number but a different atomic number.
No isotopes are present.
4. D is noble gas argon with atomic number 18 as its electronic configuration is complete and stable i.e 2,8,8.