Assertion: Elements ¹₁H, ²₁H, ³₁H have different number of neutrons.

Reason: Isotopes have the same atomic number.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true and R is not the correct explanation of A.
3. A is true and R is false.
4. A is false and R is true.

The element 'X' has same number of protons, neutrons & electrons, with same number of electrons in 'K' shell & 'N' shell. The element -

1. Is a non-metal with valency -2.
2. Attains stable octet structure of the nearest inert gas 'Ar', by gain of two electrons in its valence shell.
3. Is a metal with valency +2.
4. X₂₊ has electronic configuration 2, 8.

Four elements A, B, C, D are given :

A shows the presence of 20 neutrons, 17 protons and 17 electrons.

B shows the presence of 18 neutrons, 17 protons and 17 electrons.

C shows the presence of 10 neutrons, 9 protons and 10 electrons.

D shows the presence of 4 neutrons, 3 protons and 2 electrons.

State which of the above is —

(a) an anion

(b) a cation

(c) a pair of isotopes.

Write the formula of the compound formed between D and C.

State the incorrect statement from the list given below:

1. Modern atomic theory differs from Dalton's atomic theory with inference to isotopes.
2. On supplying energy initially to an atom, an electron may move from K shell to L shell of the atom.
3. An atom 'X' having equal no. of protons & neutrons in its atom, has electronic configuration-2, 8, 10.
4. Atoms of the same element having atomic mass 39 & 41 respectively, have the same electronic configuration.