Question

Five atoms are labelled V to Z

Atoms	Mass Number	Atomic Number
V	40	20
W	19	9
X	7	3
Y	16	8
Z	14	7

1. Which one of these atoms 
   (1) contains 7 protons;
   (2) has an electronic configuration 2, 7?
2. Write down the formula of the compound formed by atoms X and Y.

Answer 1

(i) As Z has atomic number of 7, hence it has 7 protons. Electronic configuration of 2, 7 means the atom has 2 + 7 = 9 electrons. Therefore, element W has electronic configuration 2, 7 since its atomic number is 9.

(ii) Number of electron in X is 3. Therefore, electronic configuration = (2, 1). As X will try to lose 1 electron to attain stable state, hence, X has valency [1+]

Number of electron in Y is 8. Therefore, electronic configuration = (2, 6). As Y will try to gain 2 electrons to attain stable state, hence, Y has valency [2−]

Since the valency of X is 1+ and valency of Y is 2−

Formula of the compound:

X¹⁺  Y²⁻

So, we get the formula as X₂Y.