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JAMB Chemistry 2019 Paper

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Question : 18

Total: 80

Elements X and Y have electronic configurations 1s

^2

^2

^4

^2

^2

^6

^2

^1

respectively. When they combine, the formula of the compound formed is

XY
X $_2$ Y
Y $_2$ X $_3$
X $_2$ Y $_3$

Solution:

Concept:

Identify the valencies from the electronic configurations and combine them by criss-crossing charges.

Explanation:

X:

1s^2\,2s^2\,2p^4

has 6 valence electrons, so it needs 2 more electrons: valency 2, charge

X^{2-}

.

Y:

1s^2\,2s^2\,2p^6\,3s^2\,3p^1

has 3 valence electrons, so it can lose 3 electrons: valency 3, charge

Y^{3+}

.

To balance charges:

2 \times (+3) + 3 \times (-2) = 0

.

Thus, the formula is

Y_2X_3

.

Answer:

C.

Y_2X_3

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