Question

Five elements A, B, C, D and E have atomic numbers of 2, 3, 7, 10 and 18 respectively. The elements which belong to the same period of the periodic table are
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E

Answer 1

Answer:

A,D and E belong to same group

Explanation:

Helium have atomic number 2 (element A), neon have 10 (element D) and argon have 18(element E). all these three elements belong to group 8 A

Answer 2

The elements which belong to the same period of the periodic table are B, C and D.

Explanation:

Electronic configuration represents the total number of electrons that a neutral element contains. We add all the superscripts to know the number of electrons in an atom.

The electrons are filled according to Afbau's rule in order of increasing energies.

$A:2:1s^2$ .

$B:3:1s^22s^1$

$C:7:1s^22s^22p^3$

$D:1:1s^22s^22p^6$

$E:18:1s^22s^22p^63s^23p^6$

The period of an element is determined by the principle quantum number of the valence electron. As the value of principle quantum number is 2 for B, C and D , they belong to the same period.

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