Question

The number of protons in Calcium is:
A. 20
B. 21
C. 22
D. 23

Answer 1

Hint: We determine the valency of an atom by knowing the number of electrons present in its outermost shell. The number electrons depends on the atomic number. Similarly, the number of protons and neutrons are also associated with atomic number.

Complete step by step answer:
We all know that protons, neutrons and electrons together constitute an atom. The total number of protons present in the nucleus of an atom gives the atomic number of that particular element.
Atomic number of Calcium is 20 i.e. the nucleus of the Calcium atom contains 20 protons.
So, A is the correct option.

Additional information:
We know that electrons can be gained or lost during chemical reactions. Hence, the number of electrons in an atom are not fixed always. Similarly, it is not always necessary that the number of neutrons will be the same as the number of protons.
 As in the case of isotopes, the number of neutrons changes but the number of protons always remains constant. Hence, atomic number is always determined by the number of protons present in the nucleus of an atom.

Modern form of the periodic table is based on atomic numbers. Elements in the modern periodic table are arranged in increasing order of their atomic number.

Note: Atomic number is the number of protons present in the nucleus and atomic mass number is the sum of protons and neutrons present in the nucleus of an atom. For calcium, the number of protons and neutrons is equal i.e. 20. Hence its atomic number is 20 and atomic mass number is 20 + 20 = 40.