Why is the atomic mass for calcium a decimal? how many protons does it have? neutrons? electrons?

atomic mass = 40.08 amu

Number of protons = 20

Number of electrons = 20

Number of electrons = 20

Explanation:

The element of group 2A in 4th period is calcium.

Its atomic number is 20.

Its atomic weight is 40.08 amu. The atomic mass is in decimal number because it is the average of isotopes of calcium.

It is third most abundant metal.

It is silvery soft metal.

It react rapidly with water and air.

It is also used as reducing agent.

it is used to make the alloy with copper, aluminium and beryllium.

Its electronic configuration is:

Ca₂₀ = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

1s² 2s² 2p⁶ 3s² 3p⁶ are inner electrons.

4s² are valance electrons. Thus it loses two valance electrons to get complete octet.

Abbreviated electronic configuration:

Ca₂₀ = [Ar] 4s²

i think   the answer is c hope i'm right