Question

The letters X, Y and Z represent different atoms.
$_{19}^{40}{\text{X }}_{19}^{39}{\text{Y }}_{20}^{40}{\text{Z}}$
What can be deducted from the proton numbers and nucleon numbers of X, Y and Z.
(A) X and Y are the same elements.
(B) X and Z are the same elements.
(C) X has more protons than Y.
(D) Z has more neutrons than Y.

Answer 1

The number of protons present in the nucleus of an atom represents the atomic number of an element and the total mass of the nucleus of atom represents the mass number for an element ( ${\text{Mass number = no}}{\text{. of protons + no}}{\text{. of neutrons}}$ ) .

Complete step by step answer:
Atomic number: The number of protons present in the nucleus of an atom describes the atomic number of an element.
Mass number: Mass number or atomic mass number is the sum of total number of neutrons and protons present in the nucleus of an atom. We can also say that the mass of the nucleus of an atom is considered as its mass number.
Number of neutrons: The difference of the mass number and number of protons gives us the number of neutrons.
Representation of an atomic symbol: $_{{\text{atomic number}}}^{{\text{Mass number}}}{\text{E}}$
The table representing whole data of X, Y and Z is as follows:

| X| Y| Z
---|---|---|---
No. of protons| 19| 19| 20
No. of electrons| 19| 19| 20
No. of protons| (40-19=21)| (39-19=20)| (40-20=20)

So from the above table we can see that X and Y have the same atomic number but different numbers of neutrons so X and Y are the isotopes of the same element.
Therefore we can say that X and Y are the same elements.
Hence option (A) is the correct answer.

Note:
Isotopes: The atoms of the same chemical element having the same number of protons but different number of neutrons.
Isobars: The atoms of different chemical elements having different atomic numbers but having the same mass number.