Question

An element has atomic number 19 and mass number 39. The number of the neutron in its nucleus is:
A.20
B.58
C.19
D.39

Answer 1

To answer this question, you should recall the basic concepts of atomic mass and atomic number.
The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom.

Complete step by step answer:
Atomic Number is defined as the total number of protons in the nucleus of an atom that gives us the atomic number of that atom while the mass Number is defined as the number of protons and neutrons combined to give us the mass number of an atom. Protons (the atomic number) and neutrons both contribute to the atomic mass.
If you, therefore, subtract the number of protons (19) from the atomic mass (39), you are left with the number of neutrons (=20).

Hence, option A is correct.

Note:
Isotopes, isotone, isobars and isoelectronic species are important concepts related to the number of neutrons, protons and electrons of an atom. Make sure you remember the difference between isotopes, isotone, isobars and isoelectronic species. Isotopes refer to the atoms of the same element having the same atomic number but different mass numbers. According to Moseley’s law, the chemical properties of an element depend on the number of electrons present and their configuration within an atom and as the isotopes have the same number of electrons it results in the fact that they exhibit similar chemical properties. Isotones refers to two nuclides which have the same number of neutrons, but different proton number. Isoelectronic species refers to the elements or ions that have an equal number of electrons. Isobars refer to the atoms of different elements with different atomic numbers but the same mass numbers. They have different chemical properties because they have different atomic numbers.