Question

An atom of the same element that differs by the number of neutrons is called:
A) anion
B) cation
C) element
D) isotope
E) isobar

Answer 1

Hint: We have learnt that molecules or compounds are made up of atoms. As we know that the number of neutrons, the number of protons and the number of electrons give the basic identity of an atom or element. We use this information to classify the atoms or elements to different categories. When electrons are lost or gained, different groups of molecules or ions are formed.

Complete step by step answer:
Atomic number is the basic information about an atom. Apart from that there is another identity for an atom which is known as a mass number. Mass number is the total number of protons and neutrons in an atom. Atomic number is the number of electrons or protons in an atom. The number of electrons and protons will be the same because the atom as a whole will be electrically neutral only if the number of electrons and protons are equal as they are elements having opposite charges.
Protons, electrons and neutrons are subatomic particles. They differ mainly in their electrical charges. Protons have a positive charge, an electron has a negative charge and a neutron has a neutral charge. Although being neutral, neutrons have a major role in determining the mass number or atomic weight of an atom.
Atoms having the same atomic number but having different mass numbers are called isotopes. For example, the hydrogen atom has mainly three isotopes and each of them differ by a value of one in their mass number. Therefore, the number of neutrons, although being neutral, are important in determining the mass number of an atom.

So, the correct answer is Option D.

Additional information:
If we know the atomic number or the number of protons and the mass number, we can calculate the number of neutrons simply by subtracting the atomic number from the mass number. The mass number is different from the atomic mass.

Note: Atoms having the same atomic number but different mass numbers are called isotopes. An element may have more than one isotope. Atoms having the same mass number but different atomic numbers are called isobars are some elements that exhibit that property.