Question

The elements in the periodic table are arranged according to their atomic number (the number of protons in the nuclei). Starting with element number 1, hydrogen. The atomic mass of the atom is the number of the protons and the neutrons in its nucleus. The discovery of the element 118-(Uuo) Ununoctium was confirmed in January 2016. One possible isotope of ununoctium is Uuo-291, which has an atomic mass of 291. How many neutrons in the nucleus of Uuo-291?
(A) 118
(B) 173
(C) 291
(D) 409

Answer 1

The number of neutrons is equal to the atomic number of the element which is also the number of protons present in the nucleus of the atom subtracted from the atomic mass of the element.

Complete Stepwise solution
An atom is composed primarily of three subatomic particles which are: the electrons, the protons and the neutrons. The electrons move in spherical shells called ‘orbits’ around the nucleus of the atom while the protons and the neutrons together form the nucleus of the atom. The electrons have negligible mass and are negatively charged while the protons are positively charged but the neutrons are chargeless, neutral particles. The mass of the atom is mainly due to the neutrons and the protons and therefore the mass number of the atom or the atomic mass is the sum of the neutrons and the protons present in the atom. Therefore it is clear from the above discussion that the number of neutrons in the nucleus of Uuo-291 is the value obtained after subtracting the mass number from the atomic number of Un-un-octium.
Number of neutrons = $291-118=173$
So, the correct answer to the above question is option B.

Note
There are elements in the periodic table that have the same atomic number but different atomic masses, i.e., the atoms of those elements differ in the number of neutrons. Such atoms are called “isotopes”. For example hydrogen has three isotopes in total with mass number 1, 2, and 3.
The atoms of the elements that have the same atomic mass but different atomic numbers are called “isobars”. For example, calcium and argon both have mass number 40 but different atomic numbers.