Question

A monatomic anion of unit charge contains 18 neutrons and 18 electrons. Find the atomic number, mass number of an ion with its identification.

Answer 1

A monatomic anion of unit charge means the element is having an extra electron over it making it to be called an anion. Atomic numbers can be easily justified by the number of protons present in an atom. To solve this type of illustration, we should have a basic idea about anion, mass number, atomic number, atomic mass, etc.

Complete step by step solution:
Let us learn the basic concepts of an element to answer the given question:
Element- It is the basic unit of a periodic table. We have many elements in nature forming cation or anion at times. A neutral element is categorised by a factor i.e. it should have equal number of electrons and protons. Number of electrons = Number of protons. When an anion or a cation is formed, the net charge on an atom is given as the difference between the number of protons and number of electrons i.e. Net charge = No. of protons – No. of electrons
Atomic number-Atomic number is the number of protons present in the nucleus of an atom. Only the number of protons define the identity of an element, it does not depend upon the number of neutrons present in the same. In nature, it is represented as Z.
Mass number-The mass number is the sum of the number of neutrons and protons present in the nucleus. It is represented as A and is given as,
A = Z + no. of neutrons
Now, let us solve the above given illustration using the basic knowledge mentioned;
Given that, Number of electrons = 18
Number of neutrons = 18
It is a monatomic anion with unit charge so, net charge = -1
Thus, By the formula mentioned above, we get,
Net charge = No. of protons – No. of electrons

& -1=x-18 \\\ & x=18-1=17protons \\\ \end{aligned}$$ Thus, the number of protons = 17 which in fact is the atomic number of an element. Hence, the anion we are discussing about is chlorine i.e. Cl. Now, the mass number is given as, A = Z + no. of neutrons $$A=17+18=35$$ Thus, mass number is 35. **Note:** Do note that the mass number is not always equal to the atomic mass of an element. As, atomic mass is the summation of the average number of neutrons and protons; if isotopes exist for that element. Whereas, mass number is specific for every isotope (if it exists); it is a whole number unlike atomic mass which can be a decimal number. Also, no. of neutrons is not always equal to the no. of protons in the nucleus.