Question

A neutral atom of aluminum has a mass number of 27 and an atomic number of 13. How many electrons does it have?
A) 14
B) 13
C) 27
D) 40
E) It is impossible to determine from the information given.

Answer 1

Hint: Since the atom is neutral, hence it must be in its ground state which means it has neither gained nor lost any electrons. Also, the number of protons and electrons in this atom are numerically equal hence making it electrically neutral.

Complete answer:
We know that the atomic Number (Z) of an element is defined as the number of protons present in its atom. Also, Z is equal to the number of electrons in the ground state, when the atom has neither gained nor lost any electrons.
On the other hand, the Mass Number (A) of an atom is defined as the number of nucleons present inside the nucleus of an atom. Nucleons represent both protons and neutrons. So basically, the mass number is equal to the sum of protons and neutrons present in the nucleus of an atom.
Mass Number = Number of Protons + Number of Neutrons
Number of Protons = Atomic Number
Therefore, the number of Protons present in this Aluminum atom are 13.
Since the number of protons is equal to the number of electrons in an electrically neutral atom, therefore, the number of electrons in this atom is 13.

So, option (B) is correct.

Note: Students might get confused between the terms Mass Number and Atomic Mass. These two terms have different meanings but the same numerical value. Atomic mass is the actual weight of an atom which is represented in u. Atomic mass of an aluminum atom is 27u.