Question

The number of neutrons in the Sodium atom is
A) 10
B) 11
C) 13
D) 12

Answer 1

Hint: Neutrons and protons together make up the nucleus of an atom.
The number of protons is equal to the atomic number (Z) of the element.
The sum of the number of protons and the number of neutrons is equal to the mass number (A) of the element.

Complete step by step answer:
Neutrons and protons together reside in the nucleus of an atom.
The Sodium atom has an atomic number of 11. By definition,
$\text{Number of protons = Atomic number of element }\left(\text{Z}\right)$
Thus, the number of protons in the nucleus of sodium is 11. --(1)
Also, Sodium has a mass number of 23. By definition,
$\text{Number of protons + Number of neutrons = Mass number of an element }\left(\text{A}\right)$
$\therefore \text{Number of protons + Number of neutrons = 23}$ --(2)
Now, using (1) in (2),
$11+\text{ Number of neutrons = 23}$
$\therefore \text{Number of neutrons = 23-11 = 12}$
Therefore, there are 12 neutrons in the sodium atom.
Hence, the correct option is D) 12.

Additional information:
The periodic table of elements is arranged with the elements in ascending order of their atomic numbers. Earlier, it used to be arranged in ascending order of the mass numbers of the elements. However, this system did not last as this was not consistent and had many flaws. Arranging elements according to their atomic number forms better-arranged groups of elements that have similar characteristics and thus correctly belong together.
Some elements have different atomic numbers but the same mass numbers. This arises due to the number of neutrons. For example, Argon (Ar, atomic number 18), and Calcium (Ca, atomic number 20) have the same mass number of 40. Such species are known as isobars.
Some elements have the same atomic number but different mass numbers. This arises due to different numbers of neutrons. For example, C-12 and C-14 are forms of carbon with atomic number 6, but mass numbers 12 and 14 respectively. Such species are known as isotopes.

Note: To solve such questions students should have a good grip on the periodic table. Learning the periodic table is imperative to scoring well in such questions and especially in competitive exams.
The periodic table can be learned in series and groups, using clever mnemonics. For the series of halogens, Fluorine, Chlorine, Bromine, Iodine and Astatine, one such mnemonic is “First Class Burger I Ate” where the first letters denote the starting letters of the elements in the correct order.