Question

A nucleus $_{11}^{24}Na$ is $\beta$-radioactive. What are the numbers 24 and 11?
A) 24−neutron number, 11−atomic number
B) 24−electron number, 11−atomic number
C) 24−mass number, 11−atomic number
D) 11−mass number, 24−atomic number

Answer 1

The question is easy to solve if one has seen the periodic table and has knowledge of how to write them. The correct way to write an element is represented as $_Z^AX$ where, A is the mass number and Z is the proton number or atomic number. This will solve our riddle.

Complete step by step answer:
Step 1:
The number of protons determines an element's atomic number and is used to distinguish one element from another. Together, the number of protons and the number of neutrons determine an element's mass number: mass number = protons + neutrons. If you want to calculate how many neutrons an atom has, you can simply subtract the number of protons, or atomic number, from the mass number.
The total number of protons present in the nucleus of every atom of a chemical element represents the atomic number of that element. Each element is characterized by a unique atomic number. It is represented by the letter Z.
Step 2:
Now coming to the question:
Generally a nucleus is represented as $_Z^AX$ where, A is the mass number and Z is the proton number or atomic number.
Here in the question we are given, a nucleus $_{11}^{24}Na$
Here mass number A =24 and atomic number Z =11.
Hence option C is correct.
Additional information:
If we need to find a mass number without a neutron number then: The atomic number of an element is available from the periodic table of elements. If the mass number is not given to us, we can still calculate the number of neutrons by determining the atomic mass of the element. This value is also available from the periodic table of elements.

Note: This question is directly asked from the formula of writing the elements in periodic tables. Sometimes proton number or neutron also asked in the question. Z is the proton or atomic number and for neutrons we need to subtract mass number with atomic or proton number, A-Z.