An atom of an element $X$ may be written as ${ }_{4}^{9} X$.
$(a)$ What does the figure $9$ indicate?
$(b)$ How many neutrons does this element $X$ contain?
$(a)$ What does the figure $9$ indicate?
$(b)$ How many neutrons does this element $X$ contain?
(N/A) The figure $9$ indicates the mass number of the element $X$,which is the sum of the number of protons and neutrons in the nucleus.
$(b)$ The number of neutrons is calculated as: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$.
Given: $\text{Mass number} = 9$,$\text{Atomic number} = 4$.
Therefore,$\text{Number of neutrons} = 9 - 4 = 5$.
$(b)$ The number of neutrons is calculated as: $\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$.
Given: $\text{Mass number} = 9$,$\text{Atomic number} = 4$.
Therefore,$\text{Number of neutrons} = 9 - 4 = 5$.
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