Question

The isotope sodium-20 has how many neutrons?

Answer 1

Hint The isotopes are those species which have the same atomic number but differ in the mass number, the atomic number of Na is 11. Mass number of an element is the sum total of the number of protons and number of neutrons present in an atom.

Complete step by step solution:
So in the above given question, it is asked how many neutrons will be present in the isotope of sodium-20.From the lower classes we are studying about the subatomic particles in an atom, its charge and mass. We are also familiar with the relation of these subatomic particles with atomic number and atomic mass of the elements. The term isotope is specified in the question lets brush up the basic idea about the isotopes.
Isotopes are the species which are having the same atomic number but they differ in their mass number. This is the basic idea of isotopes. We have to relate this idea with the concept of atomic mass.
We know that the atomic number is the unique number for the element which is the identity of a specific element. And the atomic number is equal to the number of electrons and protons present in the atom. So if we know the atomic number, we could tell about the number of protons and electrons present in the atom
Now as discussed earlier isotopes differ only in mass, the atomic mass is contributed by the protons and neutrons.
Since the atomic number remains the same the number of protons remains the same and the change in mass in isotopes is by the number of neutrons present in the atom.
The atomic number of sodium is 11, so there are 11 electrons and protons.
The given isotope has an atomic mass of 20.
So the number of neutrons,
$\text{No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{=Atomic}\,\text{mass-No}\text{.}\,\text{of}\,{{\text{p}}^{\text{+}}}\text{s}$
$\text{No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{=20-11=9}{{\text{n}}^{\text{0}}}\text{s}$

Hence there are only 9 neutrons in the isotope sodium-20

Note: The atomic mass mas is equivalent to the weight of the nucleus and atomic mass can be calculated using the equation,
$\text{Atomic}\,\text{mass=No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{s+No}\text{.}\,\text{of}\,{{\text{p}}^{\text{+}}}\text{s}$
Sodium-23 is the naturally occurring isotope and there are six more isotopes for sodium but among them two of the isotopes have great commercial purpose.
Sodium-22 and sodium -24 have great applications in the medical field and they are radioactive isotopes.
Another type of question that may be asked is about the isobars. Isobars are the compounds in which the atomic mass will be the same but differ in atomic number.
Example Ar and Ca have different atomic numbers but same mass number.