Question

Calculate the number of neutrons the following element contain and state the formula used.

${{}_{19}\mathrm{K}}^{39}$

Answer 1

• An atom of an element is represented as ${{}_{\mathrm{Z}}\mathrm{X}}^{\mathrm{A}}$
• Where Z is the atomic number, and A is the mass number of element X.
• Atomic number(Z)- The total number of protons in the nucleus of an atom gives us the atomic number of that atom.
• Mass number(A)- The number of protons and neutrons combine to give us the mass number of an atom.
• Any neutral atom has the same number of electrons as the number of protons.
• For a neutral atom Z = atomic number = number of protons = number of electrons.
• Number of neutrons = A - Z
• In ${{}_{19}\mathrm{K}}^{39}$, 19 is the atomic number, and 39 is the mass number of Potassium.
• Hence, the Number of neutrons = A - Z = 39 - 19 = 20.