Question

If an element has atomic number $9$ and atomic mass $19$ ,how many neutrons will it have?

Answer 1

In order to solve this equation we first need to know, the relationship between protons, electrons and neutrons of an element. The number of protons is equal to the atomic number of an element. In a neutral atom, the number of electrons is equal to the number of protons and the atomic mass is the sum of the number of protons and neutrons.

Complete step-by-step answer: If you have mass number and atomic number we can subtract the atomic number from the mass number to get the number of neutrons.
$\ Number{\text{ of neutrons = Atomic mass - Atomic number}} \\\ {\text{ = 19 - 9}} \\\ {\text{ = 10neutrons}} \\\ \$
Hence there are $10neutrons$in an element which has an atomic number $9$ and atomic mass $19$ .
As the atomic number is $9$ we can say that the element in question is fluorine. Since the isotopes are named using the mass number, this particular isotope is named as $fluorine - 19$

Additional information: Since an element’s isotopes has different atomic masses, scientists also determine the relative atomic mass which is also sometimes called as the atomic weight for an element. The relative atomic mass is an average of the atomic masses of all the different isotopes contained in a sample, with each isotope's contribution to the average determined by how big the fraction of the sample it makes up.

Note: The mass number of an atom is equal to the sum of the number of protons and the number of neutrons contained in its nucleus. The atomic number of an element gives us information about how many electrons there are in the atoms. We must always note that the number of neutrons is the difference between atomic mass and atomic number. We can add the mass of protons and neutrons to calculate the atomic mass of a single atom of an element.