Published : 7 years ago, on 03/01/2018

In chemistry, Atomic mass is the sum of protons, electrons, and neutrons present in a single atom or molecule. However, as the particles stay in constant motion on their orbits, their mass became negligible. In other words, the Atomic mass is the sum of Protons and Neutrons present in a single atom or a molecule.

The term, atomic mass, is used to refer to the various isotopes of the atom though technically it’s incorrect. Let’s get to know, How to find the nuclear mass of a given atom or molecule.

Using The Periodic Table:

The mass of an atom is typically expressed with general SI units, i.e. grams, kilograms, etc. However, the expressed amount is minimal in quantity. So for easy calculation and other use, the mass is often expressed in unified atomic mass units (“u” or “amu”) or Daltons (Da).

(*Remember that the standard for one atomic mass unit is equal to 1/12th of the mass of a standard carbon-12 isotope)

When it comes to atomic mass, there is another term, Relative nuclear mass. It is defined as the nuclear weight that takes into account the average of the masses of naturally occurring isotopes of the same element.

Mostly, in a standard periodic table, the relative atomic masses (atomic weights) of each element is already given. It is mainly written at the bottom of the element’s square on the table, below the one or two letter chemical symbol. The relative atomic mass is mostly presented in the form of decimal no instead of a whole no. But remember that the given relative atomic masses on a periodic table are average values of the element. As the chemical elements show different isotopes depending upon the addition and subtraction of one or more than one neutrons to the atom’s nucleus, we can take the given relative atomic mass as an average value for the associated element instead of its single nuclear matter.

Generally, the given relative atomic masses in a periodic table are technically unitless. But we can get a workable quantity (the molar mass) by multiplying an atomic mass by 1 g/mol.

As I said earlier, the relative atomic masses, given on the periodic table, are average values for an atom’s isotopes. It helps in many ways for practical calculations like we can say while calculating the molar mass of a molecule. But when its come to individual atoms, the given number became insufficient.

For an Individual Atom:

The atomic number of a chemical element also refers to its proton count. The proton number in an atom never varies. For example, the atomic number of the element C (carbon) is 6. So, the photon count for coal is 6.

But the neutron count in an atom varies as per the given isotopes.We can count the no of neutrons present in an isotope by subtracting the atomic number from the isotope number. As per example, we can take carbon-14. So, the no. of neutrons present in a C-14 is 14 – 6= 8.

To calculate the atomic mass of the individual atom, Add the no of protons with the no of neutrons of the molecule. The combined mass of all the electrons is excessively small that we can neglect them.

For Example, the atomic mass of C-14 is,

8 + 6=14

Not to forget, that the atomic mass of an isotope is relatively quite close to the isotope no. So, for basic calculation, we can take isotope number as the atomic mass.

In this above explanation, we got to know the way to find out the Atomic mass of an Atom from Periodic Table as well as for an individual atom.