Atomic Mass And How It Is Measured

                John Dalton was the first scientist who postulated in his Dalton’s Atomic Theory that each element had a characteristic atomic mass. Atoms are extremely tiny particles which are even difficult to see through microscopes. It became difficult for scientists to measure their individual masses.

We have already seen about the formation of ions in the previous post in this blog. Similar to how ion is formed where atoms gain and loose electrons; even though the atoms are of less electrons it is still the same element.

In the same way, if an atom is losing a neutron or gaining a neutron that type of atom is called as ISOTOPE. They are still the same element.

Finally scientists thought of finding the atomic mass by comparing with standard atomic mass of some other element. Before 1961, the physicists and chemists considered two sets of atomic mass of Oxygen (atomic mass 16).

Chemists used naturally occurring oxygen which is the mixture of isotope of Oxygen-16, Oxygen-17 and Oxygen-18. Physicists used the isotope of Oxygen-16, because it given their measurement according to their mass spectrometry.

So it confused the scientists which isotope of oxygen should be taken for the mass comparison. In 1961, their confusion came to an end by instead taking the isotopes of oxygen or hydrogen, the isotope of Carbon was chosen as it contains 6 protons and 6 neutrons giving a mass of exactly 12. Carbon- 12 atomic mass is measured accurately compared with other elements in the periodic table.

So, now it is used as a standard reference to measure the atomic mass of other elements. Carbon-12 is divided into 12 equal parts where each part represents 1/12 of atomic mass of crabon-12.

Atomic Mass:

              The number of times one atom of given element is heavier than 1/12^th part of atomic mass of Carbon-12 is known as Atomic mass. Atomic mass of an element is defined as the average mass of all the isotopes of the element as compared to 1/12^th of the mass of one carbon-12 atom.

Atomic mass is measured in “amu” (atomic mass unit). But through latest recommendations of IUPAC, the amu has been replaced with “u” which is known as unified mass.

                        Atomic mass of some elements