Atomic Number and Mass Number, Isotopes and Isobars

The discovery of subatomic particles revealed a lot about the properties of chemical elements and isotopes. Moseley suggested that the modern periodic table proposed by Mendeleev was based on the number of protons in a nucleus. Moseley's hypothesis was based on the study of X-ray wavelengths emitted by chemical elements. This hypothesis was the bedrock for the atomic number.

The Atomic Number

The atomic number is the number of protons in a nucleus.
It is commonly represented by the letter Z.
It has been established that a proton's charge is equal to but opposite to that of an electron.
Because an atom is electrically neutral and neutrons have nil electrical charge, the number of protons and electrons in a neutral atom are equal.
This implies that the atomic number is the same as the number of protons or electrons in an atom.
Because elements have the tendency to gain or lose electrons during many chemical reactions, the number of protons is used to represent an element's atomic number.
Hydrogen has an atomic number of one because its atom contains only one proton.

Atomic Number (Z) = the number of protons in a nucleus.
For eg. Carbon has an atomic number of 6 and a total of 6 protons, while Iron has an atomic number of 25 and a total of 26 protons.

Read atomic theory in detail.

Mass number

It is the total number of neutrons and protons in a nucleus.
Neutrons and protons in a nucleus are collectively referred to as nucleons.
Thus the mass number is the number of nucleons present in an atom.
‘A’ represents the mass number. Nitrogen, for example, has a mass number of 14 because its atom contains 7 protons and 7 neutrons.

Mass Number (A) = Number of neutrons + Number of protons
Example: In a Carbon atom,
Number of Protons =6 , Number of Neutrons= 6,
Mass Number =12

Isotopes

Isotopes are elements that have the same atomic number but a different mass number.
Isotopes form when elements with the same atomic number have a different number of neutrons, as the mass number is the total of the number of protons and neutrons.
Many elements, but not all, have isotopes.
Protium (no neutrons and one proton ), deuterium (one neutron and one proton), and tritium are hydrogen isotopes (has one proton and two neutrons).
Isotopes have the same chemical properties because they have the same number of protons.
Thus the same number of electrons, which determines an element's chemical properties.

Isobars

Isobars, on the contrary, are atoms that share the same mass number but have a different atomic number.
For example, Argon has a mass number of 40 and an atomic number of 18 , while Potassium has a mass number of 40 and an atomic number of 19.
Calcium has a mass number of 40 and an atomic number of 20.
Argon, Calcium, and Potassium have the same mass numbers but different atomic numbers, so they are isobars.
Carbon and nitrogen have atomic numbers of 6 and 7, respectively.
Carbon-14, an isotope of carbon, has the same mass number as nitrogen (14), and thus carbon-14 and nitrogen are isobars.