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.