Atomic number and Mass number
The discovery of subatomic particles provided a lot of information about the properties of the chemical elements and their isotopes. Moseley gave the hypothesis that the modern periodic table proposed by Mendeleev was based on the number of protons in the nucleus of an atom. Moseley’s hypothesis was based on the study of wavelengths of X-rays that were emitted by chemical elements. It was this hypothesis that laid the foundation for the atomic number.
The number of protons presents in the nucleus of an atom is called the atomic number. It is generally represented by Z. It has been established that the charge of a proton is equal but opposite to that of an electron. Since an atom is electrically neutral and neutrons carry no electrical charge, the number of electrons and protons in a neutral atom is the same. This implies that the atomic number is equal to the number of protons or the number of electrons in an atom. As we know that elements tend to lose or gain electrons during many chemical reactions, the number of protons is conventionally used to represent the atomic number of an element. Hydrogen has the atomic number of one as it has only one proton in its atom.
It is the total number of protons and neutrons present in the nucleus of an atom. Protons and neutrons present in the nucleus of an atom are together called as nucleons and hence the mass number is the number of nucleons present in an atom. The mass number is represented by A. Example: Nitrogen has a mass number of 14 as it has 7 protons and 7 neutrons in its atom.
Isotopes and Isobars
Those elements that have the same atomic number but a different mass number are referred to as isotopes. Isotopes occur due to the presence of a different number of neutrons in elements having the same atomic number as mass number is the sum of the number of neutrons and protons. Many but not all elements have isotopes. The isotopes of hydrogen are protium (has one proton and no neutrons), deuterium (has one proton and one neutron) and tritium (has one proton and two neutrons). The chemical properties of isotopes are the same owing to the fact that they have the same number of protons and hence the same number of electrons which determines the chemical properties of an element.
Isobars, on the other hand, are the atoms having the same mass number but a different atomic number. For example, the atomic number of carbon and nitrogen is 6 and 7 respectively. Carbon-14 an isotope of carbon has a mass number of 14 which is same as that of nitrogen and hence carbon-14 and nitrogen are isobars.
Atomic number plays a very important role in the placement of elements in the modern periodic table. It represents the number of protons and hence the number of electrons in an atom. The number of electrons in an atom governs its chemical behaviour. The study of elements has revealed their applicability in different forms. Learn more about these elements with the expert faculty at BYJU’S.