Valency, Atomic number, Mass number

Introduction

Electrons are distributed in various shells with discrete energy around the nucleus.

If the atomic shells are complete, then the atoms are going to be stable and less reactive.

• Only certain special orbits known as discrete orbits of electrons, are allowed within the atom
• While revolving in distinct orbits the electrons do not radiate energy
• These orbits or shells are known as energy levels and are either designated as K, L, M, N etc. or numbered as n = 1, 2, 3, 4, etc.

Distribution of electrons in numerous orbits (shell):

• The maximum variety of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2, 3, and so on

The maximum variety of electrons in different shells are as follows:

• First Orbit or K-Shell (n = 1) = 2n2 = 2 x 12 = 2 electrons
• Second Orbit or L-Shell (n = 2) = 2n2 = 2 x 22 = 8 electrons
• Third Orbit or M-Shell (n = 3) = 2n2 = 2 x 32 = 18 electrons
• Fourth Orbit or N-Shell (n = 4) = 2n2 = 2 x 42 = 32 electron

• The maximum number of electrons that can be accommodated in the outermost orbit is 8
• Electrons are not accommodated in a given shell unless the inner shells are filled. That is, the shells are filled in a stepwise manner

Valency:

• An outermost shell that has eight electrons is said to possess an octet
• Atoms react to achieve an octet within the outermost shell by sharing, gaining, or losing electrons
• The number of electrons gained, lost, or shared to form the octet of electrons in the outermost shell is the combining capacity of the element and is referred to as valency

Atomic number:

It’s calculated as the total number of protons present in the nucleus of an atom and is denoted by ‘Z’. All atoms of an element have the same atomic number. As an example, carbon, Z = 6.

Mass number:

It’s calculated as the sum of the total number of protons and neutrons present within the nucleus of an atom and is denoted by ‘A’. 

• Mass of an atom resides in its nucleus and is practical because of protons and neutrons solely
• Both protons and neutrons are present within the nucleus of an atom, therefore known as nucleons

Isotopes:

Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.

• Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance
• The chemical properties of isotopes are similar, however, their physical properties are different

Examples:

• Hydrogen atoms have three atomic species namely protium (1H1), deuterium (1H2 or D) and tritium (1H3 or T). Atomic number of all is 1, but the mass number is 1, 2 and 3, respectively
• Similarly, Carbon has 6 C12 and 6 C14 and Chlorine has 17 Cl35 and 17 Cl37
• Apart from these, tin has 22 isotopes, zinc has 21 known isotopes, neon is a mixture of 3 isotopes, natural xenon has a stable combination of 9 isotopes, and nickel has 14 isotopes

Applications of Isotopes: 

• An isotope of uranium is employed as a fuel in nuclear reactors
• An isotope of cobalt is employed in the treatment of cancer
• An isotope of iodine is used in the treatment of goitre

• Isotopes have physical and chemical properties

When discussing the chemical properties of isotopes of a given object, they are nearly identical or similar. Chemically, different isotopes behave almost identically. When it comes to physical properties of isotopes, such as mass, melting or boiling point, density, and freezing point, they are all different.

Isobars:

Atoms of different elements with different atomic numbers, which have the same mass number are referred to as isobars. For instance, Calcium and Argon have different atomic numbers (20 and 18 respectively) however the mass number of each of these elements is 40.

Examples of isobars

Argon, potassium, and calcium all have the same number of atoms (40).

18Ar40, 19K40, 20Ca40, where the atomic numbers of three things in a row are shown as payloads and 18, 19, and 20 are the atomic numbers of three things in a row are isobars. Because atomic numbers differ, chemical properties differ as well. The term isotope comes from two Greek words, isos and topos, which both mean “in the same place.” The idea behind this term is that all of an object’s isotopes can be found in the same place in the timeline. Margaret Todd, a Scottish physician and author, coined the term during a consultation with radiochemist Frederick Soddy. In 1913, the name was coined.

Conclusion

The placement of elements in the modern periodic table is largely determined by their atomic numbers. It indicates the number of protons in an atom, and thus the number of electrons. An atom’s chemical behaviour is determined by the number of electrons in it. The isotopes and isobars of different elements has revealed that they can be used in a variety of ways.