JOURNEY INSIDE THE ATOM-3

PRERNA FOR IAS

         Mehnat Aapki, Guidance Humari

         Roz ki Prelims Practice — Experts ke Saath

JOURNEY INSIDE THE ATOM-3

1. Mass Number (A)

The mass number (A) of an atom is the total number of protons and neutrons present in its nucleus. It represents the total mass of the nucleus because the mass of electrons is negligible. The mass number is calculated using the formula: Mass Number = Number of Protons + Number of Neutrons. For example, carbon-12 has 6 protons and 6 neutrons, giving it a mass number of 12. Mass number helps distinguish different isotopes of the same element. It is an important concept in chemistry, nuclear physics, and atomic structure studies, as it determines the atom's overall nuclear composition.

2. Electronic Configuration

Electronic configuration is the arrangement of electrons in different energy levels or shells around the nucleus of an atom. Electrons are distributed according to specific rules, filling lower energy levels before higher ones. The maximum number of electrons that a shell can hold is given by the formula 2n², where n is the shell number. For example, oxygen (atomic number 8) has the electronic configuration 2, 6, while sodium (11) has 2, 8, 1. Electronic configuration determines an element's chemical properties, valency, and reactivity. It is essential for understanding chemical bonding and periodic table trends.

3. Electronic Shells (K, L, M, N)

Electrons revolve around the nucleus in fixed energy levels, also called electronic shells. These shells are named K, L, M, N, corresponding to n = 1, 2, 3, and 4. The K shell can hold a maximum of 2 electrons, L holds 8, M holds 18, and N holds 32 electrons, according to the 2n² rule. Electrons occupy lower-energy shells first before filling higher ones. The outermost shell is known as the valence shell and plays the most important role in chemical bonding. Electronic shells help explain atomic stability and electron distribution.

4. Valency

Valency is the combining capacity of an atom or element. It indicates the number of electrons an atom loses, gains, or shares to achieve a stable electronic configuration, usually by completing its outermost shell. Elements with one, two, or three valence electrons generally lose electrons, while those with five, six, or seven gain electrons. Noble gases have a valency of zero because their outermost shell is already complete. For example, sodium has a valency of 1, oxygen has a valency of 2, and carbon has a valency of 4. Valency helps predict chemical bonding and compound formation.

5. Isotopes