Class 7 Chemistry Chapter 2: The Atom

Study Notes

An atom is the smallest unit of a substance that retains the properties of that substance.
An atom consists of a central nucleus containing protons (positively charged particles) and neutrons (neutral particles with no charge).
Electrons revolve around the nucleus in different shells or orbits, which are imaginary paths surrounding the nucleus.
Atoms are made up of three types of particles: electrons, protons, and neutrons, which have different properties:
- Electrons: tiny, very light particles with a negative electrical charge (-).
- Protons: much larger and heavier than electrons with a positive charge (+).
- Neutrons: large and heavy like protons, but with no electrical charge.

Mass number or atomic mass (A) is the total number of protons and neutrons present in the nucleus of an atom.
Mass number is calculated by adding the number of protons and neutrons (A = p+ + n0).

Electrons revolve around the nucleus in imaginary paths called orbits or shells.
The distribution of electrons in various shells of an atom is known as electronic configuration.
Examples of electronic configurations are given for different elements, such as Sodium (Na), Hydrogen (H), and Helium (He).

A symbol is the short form or abbreviated name of an element.
Symbols are generally represented by the first capital letter or alphabet of the atom of that element.
Some symbols are derived from Latin names of elements, such as Na for Sodium (Natrium) and Fe for Iron (Ferrum).

A radical is a group of atoms of elements that behave like a single unit and show valency.
Examples of radicals include Ammonium (NH41+) and Hydroxide (OH1-).

Valency is the number of electrons an atom can donate, accept, or share to achieve stable electronic configuration of the nearest noble gas.
Valency is always a whole number and can be positive (for metals and hydrogen) or negative (for non-metals).
Examples of valencies are given for different elements, such as Sodium (Na+, monovalent), Zinc (Zn2+, divalent), and Aluminium (Al3+, trivalent).

An ion is any atom or group of atoms carrying a positive or negative charge due to loss or gain of electrons.
Examples of ions include Sodium ion (Na+) and Chloride ion (Cl-).
A valency chart is provided, showing different elements with their symbols, ions, and valencies.

A formula is the representation of the chemical name of a substance by means of symbols.
A chemical formula denotes the number of atoms of each element present in a compound.
Steps are given for writing a chemical formula, including writing the symbols of the elements, their valencies, and interchanging and canceling out the valencies to get the final formula.