Chapter 4: Structure of the Atom
1. The Subatomic Particles
Initially, atoms were thought to be indivisible. However, experiments proved they contain
three sub-particles:
● Protons: Positively charged particles found in the nucleus. They have a mass of 1
unit.
● Electrons: Negatively charged particles that revolve around the nucleus. Their mass
is considered negligible.
● Neutrons: Neutral particles (no charge) found in the nucleus. Their mass is 1 unit.
(Note: Hydrogen is the only atom that typically doesn't have a neutron).
2. Atomic Models and Discoveries
● Thomson’s Model: He proposed the "Christmas Pudding" model. He suggested the
atom is a sphere of positive charge with electrons embedded in it like dry fruits. The
main takeaway was that the atom is electrically neutral.
● Rutherford’s Gold Foil Experiment: By firing alpha particles at a thin gold sheet, he
discovered that most of the atom is empty space. He found that the positive charge
and mass are concentrated in a tiny center called the Nucleus.
● Bohr’s Model: He corrected Rutherford by stating that electrons revolve in fixed paths
called Orbits or Energy Levels. As long as electrons stay in these orbits, they do not
lose energy.
3. Distribution of Electrons (Rules)
The distribution of electrons in different shells (K, L, M, N) follows specific rules:
The maximum number of electrons in a shell is calculated by the formula 2n^2, where n is
the shell number.
The first shell (K) can hold up to 2 electrons.
The second shell (L) can hold up to 8 electrons.
The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has
the capacity for more.
4. Valency
The electrons present in the outermost shell are called Valence Electrons.
Valency is the combining capacity of an atom. Atoms react to achieve a stable state (an
"Octet" or 8 electrons in the outer shell).
If an atom has 1 to 4 electrons in its outer shell, its valency is usually equal to those
electrons.
If it has 5 to 7 electrons, the valency is calculated by subtracting that number from 8 (e.g.,
Chlorine has 7 outer electrons, so its valency is 8 - 7 = 1).
5. Atomic Number and Mass Number
Atomic Number (Z): This is the total number of protons in an atom. Every element has a
unique atomic number.
Mass Number (A): This is the sum of protons and neutrons in the nucleus. These two are
together called Nucleons.
Representation: In chemical notation, the Mass Number is written at the top left of the
element symbol, and the Atomic Number is written at the bottom left.
1. The Subatomic Particles
Initially, atoms were thought to be indivisible. However, experiments proved they contain
three sub-particles:
● Protons: Positively charged particles found in the nucleus. They have a mass of 1
unit.
● Electrons: Negatively charged particles that revolve around the nucleus. Their mass
is considered negligible.
● Neutrons: Neutral particles (no charge) found in the nucleus. Their mass is 1 unit.
(Note: Hydrogen is the only atom that typically doesn't have a neutron).
2. Atomic Models and Discoveries
● Thomson’s Model: He proposed the "Christmas Pudding" model. He suggested the
atom is a sphere of positive charge with electrons embedded in it like dry fruits. The
main takeaway was that the atom is electrically neutral.
● Rutherford’s Gold Foil Experiment: By firing alpha particles at a thin gold sheet, he
discovered that most of the atom is empty space. He found that the positive charge
and mass are concentrated in a tiny center called the Nucleus.
● Bohr’s Model: He corrected Rutherford by stating that electrons revolve in fixed paths
called Orbits or Energy Levels. As long as electrons stay in these orbits, they do not
lose energy.
3. Distribution of Electrons (Rules)
The distribution of electrons in different shells (K, L, M, N) follows specific rules:
The maximum number of electrons in a shell is calculated by the formula 2n^2, where n is
the shell number.
The first shell (K) can hold up to 2 electrons.
The second shell (L) can hold up to 8 electrons.
The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has
the capacity for more.
4. Valency
The electrons present in the outermost shell are called Valence Electrons.
Valency is the combining capacity of an atom. Atoms react to achieve a stable state (an
"Octet" or 8 electrons in the outer shell).
If an atom has 1 to 4 electrons in its outer shell, its valency is usually equal to those
electrons.
If it has 5 to 7 electrons, the valency is calculated by subtracting that number from 8 (e.g.,
Chlorine has 7 outer electrons, so its valency is 8 - 7 = 1).
5. Atomic Number and Mass Number
Atomic Number (Z): This is the total number of protons in an atom. Every element has a
unique atomic number.
Mass Number (A): This is the sum of protons and neutrons in the nucleus. These two are
together called Nucleons.
Representation: In chemical notation, the Mass Number is written at the top left of the
element symbol, and the Atomic Number is written at the bottom left.
