What Is Atom Class 12 Notes: Complete Guide for Physics Chapter Atoms

Definition and Basic Structure of an Atom

An atom is the fundamental unit of matter that defines the chemical elements. It consists of three main subatomic particles:

• Protons: Positively charged particles found in the nucleus.
• Neutrons: Neutral particles also located in the nucleus.
• Electrons: Negatively charged particles orbiting the nucleus in electron shells.

The nucleus contains protons and neutrons, making it dense and positively charged, while electrons move around the nucleus in defined energy levels. The number of protons determines the atomic number ($Z$), which defines the element.

Key terms:

• Atomic Number ($Z$): Number of protons in the nucleus.
• Mass Number ($A$): Total number of protons and neutrons.

Understanding these basics is essential for grasping atomic models and properties.

Historical Development of Atomic Models

The concept of the atom has evolved through various models:

1. Dalton’s Atomic Model (1808):

• Atoms are indivisible spheres.
• Atoms of the same element are identical.

2. Thomson’s Plum Pudding Model (1897):

• Atom is a positively charged sphere with embedded electrons.

3. Rutherford’s Nuclear Model (1911):

• Atom has a small, dense, positively charged nucleus.
• Electrons orbit the nucleus.

4. Bohr’s Model (1913):

• Electrons move in fixed orbits with quantized energies.
• Explains atomic spectra.

Each model improved understanding, leading to the modern quantum mechanical model. For Class 12 exams, focus on Bohr’s model and its significance.

Bohr’s Model of the Atom: Key Concepts and Formulas

Bohr’s atomic model introduced the idea of quantized electron orbits:

• Electrons revolve around the nucleus in circular orbits without radiating energy.
• Each orbit corresponds to a specific energy level.
• Electrons can jump between orbits by absorbing or emitting energy equal to the difference between energy levels.

Important formulas:

• Radius of nth orbit:

$$r_n = n^2 a_0$$ where $a_0 = 0.529 \times 10^{-10}$ m (Bohr radius), $n$ = orbit number.

• Energy of nth orbit:

$$E_n = - \frac{13.6}{n^2} \text{ eV}$$

• Energy difference between two levels (for photon emission/absorption):

$$\Delta E = E_i - E_f = h \nu$$

Bohr’s model successfully explains the hydrogen atom spectrum and is a crucial part of Class 12 Physics.

Isotopes and Isobars: Differences and Examples

Atoms of the same element can differ in the number of neutrons, leading to isotopes and isobars:

Isotopes have identical chemical properties but different physical properties. Isobars are atoms of different elements with the same mass number.

Atomic Number, Mass Number, and Atomic Mass

Understanding atomic number, mass number, and atomic mass is vital:

• Atomic Number ($Z$): Number of protons in the nucleus. It defines the element.
• Mass Number ($A$): Total number of protons and neutrons.
• Atomic Mass: Weighted average mass of all isotopes of an element, expressed in atomic mass units (u).

Example:

For chlorine, which has two main isotopes:

• $^{35}Cl$ (75% abundance)
• $^{37}Cl$ (25% abundance)

Atomic mass = $(0.75 \times 35) + (0.25 \times 37) = 35.5$ u

This explains why atomic masses on the periodic table are often decimal values.

Important Formulas and Numerical Example

Here are essential formulas related to atoms:

• Number of neutrons:

$$N = A - Z$$

• Energy of electron in nth orbit (Bohr’s model):

$$E_n = - \frac{13.6}{n^2} \text{ eV}$$

• Radius of nth orbit:

$$r_n = n^2 a_0$$

Worked Example:

Calculate the radius of the 3rd orbit of a hydrogen atom.

Given: $a_0 = 0.529 \times 10^{-10}$ m, $n=3$

Solution:

$$r_3 = 3^2 \times 0.529 \times 10^{-10} = 9 \times 0.529 \times 10^{-10} = 4.761 \times 10^{-10} \text{ m}$$

This helps in understanding electron distribution and energy levels.