What Is Atom Class 12 Notes: Complete Physics Guide for CBSE

Definition and Basic Structure of an Atom

An atom is the fundamental unit of matter that defines the chemical properties of an element. It consists of a tiny, dense nucleus surrounded by electrons. The nucleus contains protons (positively charged) and neutrons (neutral), while electrons (negatively charged) revolve around the nucleus in specific energy levels or shells.

Key points:

• Proton charge = +1, neutron charge = 0, electron charge = -1
• Atomic number ($Z$) = number of protons
• Mass number ($A$) = protons + neutrons

Understanding the atom’s structure is essential for grasping chemical reactions and physical properties studied in Class 12 Physics.

Historical Development of Atomic Models

The concept of the atom has evolved over time through various models:

• Dalton’s Model: Atoms are indivisible spheres.
• Thomson’s Model: Atom is a positively charged sphere with embedded electrons (plum pudding model).
• Rutherford’s Model: Atom has a small, dense nucleus with electrons orbiting around it.
• Bohr’s Model: Electrons move in fixed orbits with quantized energy levels.

Bohr’s model successfully explained atomic spectra and stability of atoms, making it a key topic in Class 12 NCERT Physics.

Bohr’s Atomic Model and Its Significance

Bohr introduced the idea that electrons revolve in fixed orbits without radiating energy. Each orbit corresponds to a specific energy level:

• Electrons can jump between orbits by absorbing or emitting energy.
• Energy of an electron in the $n^{th}$ orbit is given by:

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

• The radius of the $n^{th}$ orbit:

$$r_n = n^2 r_1$$

where $r_1 = 0.529 \times 10^{-10} m$ (Bohr radius)

This model explains the hydrogen atom’s emission spectrum and is vital for Class 12 students to understand atomic behavior.

Atomic Number, Mass Number, and Isotopes

Understanding atomic number and mass number is crucial:

Isotopes are atoms with the same atomic number but different mass numbers. For example, Carbon-12 and Carbon-14 are isotopes:

• Carbon-12: 6 protons, 6 neutrons
• Carbon-14: 6 protons, 8 neutrons

Isotopes have similar chemical properties but different physical properties, important for nuclear physics and chemistry.

Worked Example: Calculating Neutrons in an Atom

Example:

Calculate the number of neutrons in an atom of chlorine with atomic number 17 and mass number 35.

Solution:

Number of neutrons = Mass number - Atomic number

$$ = 35 - 17 = 18 $$

So, chlorine atom has 18 neutrons.

This simple formula helps Class 12 students quickly solve related problems in exams.

Summary of Important Formulas in Atomic Physics

Here are key formulas every Class 12 student should remember:

• Atomic number: $Z = $ number of protons
• Mass number: $A = $ protons + neutrons
• Number of neutrons: $N = A - Z$
• Energy of electron in $n^{th}$ orbit: $$E_n = - \frac{13.6}{n^2} \text{ eV}$$
• Radius of $n^{th}$ orbit: $$r_n = n^2 r_1$$ where $r_1 = 0.529 \times 10^{-10} m$

Mastering these formulas is essential for solving numerical problems in the NCERT Class 12 Physics chapter on atoms.