Introduction: What Are Atoms?
Everything around us is made of matter—your water bottle, books, clothes, air, and even your own body! But what is matter made of? Atoms.
Atoms are incredibly tiny. You can’t see them with your eyes or even with a normal microscope. Yet, they are the basic units that make up all matter in the universe.
To understand what atoms are made of, scientists came up with different models. One of the most important and useful models was given by Niels Bohr in 1913. His model explains the structure of an atom in a simple and understandable way.
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What Is an Atom?
- The atom is the smallest unit of an element that still shows the properties of that element.
- Atoms are made up of subatomic particles: protons, neutrons, and electrons.
- Atoms combine to form molecules, and molecules make up everything around us.
Bohr’s Model of the Atom
Before Bohr, models of atoms weren’t very accurate or easy to use. Bohr improved them by introducing the idea of electrons moving in fixed circular paths.
Main Points of Bohr’s Model:
- The atom has a central nucleus.
- The nucleus contains:
- Protons (positive charge)
- Neutrons (no charge)
- Protons (positive charge)
- Electrons (negative charge) move around the nucleus in fixed paths called orbits or shells.
- These shells are named as K, L, M, N… (starting from the one closest to the nucleus).
- Each shell has a fixed energy and electrons don’t lose energy while revolving.
Subatomic Particles
Atoms are made up of three kinds of particles:
| Particle | Charge | Mass | Location |
| Proton | +1 | 1 atomic mass unit (amu) | Inside the nucleus |
| Neutron | 0 | 1 amu | Inside the nucleus |
| Electron | –1 | 1/1836 of a proton | Outside the nucleus (in orbits) |
Atomic Number and Mass Number
Atomic Number (Z)
- Number of protons in an atom.
- Also equal to the number of electrons in a neutral atom.
- It defines the identity of the element.
Mass Number (A)
- Total number of protons + neutrons in the nucleus.
Formula:
Mass Number (A) = Number of Protons + Number of Neutrons
Example:
Oxygen Atom
- Atomic Number = 8 → 8 protons, 8 electrons
- Mass Number = 16 → 8 protons + 8 neutrons
How Are Electrons Arranged?
Electrons don’t just spin randomly. Bohr said electrons revolve in shells or energy levels and follow a fixed rule.
Electron Shell Capacity (2n² Rule):
The number of electrons a shell can hold depends on its position from the nucleus.
| Shell | Symbol | n (shell number) | Max electrons = 2n² |
| 1st | K | 1 | 2 |
| 2nd | L | 2 | 8 |
| 3rd | M | 3 | 18 |
| 4th | N | 4 | 32 |
Example:
Sodium (Atomic number = 11)
→ Electron distribution: 2 in K, 8 in L, 1 in M
→ Written as 2, 8, 1
Core Concepts Table
| Concept | Description |
| Atom | Smallest unit of matter |
| Nucleus | Dense center of the atom containing protons and neutrons |
| Electron | Negatively charged particle orbiting the nucleus |
| Proton | Positively charged particle inside the nucleus |
| Neutron | Neutral particle inside the nucleus |
| Atomic Number (Z) | Number of protons (also electrons in neutral atom) |
| Mass Number (A) | Number of protons + neutrons |
| Shells/Orbits | Fixed paths where electrons revolve around the nucleus |
Why Is Bohr’s Model Important?
- Helps explain how atoms bond in chemistry.
- Tells us how electrons behave during reactions.
- Foundation of modern atomic theory and periodic table.
- Basis for electronic configuration of elements.
Real-Life Examples of Bohr’s Model in Action
- Neon Lights: Electrons jump between energy levels and emit light.
- Fireworks: Different elements emit different colors when electrons change shells.
- Chemical Reactions: Atoms react based on outer shell electrons.
- Semiconductors: Work based on controlled electron flow.
Frequently Asked Questions
Q1. Who gave Bohr’s Model of the atom?
Niels Bohr, in 1913.
Q2. What is the K shell?
The first energy level or shell closest to the nucleus.
Q3. How many electrons can fit in the L shell?
Maximum 8 electrons (2 × 2² = 8)
Q4. What is the atomic number of carbon?
6 (Carbon has 6 protons and 6 electrons)
Q5. Which particles are in the nucleus?
Protons and neutrons.
Fun Facts for Students
- If an atom were the size of a stadium, the nucleus would be as tiny as a pea in the center!
- Bohr’s model was the first to explain atomic stability—why electrons don’t fall into the nucleus.
- Hydrogen is the only element with no neutrons in its most common form.
- The periodic table is arranged based on atomic number, not mass number.
Conclusion
Bohr’s Model gives us a simple way to see what’s happening inside atoms. It explains how electrons are arranged and how they move. These small particles decide how substances react, bond, and form everything in our world.
Understanding the structure of the atom is like unlocking the code of the universe. Once you learn it, chemistry and science make much more sense.
