Ridha Fathima

🔹 What is an Atom?An atom is the smallest particle of an element that still has the same properties as that element. Everything around us—air, water, plants, animals, and even you—is made up of atoms.

🔹 Main Parts of an AtomAn atom is made up of three main subatomic particles:

Protons (⁺)Found in the nucleus (center) of the atom.Have a positive charge.The number of protons = atomic number (this tells which element it is).

Neutrons (0)Also found in the nucleus.Have no charge (they are neutral).They add mass to the atom and help keep the nucleus stable.

Electrons (⁻)Move around the nucleus in regions called energy levels or shells.Have a negative charge.They are very light compared to protons and neutrons.

🔹 Structure of the AtomAt the center of every atom is the nucleus, which is very small but contains almost all the atom’s mass. It holds the protons and neutrons tightly together.Around the nucleus, electrons move in shells or orbits, like planets moving around the sun.Each shell can hold only a certain number of electrons:1st shell → 2 electrons2nd shell → 8 electrons3rd shell → 18 electrons

🔹 Example: Atom of OxygenProtons: 8Neutrons: 8Electrons: 8So oxygen has 8 protons and 8 neutrons in its nucleus, and 8 electrons orbiting in two shells.

🔹 How Scientists Discovered the Atomic ModelThe idea of the atom has changed over time as scientists made new discoveries.John Dalton (1803) first said atoms are solid spheres that make up all matter.Later, J.J. Thomson (1897) discovered electrons and said atoms are positive spheres with negative electrons inside (the “plum pudding model”).Then, Ernest Rutherford (1911) found that atoms have a tiny, dense, positively charged nucleus at the center with electrons around it.After that, Niels Bohr (1913) explained that electrons move in fixed orbits called energy levels.Finally, in the modern atomic model, scientists like Schrödinger showed that electrons don’t move in fixed paths but in cloud-like regions around the nucleus.