Back to: Inorganic Chemistry 100 Level
Welcome to class!
Good to have you here today. I’m excited to take you through another interesting journey in Chemistry. You know how people always look for ways to explain what they cannot see, like when you look at the sky and wonder how the stars shine, or when you drink water without ever seeing the tiny particles inside it? That same curiosity is what pushed scientists to develop different ideas about the atom—the smallest unit of matter. Today, we will be learning about how the atomic theory developed, from Dalton’s time up to the quantum mechanical model.
Development Of Atomic Theory (Dalton, Thomson, Rutherford, Bohr, Quantum Mechanical Model)
Dalton’s Atomic Theory
Let’s start with John Dalton in the early 1800s. He was like a good student who wanted to organise ideas properly. Dalton said matter is made up of tiny particles called atoms, which are indivisible and indestructible. Imagine small grains of garri in a cup; Dalton believed each grain was like an atom—solid and not breakable. He also said atoms of the same element are identical, and they combine in simple ratios to form compounds. This was a brilliant starting point.
Thomson’s Model
Later, J. J. Thomson discovered something smaller inside the atom—electrons. His experiment with cathode ray tubes showed that atoms are not solid lumps but contain tiny negatively charged particles. He suggested the “plum pudding model,” where electrons are scattered like raisins inside a spherical cake. Think of agege bread with groundnut sprinkled inside—that was Thomson’s idea of an atom.
Rutherford’s Model
Then came Ernest Rutherford with his gold foil experiment. He fired alpha particles at thin gold foil and discovered that most went straight through, but a few bounced back. This showed that atoms have a small, dense, positively charged centre called the nucleus, while the rest is mostly empty space. Picture a football stadium: the nucleus is like a small marble at the centre, and the empty stadium space represents where the electrons move.
Bohr’s Model
Niels Bohr improved on Rutherford’s idea. He said electrons move in fixed paths or energy levels around the nucleus, just like cars moving on express lanes without crashing into each other. Each orbit has a certain amount of energy. This model explained why atoms give off light in specific colours when heated.
Quantum Mechanical Model
Finally, modern scientists built on Bohr’s idea. The quantum mechanical model says electrons don’t just move in fixed paths but in regions called orbitals, where they are most likely to be found. Imagine bees buzzing around a hive—you can’t tell their exact path, but you know the region they hover around. This is the most accepted model today.
Summary
- Dalton: atoms are indivisible and identical for each element.
- Thomson: discovered electrons, “plum pudding model.”
- Rutherford: nucleus at the centre, mostly empty space.
- Bohr: electrons move in fixed energy levels.
- Quantum model: electrons found in orbitals, not fixed paths.
Evaluation
- What did Dalton believe about atoms?
- How did Rutherford’s experiment prove the existence of the nucleus?
- Explain the difference between Bohr’s model and the quantum mechanical model.
Always remember, you are capable of understanding great scientific ideas. Each scientist built on the work of others, just like you will keep building on what you learn today. Stay curious, stay confident, and keep shining with Afrilearn.