The atom is the basic unit of matter that makes up everything we see. But how did scientists discover the structure and properties of atoms? In this article, we will review some of the key experiments and models that shaped our understanding of the atom.
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One of the first scientists to propose that matter was made of tiny particles was John Dalton, an English chemist and teacher. In 1803, he published his ideas about atoms, which he imagined as tiny spheres that could not be divided. He based his theory on four postulates:
- Elements are made of tiny particles called atoms.
- All atoms of a given element are identical.
- Atoms of different elements can combine in simple whole-number ratios to form compounds.
- Atoms are indivisible in chemical reactions. They are neither created nor destroyed.
Dalton's atomic theory explained many observations about chemical reactions, such as the law of conservation of mass and the law of definite proportions. However, it could not account for some phenomena, such as electricity and magnetism.
In 1897, J.J. Thomson, a British physicist, discovered the electron, a negatively charged subatomic particle. He used a device called a cathode ray tube, which produced a beam of electrons that could be deflected by electric and magnetic fields. Thomson measured the ratio of the charge to the mass of the electron and found that it was much smaller than that of any known atom. This meant that electrons were much lighter than atoms and that atoms must contain other particles.
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Thomson proposed a new model for the atom, called the plum pudding model. In this model, the atom is a ball of positive charge with negative electrons embedded in it - like currants in a Christmas pudding. He thought that the positive and negative charges balanced each other out, making the atom electrically neutral.
In 1909, Ernest Rutherford, a New Zealand-born physicist, designed an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. According to the plum pudding model, most alpha particles should pass through the foil with little or no deflection. However, Rutherford observed that some alpha particles were deflected at large angles and some even bounced back. This was very surprising and led him to conclude that:
- The mass of an atom is concentrated at its center, called the nucleus.
- The nucleus is positively charged and contains most of the atom's protons.
- The electrons are located outside the nucleus in a large empty space and orbit around it.
Rutherford's nuclear model explained why most alpha particles passed through the foil (because they encountered mostly empty space) and why some were deflected or bounced back (because they hit the dense nucleus). However, it could not explain why electrons did not fall into the nucleus due to electrostatic attraction or how they emitted light when heated. 8cf37b1e13