History of the Atom

Scientists have studied atoms for many years. As more experimental evidence has been collected, our understanding of atomic structure has improved.

Atoms are the building blocks of matter and come in many different types, known as elements, each with unique properties. Let’s look at what scientists have learned about atoms over time.

Dalton’s model 1803

John Dalton was a scientist who lived in the early 19th century and he made a big contribution to our understanding of atoms. In 1803, before the discovery of the electron, Dalton introduced his atomic theory. This had a big impact on the world of science.

According to Dalton:

  • Atoms are the small building blocks of all substances and they cannot be created, divided, or destroyed.
  • Each element has its own unique type of atom. Atoms of the same element are exactly alike, while atoms of different elements are different.
  • When atoms of different elements come together, they form new substances.

So, in Dalton’s model, atoms were seen as tiny solid balls that couldn’t be broken down into smaller parts.

A plain circle depicting Daltons "Solid sphere model" of the atom in 1803.

This was a groundbreaking idea at the time, and it helped to lay the foundation for future studies on atomic structure. However, as scientific experimentation advanced, some parts of his theory were found to be incorrect.

Thomson’s Plum Pudding Model (1897)

In 1897, physicist J.J Thomson challenged John Dalton’s atomic theory by conducting an experiment with a cathode ray tube. This experiment revealed that atoms could indeed be divided into smaller parts. As a result, Thomson discovered the electron as a negatively charged subatomic particle.

He proposed the “plum pudding” model, where atoms were made up of negatively charged electrons inside a positive charge.

As the electrons have a negative charge, Thomson assumed that the rest of the atom had a positive charge. So, this arrangement resulted in a neutral overall charge for the atom.

A big yellow circle with many smaller blue circles with a negative symbol inside representing electrons; depicting Thomsons "Plum pudding model" in 1897.

This was a huge step forward in our understanding of atomic structure, as we gained an understanding of the role of electrons within atoms.

Rutherford’s Nuclear Model (1911)

The scientist who made the next major change to the atomic model was Ernest Rutherford. In 1909, Rutherford tested the plum pudding model. He directed a beam of positively charged alpha particles at thin pieces of gold foil.

In the plum pudding model, the positive charge of the nucleus is spread out. So, Rutherford expected the charged particles to pass through the sheet of foil. However, the results were different to what he expected:

  • Most of the particles continued in a straight line.
  • Some of the particles deflected a bit to the side.
  • A few of the particles bounced straight back.

During this experiment, he made a number of observations and came to new conclusions:

A block representing a radioactive source emitting (alpha) particles at a thin gold foil. Some particles are deflected back and some go through while some are slightly deflected but still go through. A silver circle around the thin gold foil represents the fluorescent screen.

These conclusions contradicted the plum pudding model. So, Rutherford developed a new model, which is known as the atomic model.

In Rutherford’s atomic model:

  • The atom is mostly empty space.
  • Most of the atom’s mass is in the dense nucleus, at the centre.
  • Negatively charged electrons travel in paths, orbiting around the positively charged nucleus.
A diagram of Rutherfords "Atomic model" in 1911.

Rutherford showed that atoms were not just tiny solid balls, but instead had a complex and dynamic structure.

Bohr’s Model (1913)

In 1913, Neils Bohr further developed Rutherford’s nuclear model. In Bohr’s model, electrons orbit the positively charged nucleus in shells, which are at certain distances from the nucleus. The series of energy shells are at increasing distances from the nucleus.

A diagram of Niels Bohr's "planetary model" in 1913 with the electron and nucleus labelled.

Bohr discovered that the closer an electron is to the nucleus, the stronger the attractive force it experiences from the positively charged nucleus. This means it’s harder to remove the electron from the atom, as it requires more energy to overcome the attractive force.

On the other hand, electrons in higher energy levels, farther away from the nucleus, experience a weaker attractive force and are easier to remove.

Discovering the Neutron

After more experiments, Rutherford confirmed that the positive charge in the nucleus is from small particles, which we now know as protons. And not long after, in 1932, James Chadwick provided evidence for particles in the nucleus with a neutral charge. We now know these particles as neutrons.

The discovery of protons, neutrons, and the energy levels of electrons allowed scientists to better understand the properties of atoms and the chemical reactions they undergo.

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