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.
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:
So, in Dalton’s model, atoms were seen as tiny solid balls that couldn’t be broken down into smaller parts.
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.
In 1897, physicist J.J Thompson 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, Thompson 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, Thompson assumed that the rest of the atom had a positive charge. So, this arrangement resulted in a neutral overall charge for the atom.
This was a huge step forward in our understanding of atomic structure, as we gained an understanding of the role of electrons within atoms.
The scientist that 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:
During this experiment, he made a number of observations and came to new conclusions:
These conclusions contradicted the plum pudding model. So, Rutherford developed a new model, which is the atomic model.
In Rutherford’s atomic model:
Rutherford showed that atoms were not just tiny solid balls, but instead had a complex and dynamic structure.
In 1913, Neil 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.
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.
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.