In the preview post we have seen how scientists correctly came up with the concepts of atoms by considering masses of elements in compounds. Now we shall take a step further and see what's an atom.
Discovery of electrons
J. J. Thomson was an important pioneer for developing our understanding on atoms. In 1897, J. J. Thomson discovered that when metals were applied with a high electrical voltage, they would emit some small particles which had some negative charge. Later he found the small particles emitted were the electrons. So he concluded that electrons must exist in atoms.
That discovery led to a question, why atoms are neutral in charge while they have negatively charged electrons inside?
Plum pudding model
To answer that question, J. J. Thomson proposed a model which described atoms as a "plum pudding" where negatively charged electrons dispersed in a cloud of positive charge. However this has been proven wrong by Ernest Rutherford in 1909.
Rutherford's model
The experiment of Rutherford was revolutionary for the time being. He used a beam of high speed alpha particles (the nucleus of \( \ce{^4_2He} \)) to hit a thin foil of gold. To his surprise, while most of the alpha particles passed through the foil with small deflections, some of them were deflected significantly and some were even reverted back. He described that as " fired a 15-inch shell at a piece of tissue paper and it came back and hit you".
Deflection of alpha particles passing through a thin foil of gold (Credit: Wikipedia)
That should never happen according to J. J. Thomson's plum pudding model. To explain his findings, Rutherford suggested that instead of having the positive charge dispersed like a pudding, there was a region is space where all the positive charges concentrated in. He further explained that this region with a high positive charge (which is the nucleus) would strongly repel the alpha particles such that if the alpha particles approached the nucleus at a certain angle, they would be deflected or even reverted.
In this model, virtually all the mass and positive charges of the atom are in the very small nucleus, while negatively charged electrons would orbit around the nucleus in the vast empty space in the atom.
Discovery of protons and neutrons
In 1919 - 1920, Rutherford further developed his atomic model after finding protons and neutrons. By using alpha particles to hit atoms, he found some streams of positively charged particles produced. So he argued that these positively charged particles must be part of the nucleus (which we now know are the protons).
However, to answer the question "why positively charged protons can exist in such a small region without repelling and falling apart", Rutherford propsed a second type of particles in the nucleus which he called neutrons to compensate the repulsion force, by forming some attractive nuclear force. The existence of neutrons were later proven by James Chadwick who was an associate of Rutherford.
The atomic model
Until now, we have reached our current atomic model where we have protons and neutrons in the nucleus while the electrons are moving outside the nucleus.
The nucleus carries virtually all the mass of the atom and all the positive charges. Yet still, nucleus only occupies a very small region in space at the center of the atom. The majority of the space in an atom is empty where electrons are constantly moving at a high speed.
Of course, as technology advances and theoretical work develops, we now have more insights of the atoms. Nevertheless, as a chemist, this simple atomic model involving electrons, protons, and neutrons is sufficient to guide our work.
How about electrons?
The only missing piece in the jigsaw now is how electrons are allocated in the atom. We'll discuss this in the later posts.