Atomic Theory and the Discovery of the Atom

Atomic Theory

The Atomic Theory was first proposed by John Dalton (1766-1844). The atomic theory states that all matter is composed of discrete units. Basically, everything is made up of distinct small units, which we now know are called atoms. Dalton used several laws as the basis for atomic theory, including: Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions.

The Conservation of Mass is an important concept that appears in many branches of chemistry and physics. Antoine Lavoisier (1743-1794) stated that the law of conservation of mass means that atoms cannot be created or destroyed, but they can be moved around or changed into different particles. To put it simply, mass is conserved.

The Law of Definite Proportions, also called Proust’s Law, was formulated by Joseph Proust (1754-1826). It states that if a compound is broken down into its constituent elements, the masses of the constituents will always be in the same proportion. For example, water has two hydrogen atoms and one oxygen atom. If we had 50 water molecules, we would have 100 hydrogen atoms and 50 oxygen atoms. The proportion or ratio of the masses stays the same.

Of course, we know that oxygen and hydrogen can react in different ways to produce different compounds. In this case, the Law of Multiple Proportions applies. It states that when one element combines with another element of fixed mass to create a compound, the masses of the elements are integer multiples. Take water and hydrogen peroxide for example. Water has two hydrogens and one oxygen, whereas hydrogen peroxide has two hydrogens and two oxygens. The elements combine in different integer ratios to form two different compounds with completely different chemical properties.

This is great, we know that all matter can be broken down into smaller units called atoms. But can we break an atom down into anything smaller?

Short answer, yes! Atoms are made up of three different subatomic particles.

Discovery of Atomic Structure

The first subatomic particle to be discovered was the electron. In the 19th century, it was known that applying a high voltage current through a sealed tube produced a light that was called a cathode ray. J.J. Thomson (1856-1940) demonstrated that this light could be bent by magnetic and electric fields. He determined that the particles in the cathode ray had a negative charge. These particles are called electrons. At this point, the atom was thought to look like this:

This is known as Thomson’s Plum Pudding model.

In later experiments, Ernest Rutherford (1871-1937) showed that there were two types of particles emitted from radioactive material. One of them was called beta particles and deflected a lot when passed by a positively charged plate. In other words, they were negatively charged and had a low mass. the second type of particle, called alpha particles, only deflected a little when passed by a negative plate. These particles were positive and had a larger mass than beta particles. If you guessed that the negatively charged particles were electrons, congratulations you were correct! This means that the positively charged particles were protons.

Rutherford conducted another experiment where he aimed alpha particles through a very thin piece of gold foil and examined the distribution of the particles on the other side. If Thomson’s model was correct, the alpha particles should pass through the gold foil. They did not. Instead, some deflected at large angles, some went straight through, and, amazingly, some came directly back in the direction of the source. This proved that there was a large dense positive charge in the centre of the atoms, negative charges dotted around the outside and in between was just empty space. The atom now looks like this:

Rutherford could not explain why this model did not fall apart due to the large amount of positive charge in the centre, called the nucleus, and so he concluded that there must be very strong forces holding the nucleus together. He also suggested that the atom contained neutral particles with approximately the same mass as the proton, however, the neutron was not discovered until 1932 by James Chadwick (1891-1974).

These concepts and ideas form the basis of all chemistry. This article covers what you need to understand for basic biochemistry, but the topic and history is fascinating and well worth looking into in more detail if you are interested. These experiments were ground-breaking as they gave so much insight into the world around us and wonderfully elegant.

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