Question

history of the atom and periodic table

Answer 1

The word ‘atom’ actually comes from Ancient Greek and roughly translates as ‘indivisible’. The Ancient Greek imagined atoms as varying in shape depending on the type of atom. They envisaged water atoms were smooth and slippery, explaining why water was a liquid at room temperature and could be poured. Though we now know that this is not the case, their ideas laid the foundations for future atomic models.

It wasn’t until 1803 that the English chemist John Dalton started to develop a more scientific definition of the atom. He drew on the ideas of the Ancient Greeks in describing atoms as small, hard spheres that are indivisible, and that atoms of a given element are identical to each other.

The first breakthrough came in the late 1800s when English physicist Joseph John (JJ) Thomson discovered that the atom wasn’t as indivisible as previously claimed. He carried out experiments using cathode rays produced in a discharge tube, and found that the rays were attracted by positively charged metal plates but repelled by negatively charged ones. From this he deduced the rays must be negatively charged.

By measuring the charge on the particles in the rays, he was able to deduce that they were two thousand times lighter than hydrogen, and by changing the metal the cathode was made from he could tell that these particles were present in many types of atoms. He discovered the electron.

In 1904, he put forward his model of the atom based on his findings. Dubbed ‘The Plum Pudding Model’ , it envisaged the atom as a sphere of positive charge, with electrons dotted throughout like plums in a pudding.

Rutherford devised an experiment to probe atomic structure which involved firing positively charged alpha particles on a thin gold sheet.

During the experiment, most of the alpha particles did pass through the foil with little or no deflection. However, a very small number of the particles were deflected from their original paths at very large angles. This was completely unexpected; the only possible explanation was that the positive charge was not spread throughout the atom, but concentrated in a small, dense centre: the nucleus. Most of the rest of the atom was simply empty space.

Rutherford’s discovery of the nucleus meant the atomic model needed a rethink. He proposed a model where the electrons orbit the positively charged nucleus. While this was an improvement on Thomson’s model, it didn’t explain what kept the electrons orbiting instead of simply spiralling into the nucleus.

Bohr was a Danish physicist who set about trying to solve the problems with Rutherford’s model. He realised that classical physics could not properly explain what was going on at the atomic level; instead, he invoked quantum theory to try and explain the arrangement of electrons. His model postulated the existence of energy levels or shells of electrons. Electrons could only be found in these specific energy levels; in other words, their energy was quantised, and couldn’t take just any value. Electrons could move between these energy levels (referred to by Bohr as ‘stationary states’), but had to do so by either absorbing or emitting energy.

Bohr’s suggestion of stable energy levels addressed the problem of electrons spiralling into the nucleus to an extent, but not entirely. The exact reasons are little more complex than we’re going to discuss here.

Bohr’s model didn’t solve all the atomic model problems. It worked well for hydrogen atoms, but couldn’t explain observations of heavier elements. It also violates the Heisenberg Uncertainty Principle, one of the cornerstones of quantum mechanics, which states we can’t know both the exact position and momentum of an electron.

In 1926 Schrödinger proposed that, rather than the electrons moving in fixed orbits or shells, the electrons behave as waves.

Schrödinger solved a series of mathematical equations to to come up with a model for the distributions of electrons in an atom. His model shows the nucleus surrounding by clouds of electron density. These clouds are clouds of probability. These are egions of space are referred to as electron clouds.

In 1932, the English physicist James Chadwick (a student of Ernest Rutherford) discovered the existence of the neutron, completing our picture of the subatomic particles that make up an atom.

History of Periodic Table

Initially elements arranged according to metallic and non-metallic nature.

Dobereiner arranged elements in increasing order of their atomic weights. Elements were arranged in groups of three called-Triads and the weight of middle element was the average of the two.

Newland arranged elements in accordance to their atomic weights and in series of eight. Properties of every eighth element was found to be a repeition of the first.

Mendeleeff arranged elements in order of their atomic weights and said that properties of elements were periodic functions of their atomic weights.

Finally, Moseley constructed the Modern Periodic Table and arranged elements according to their atomic number. The properties of elements were periodic properties of their atomic number. And this removes all the defects that was found in that of Mendeleeff's Periodic Table.