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370 BCE
Democritus
The Greek Philosopher Democritus (460 BC - 370 BC) was among the first to suggest the existence of atoms from the greek word «atomos» which means «uncuttable». He reasoned that you could take a piece of matter, cut in half, cut the half piece in half again, and continue to cut again and again. Eventually, you wouldn’t be able to cut any more. You would have only one particle left. -
Antoine Lavoisier
Lived 1743 – 1794.
Antoine Lavoisier revolutionized chemistry. He named the elements carbon, hydrogen and oxygen; discovered oxygen’s role in combustion and respiration; established that water is a compound of hydrogen and oxygen; discovered that sulfur is an element, and helped continue the transformation of chemistry from a qualitative science into a quantitative one. -
Antoine Lavoisier
In 1778 Lavoisier found that when mercury oxide is heated its weight decreases. The oxygen gas it releases has exactly the same weight as the weight lost by the mercury oxide.
Lavoisier announced a new fundamental law of nature: the law of conservation of mass:
The total mass of a chemical reaction’s products is identical to the total mass of the starting materials -
John Dalton
Combined the idea of elements with the earlier theory of the atom.
He proposed the following ideas about matter: Matter is made of atoms.
Atoms cannot be divided into smaller pieces.
All the atoms of an element are exactly alike.
Different elements are made of different kinds of atoms. Dalton pictured an atom as a hard sphere that was the same throughout, something like a tiny marble. The figures show examples of atoms of different elements. -
John Dalton
The Behavior of Gases.
Dalton stated correctly that he had no doubt that all gases could be liquefied provided their temperature was sufficiently low and pressure sufficiently high. In 1803, Dalton published his Law of Partial Pressures, which states that in a mixture of non-reacting gases, the total gas pressure is equal to the sum of the partial pressures of the individual gases. https://www.biografiasyvidas.com/biografia/d/dalton.htm -
William Crooks
In 1870, the English scientist William Crooks did experiments with a glass tube that had almost all the air removed from it. The shadow showed Crookes that something was traveling in a straight line from the cathode to the anode, similar to the beam of flashlight. Crookes hypothesized that the green glow in the tube was caused by rays, or stream of particles. These rays were called cathode rays because they were produced at the cathode. -
Joseph John Thomson
Was the greenish glow light, or was it a stream of charged particles?
In 1897, J.J. Thomson, an English Physicist, tried to clear up the confusion. His research in cathode rays led to the discovery of the electron, and he pursued further innovations in atomic structure exploration. Thomson won the 1906 Nobel Prize in Physics, among many accolades. He died on August 30, 1940. https://www.biografiasyvidas.com/biografia/t/thomson.htm -
Thomson's Model
He pictured a sphere of positive charge. The negatively charged electrons were spread evenly among the positive charge. -
Ernest Rutherford
Since atoms are generally neutral, that meant that the atom itself had to somehow have a positive charge to balance the negative charge out. But where in the atom were the positive charges needed to offset those negative electrons? And a related question, since people knew by this point that electrons were so much less massive than the atoms themselves, where was the mass distributed? -
Ernest Rutherford
They aimed a beam of alpha particles at an ultrathin sheet of gold foil. Most of the time, these alpha particles would sail right through. But every now and then, some of these projectiles would actually bounce practically right back in their faces. If the atom’s positive charge and most of its mass were concentrated in a tiny central core, it would let most particles sail through but repel any positive charge that came near the center. -
Henry Gwyn Jeffreys Mosely
The conclusion was inescapable: The X-ray spectra of the elements didn’t depend on their atomic weights but on something even simpler.
The result of these measurements was absolutely extraordinary.
Moseley had discovered that the nucleus was not one big positive blob … but a collection of positively charged particles that increased in number with each heavier element. -
Henry Gwyn Jeffreys Moseley
Moseley set out to learn if each element had a unique X-ray spectrum – a bar code like the ones that had been discovered a half century earlier using light. When a beam of electrons struck the sample, the element gave off X-rays. Moseley could then determine the element’s X-ray spectrum. Moseley found – just as he had hoped – that each element had a unique X-ray spectrum. There is here a whole new branch of spectroscopy.
