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440
Democritus proposed the idea of an atom
In 440 BC, an ancient Greek philosopher named Democritus proposed the idea of an atom. An atom was an indivisible particle that all matter was made up of.
He said: “Nothing exists except atoms and empty space. Everything else is opinion.”
His theory of the atom was however, incorrect because we now know that an atom is not indivisible, how was very close to the truth though but being a philosopher and not a scientist, he didn’t make an attempt to see whether his theory was right or not. -
"The Sceptical Chymist"
Robert Boyle, a natural philosopher, chemist, physicist, and inventor published "The Sceptical Chymist" which showed the differences between the fields of chemistry and alchemy. The work he did also features some of the earliest recorded ideas of atoms, molecules, and chemical reactions. -
First list of Elements
A French nobleman, Antoine Lavoisier, made the first extensive list of elements. The list featured 33 different elements and differentiated between metals and non-metals -
Dalton's law
In 1803, John Dalton proposed ‘Daltons Law’. This law describes the relationships between the different components in a mixture of gases. This discovery helped the future scientist isolate a mixture of gases for their individual study. -
Table of Atomic Weights
In 1828, a Swedish chemist named Jakob Berzelius developed a table of atomic weights and introduced the letters to symbolize elements. This was the first table the represented different elements with letters. -
Valence introduced to the periodic table
In 1864, a German chemist named Lothar Meyer demonstrated the connection between atomic mass and elemental properties an early version of the periodic table with 28 elements organised by valence, which is the order of the quality that will determine the number of atoms with any single atom or group will unite chemically.
This however, did not receive as much credit as Dmitri Mendeleev because he didn’t publish this organisation scheme first. -
Dmitrti Mendeleev
Russian chemist, Dmitri Mendeleev in 1864 demonstrated the connection between atomic mass and elemental properties. He, along with Newlands, noticed that when elements were put in order of atomic mass, there was a periodic repetition of the properties of the elements.
Mendeleev arranged the elements in order of increasing atomic mass, into columns with similar properties. This was the first periodic table and it featured 66 known elements.
This table was widely accepted because it predicted th -
First 14 elements organised
In 1864, an English chemist named John Newlands arranged the known elements in order of atomic weight and observed the similarities between some of the elements already discovered.
He organised the first 14 known elements into his own table, his table was organised with elements with similar properties were in the same row.
He also recognised that the elements of every 8th element shared similarities. He called this ‘The law of Octaves’. -
Problem Solved
An English Chemist in 1913 named Henry Mosley figured out the problem with Mendeleev’s table. He arranged the elements in order of increasing atomic number and not increasing atomic mass. These arrangements fixed the problem with Mendeleev’s table and resulted in clear periodic patterns of properties. This finally led to the final periodic law that there is a periodic repetition of chemical and physical properties of the chemical and physical properties of the elements when they’re arranged in i -
The neutron is discovered !
In 1932, Sir James Chadwick discovered the neutron, which identifies isotopes. This demonstrated why there was a discrepancy between atomic number and atomic weight. This helped in the placement of the elements atomic mass and number and contributed to the placement of elements. -
Lanthanides and actinides
Glenn Seaborg identified that lanthanides and actinides (atomic number greater than 92) and placed them below the periodic table.
