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450 BCE
Empedocles
A Greek scholar named Empedocles proposed that matter was composed of four elements: Earth, air, fire, and water. He demonstrated that, even though air is invisible, it is not just "nothing. " because it takes up space, it must be a form of matter. -
400 BCE
Democritus
Democritus suggested that matter is made of very small particles called atoms, that cannot be divided down further. He called the particles atoms, after the Greek word atomos, which means "indivisible.” -
350 BCE
Aristotle
The philosopher Aristotle believed in particles "four-element" model despite the more recent "Atomic" model. Aristotle's influence was so great, and his writings read by so many people, that the” four-element” model was accepted for almost 2000 years. -
500
Chemical Symbols and Labratory Tools
Many alchemists believed that metals grew like plants, ripening into gold. For centuries they performed numerous experiments trying to turn cheap metals like iron & lead into gold. They created chemical symbols for substances we now know as elements & compounds. They also invented many lab tools that we still use today: beakers, filters, stirring rods and distillation apparatus. Despite finding many new substances, they still accepted the four-element model and no one ever turned lead into gold. -
1250
Discovery of Arsenic -Albert Magnus
Albert Magnus German Catholic and bishop. He discovered the element Arsenic. He discovered it by heating soap together with orpiment. He also believed that all-natural things were compositions of matter and form. -
Creation of the Modern Definition of an Element- Robert Boyle
Robert Boyle was an English scientist, he did not believe in the four-element model. He came up with a new definition for the word element: "I mean by element, simple unmitigated bodies." This became the modern definition of an element: a pure substance that cannot be chemically broken down into simpler substances. He also believed air was a mixture and not an element. -
The First Person to Isolate Oxygen Scientifically- Joseph Priestly
Joseph Priestly prepared oxygen by heating mercury oxide with a burning glass. He found that oxygen did not dissolve in water and it made combustion stronger but he did not know that oxygen was an element. Antoine Lavoisier soon recognized by experimenting with Priestley's oxygen, he concluded that air must be a mixture of at least two gases one of which was oxygen. -
Discovery of Hydrogen- Henry Cavendish
After Priestley's discovery, Henry Cavendish experimented by mixing a metal with acid which resulted in a flammable gas that was lighter than air. He had no clue that the gas he had just prepared was hydrogen, he called the gas phlogiston. He discovered that when he burned this gas Priestley's oxygen they formed water. Until that time, everyone believed that water was an element (could not be made from other simple compounds). -
Dalton's Atomic Model
By this time, it was accepted that matter was made of elements. John Dalton an English chemist published a theory of why elements differ from each other and from non elements. Dalton's atomic model for matter concluded that:
-All matter is made from atoms, which are particles too small to see
-Each element has its own kind of atom, with its own mass
-Compounds are created when atoms of different elements link to form molecules
-Atoms cannot be created, destroyed or subdivided in chemical changes -
Modified Verson of Dalton's Model - Michael Faraday
Dalton's atomic model, could not explain how you could get a spark when you touch a metal doorknob. Michael Faraday found that electric current could cause chemical changes in some compounds in solutions. The atoms could gain electric charges and form charged atoms, called ions. A modified version of Dalton's model:
-Matter must contain positive and negative charges
-Opposite charges attract and like charges repel
-Atoms combine to form molecules because of electrical attractions between atoms -
The Periodic Classification of the Elements - Dmitri Mendeleev
Dmitri Mendeleev was a Russian chemist. He created the Periodic Law and created the periodic table of elements. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. He used it to correct the properties of some already discovered elements and he also predicted eight elements that were yet to be discovered. -
Noble Gases Discovery - William Ramsay
William Ramsay was a British chemist who discovered the noble gases. He discovered the inert gaseous elements in air. Ramsay worked on argon, helium, neon, krypton and xenon because of the discovery's it led the development of a new section in the periodic table. -
Discovery of Electrons and Protons - J.J. Thomson
In 1897 Dalton's atomic theory was shattered by a chemist named J.J. Thomson. He discovered very light negative particles, called electrons. He also did experiments with beams of much heavier positive particles (later identified as protons). The new model became known as the "raisin bun" model. -
Raisin Bun Model
The new raisin bun model:
- Atoms contain particles called electrons
- Electrons have a small mass and negative charge
- The rest of the atom is a sphere of positive charge
- The electrons are embedded in this sphere so that the resulting atoms are neutral or uncharged
H. Nagaoka a Japanese scientist working at the same time modelled the atom as a large sphere surrounded by a ring of negative electrons. -
Gold Foil Experiment - Ernest Rutherford
Rutherford was amazed and described the result as being similar to firing bullets at a piece of tissue paper and having one of them bounce back. Rutherford had to come up with another new model – the nuclear model:
- An atom has a tiny, dense, positive core called the nucleus (which deflected the alpha particles and contains protons)
- The nucleus is surrounded mostly by empty space, containing rapidly moving negative electrons (through which the alpha particles passed unrestrained) -
Gold Foil Experiment
Ernest Rutherford created an experiment to test Thomson's and Nagaoka's models. He aimed a type of radiation called alpha particles (positively charged particles smaller than most atoms) at a thin sheet of gold foil. he predicted, based on Thomson's raisin-bun model, that the particles would pass straight through the gold foid, as most of them did. However, a very small number of the alpha particles bounced almost straight back from the gold foil. -
Ernest Rutherford's Atomic Theory
Using the information that he had gotten from the gold foil experiment, along with the information that had been presented from Dalton’s and Thomson’s models, Rutherford came up with this image known as the planetary model of the atom. -
Neils Bhor's Experiment
Neils Bhor added to Rutherford's experiment that not only do electrons orbit the nucleus, but they orbit on specific levels or quanta. -
Neils Bhor's Experiment
One of Rutherford students, Neil Bohr studied the emission spectrum of atoms. He found that when a hydrogen atom had its electrons jump up into an excited state (by adding electricity) they release certain lines on an emission spectrum. The important thing is that they only released specific amounts of light. He found that when the electrons jump, they jump up to only specific levels and release a specific amount of energy (which appears in the spectrum) called quanta (little packets a light). -
Movement of Electrons - Werner Heisenberg
Werner Heisenberg was a German who took an interest in theoretical physicists. Heisenberg developed new theories in quantum mechanics about the behaviour of electrons. The theories were proven by previous experiments. He helped explain the movement of electrons. -
Modern Cloud Theory - Erwin Schrödinger
Schrodinger, Heisenberg, Einstein and many other scientists invented the cloud theory. According to today’s atomic theory, electrons do not orbit the nucleus in neat planet-like orbits but move at high speeds in an electron cloud around the nucleus. In the electron cloud, electrons whirl around the nucleus billions of times in one second. They are not moving around in random patterns; an electron’s location depends upon how much energy the electron has. -
Discovery of the Neutron - James Chadwick
James Chadwick was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. Chadwick researched on radioactivity. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. -
Modern Cloud Model
The electron cloud model is currently the accepted model of an atom.
