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500 BCE
The Alchemists
The Alchemists believed that all metals are made of mercury and sulfur and that they can be changed into gold. They were the first to use the word element. -
428 BCE
Plato
Plato believed that that atoms of matter were made of 5 different solids. People before him thought that matter was made of 4 different solids: earth, water, air and fire. His fifth atomic type was later called ether. -
300 BCE
Aristotle
Aristotle did not believe that all materials on Earth were made of atoms. He believed that all materials were made of the four elements: Earth, Fire, Water and Air. Everything could be made of these four things -
490
Democritus
Democritus thought that atoms were just one sphere and that everything is made of atoms. In between atoms there is empty space and atoms are indestructible. -
Robert Boyle
Robert Boyle believed that everything is made up of atoms. He discovered Boyle's Law. It says that if the volume of a gas is decreased then the pressure increases. -
Lavoisier
He discovered the Law of Conservation of Mass. This tells us that matter can not be created or destroyed. -
John Dalton
John Dalton was the first person to try to describe matter as having atoms. He said that all matter is made of atoms and that atoms can be combined or rearranged. He pictured atoms to look like solid spheres, kind of like billiard balls. His theory was called the solid sphere or billiard ball model. -
Dmitri Mendeleev
Mendeleev wrote down all of the elements in order of weight. He created the first periodic table of elements. -
J.J. Thomson
J.J. Thomson experimented with cathode ray tubes and learned that atoms have electrons. His model of the atom was called the Plum Pudding Model. His mode showed negatively-charged electrons inside of a positively-charged soup. -
Pierre and Marie Curie
Pierre and Marie Curie discovered polonium and radium which are radioactive elements. Marie came up with the term radioactivity. -
Max Planck
Planck’s Quantum Theory of Light says that atoms can emit or absorb energy in discrete quantities only. -
Albert Einstein
Albert Einstein mathematically proved that atoms existed. He also proved the photoelectric effect. The photoelectric effect happens when electrically charged particles are released from a material after is absorbs electromagnetic radiation. -
Ernest Rutherford
Ernest Rutherford is known for the gold foil experiment. This experiment taught him that the atom is mostly empty space. Inside there is a positively-charged nucleus. Electrons surround the nucleus. -
Henry G. J. Mosely
Henry Mosley found that the atomic number of elements is the number of positive charges in the nucleus. -
Niels Bohr
Niels Bohr discovered that electrons orbit around the nucleus. The electrons can move to different energy levels. Radiations is given off if the electrons move to different levels. Bohr also created the solar system model of the atom. It is sometimes called the Rutherford-Bohr model because it was a change to Rutherford's model. The model is like a solar system because electrons revolve around the nucleus like planets revolve around the sun. -
Robert Millikan
Robert Millikan used the oil drop experiment and this measured the charge of an electron. He discovered that electrons are negatively charged. -
Werner Heisenberg
Werner Heisenberg discovered the uncertainty principal. This tells us that the position and speed of an atom cannot be measured exactly. He also created the electron cloud model with Erwin Schrodinger. This model says that we cannot know exactly where electrons are at any given time. -
Erwin Schrodinger
Erwin Schrodinger discovered the Schrodinger Equations. He saw electrons as particles and as waves. His equation could calculate the energy levels in electrons. He also created the electron cloud model with Werner Heisenberg. This model says that we cannot know exactly where electrons are at any given time. -
James Chadwick
James Chadwick discovered neutrons. Neutrons are a particle that do not have any electric charge. He also discovered that an atom's nucleus is made of neutrons and protons.
