-
500 BCE
AlChemists
AlChemists developed the theory that all metals are made of mercury and sulfur, and that the base of these metals could be changed to gold. Even though his theory is wrong, he opened a new field that people could think about. -
442 BCE
Democritus
Democritus, a Greek philosopher said that all matter made of small and indivisible elements. -
430 BCE
Empedocles
Empedocles believed that everything in the universe, including the living organism, was made of four elements, earth, air, water, and fire. He also described that matter was held together by a force. -
Antoine Lavoisier
Lavoisier stated the definition of elements. He stated that elements are indivisible particles. He also found out that compounds made of simple elements. lastly, he predicted some elements that might exist in nature. Which later they found that the elements that he predicted are actually exist. -
John Dalton
Around 2000 years after Democritus stated his belives of atoms and matter, Dalton took the idea of atoms and substances from Democritus and pushed it forward to state his atomic theory. Dalton atomic theory state:
1) Matter made of indivisible atoms ( Both Democritus and Dalton believed)
2) All atoms of an element are identical.
3) Compounds are made of different combinations of atoms (Democritus had this idea but in a mechanical sense)
4) Different elements have different atoms -
Marie Sklodowska Curie
Curie studied the radioactivity in other elements than uranium. in 1896 Henri Becquerel discovered the radiation that comes from the element uranium. in 1898 she found that other elements produce the same energy that uranium produces. Therefore, she concluded that radioactivity does not depend on how atoms are arranged into molecules, but rather that it originates within the atoms themselves. -
Max Plank
Before 1900, Physicities had a problem. They could not understand the results that they got from heating surfaces that absorb all frequencies of light, otherwise known as black bodies. In 1900, Plank came up with an equation that explains this problem. E=Nhf, which 'E' is energy, 'N' is an integer, 'h' Plank's constant, 'f' is the frequency. In determining this equation, Planck came up with the constant 'h', which is now known as "Planck's constant." -
J.J. Thomson
Thomson began experimenting with the cathode ray tube and he discovered the electrons. He proposed a model of the atom consisting of an equal amount of electrons, the negatively charged particles, and protons, the positively charged particles. He also stated the shape of the atom which is a sphere and both electrons and both electrons and protons were embedded within it. To show his idea he came up with a model called "Pulm Pudding" and it represents the charged particles in the atom. -
Hantaro Nagaoka
Nagaoka disagreed with Thomson's model. Which was that the electrons and the protons were held together. Nagaoka proposed a new model he called it "The Saturnian model". His model shows the atom with a center-positive charged particles (Protons) and surrounded by rings of electrons. -
Albert Einstein
Before Einstein, the idea of atoms was only a hypothesis. No one could prove that they actually exist. In 1095, Einstein created a mathematical method to calculate the size of atoms and therefore proved that they exist. -
Ernest Rutherford
Thomson's model showed that the atom consists of a massive blob of positive charge dotted with negative charges. Rutherford came to test this model. In his experiment, he shot alpha particles to a gold foil. If Thomson's model was right, the alpha particles would go through the gold foil with no deflection. However, some of the alpha particles deflected. He argued that Thomson's is incorrect and the atom is mostly empty space and the electrons revolve around a positively charged center. -
Niels Bohr
Rutherford couldn't explain how do the electrons stay in a circular orbit without collapsing with the positive nucleus. Bohr suggested that electrons can be found in 'shells' around the nucleus. Each shell is associated with quantized energy and can only contain a specific amount of electrons. It takes less energy to stay in the lower shells and more energy to move up a shell. However, no matter how little the energy that the electron has, it can't move less than the first shell. -
James Chadwick
In 1911 Rutherford discovered the protons in the nucleus, but other researchers found that there could be other practicals with the protons. The problem they had that they found the atomic number (Number of protons) of an element is always less than the atomic mass (average mass of the atom). Since electrons almost have no mass, there must be other neutral particles with the protons. In 1932 Chadwick's experiments on tracking particles radiation found the neutral particles which is Neutron -
Louis deBroglie
Einstein suggested that light has waves properties which means that it behaves like waves as well as a particle. deBroglie wondered that if other particles such as electrons and protons have wave properties. in 1924, he found that all the material particles 'in motion' have wave characteristics. He expressed his found in an equation that shows that particles also have waves properties. -
Werner Heisenberg
Heisenberg contributed to the atomic theory by his uncertainty principle. His principle states that it is not possible to find the electron's velocity and its location at the same time. that's because an electron has almost no mass and when anyone tries to observe it with 'light', it would move because of the light. -
Wolfgang Pauli
Pauli contributed to the atomic theory by his "Exclusion principle". His principle states that when an electron has a set of quantum numbers, then there are no other electrons in the same atom that have the same set numbers. He also found that an electron spins up or down within its orbit. He proved his point by finding that the helium has a maximum of two electrons in its shell one spins up and the other one spins down. -
Erwin Schrödinger
Bohr's model was a one-dimensional circular orbits that used one quantum number to describe the place of the electrons. In Schrödinger's model, it allowed the electron to be in a three-dimensional space. In 1926, Schrödinger combined the equation of the behavior waves with deBroglie equation to make a model for the place of electrons in an atom. Schrödinger's model describes the probability of an electron that could be found in a specific region of space at a given time. -
Richard Badar
Badar contributed to the atomic theory when he published his theory in 1991. His theory state that there are no orbitals in molecules. He also discovered that electrons density is very important in order to explain the behavior of the atoms in a molecule.
