The Modern Atomic Theory states that: 1. Atoms are no longer considered indivisible or indestructible; 2. Atoms of the same element can have different weights and atoms of different elements can have the same weight; 3. Atoms are the smallest particles that take part in chemical reactions; and 4. Atoms do not always have to combine in simple ratios (Wiz IQ). The development of this theory can be contributed to the works of John Dalton, J.J. Thomson, Robert Millikan, Ernest Rutherford, Henry Moseley, Niels Bohr, Gary Motherfucking Oak, and Sir James Chadwick.
John Dalton was the first scientist to propose a version of the modern atomic theory. He based his atomic theory off of ancient Greek philosophers’ theories on atoms, although his theory was much more accurate than theirs. Dalton did not have any experimental designs for his theory, but he did have a lot of quality research. He proposed a standard symbol for each element (right). He found that chemical elements were composed of atoms, and that the atoms in one element were all identical in weight, but atoms of different elements had different weights (Atomic Theory). John Dalton was able to prove this information because of his development of a method used to find atomic weights. He also found that atoms only combined in small whole-number ratios. This information is related to the formulation of the Law of Multiple Proportions (Historical Outline). The Law of Multiple Proportions states that when two elements can combine to form more than one compound, the mass of one element and the mass of the other element are in a ratio of small whole numbers. He created the Law of Multiple Proportions as he was experimenting with finding the atomic weights of elements (Britannica). With all of his work, John Dalton came up with his own atomic theory. It stated that 1. Matter is made up of very small, indivisible parts known as atoms; 2. Atoms of the same element are identical in every way; 3. Atoms of different elements are different in all ways; 4. When atoms of the same or different element combine they form compound atoms; 5. When combined, they do so in simple, whole-number ratios; 6. An atom is the smallest particle to have a part in chemical reactions; and 7. An atom can neither be created nor destroyed (Wiz IQ).
When J.J Thomson was only 14 years when he enrolled at Owens College, Manchester. He then went on to become a Fellow of Trinity College, after that he became a professor at Cambridge. As a professor, Thomson made a few trips to America, and when he returned, he made the most brilliant discovery of his career. Thomson had discovered the electron. While he was on another trip to America, he lectured on some things he thought the structure of an atom would be (Thomson Nobel Prize). Before him, Dalton had done a lot of research with atoms, but his theory had nothing about the physical and internal structures of atoms. The evidence that led him to believe Dalton’s theory was not the most accurate was his own discovery of the electron, or negatively charged particles (Historical Outline). Thomson discovered the electron while experimenting with cathode rays; he concluded that a cathode ray is a stream of negatively charged particles that has a mass about 1000 times smaller than a hydrogen atom. With this information he proposed that cathode rays are streams of particles much smaller than atoms, and that theses small particles were pieces of atoms (American Institute of Physics). In 1904, Thomson made a model of what he believed the internal structure of an atom looked like (Chemical Heritage Foundation). He called this model, “Plum Pudding”, because there was a positively charged “bowl” with negatively charged electrons in the atom that were like plums in pudding (History of Atoms).
Robert Millikan grew up in rural America as the son of Reverend Silas Millikan. He attended Oberlin College. After he graduated, he taught elementary physics. He became a professor at the University of Chicago after being a teacher in every grade level. Millikan made many advancements in his career, but by far, his greatest discovery was that of the charge of an electron. Millikan determined the unit charge of the electron Thomson discovered (Millikan Nobel Prize). With the unit charge of the electron, the mass of electrons and positively charged atoms could be calculated. He found the unit charge of the electron by using his oil drop experiment. In the oil drop experiment, Millikan dropped oil drops a certain distance in air and measured how long it took the drop to fall. Then he turned on an electric field, and measured how long it took for the oil drops to rise the same distance upward. With the two times-up and down-of the oil drops, Millikan could find the mass of the drop of oil and its total charge. Sometimes the charge of the oil drop would change and he concluded that these changes were small multiples of e, an important unit charge (Franklin).
Ernest Rutherford grew up with a father who was a wheelwright and a mother who was a schoolteacher. When he was 16, Rutherford entered Nelson Collegiate School then went on to study at the University of New Zealand, where he earned a masters degree in mathematics and physics. Rutherford was awarded the Exhibition Science Scholarship, which enabled him to study at Trinity College where he would work closely with Thomson, the man who discovered the electron and found that atoms were made of electrons. Another Scientist who did work with atoms was Millikan did who found the charge of an electron; Rutherford went in depth to find a more accurate depiction of the internal structure of an atom and how the electrons play a role in its structure (Rutherford Nobel Prize). He experimented with alpha rays and concluded, with their help, that the atom was a small, heavy nucleus with electrons in orbit around it. Rutherford developed a model of this atom with a nucleus and orbiting electrons; the picture to the right is the Rutherford model. This would become the base for the model that is still accepted today (Atomic Theory). Besides stating that electrons orbit a nucleus, Rutherford also discovered the proton, which makes up the nucleus of a hydrogen atom. He found that a proton has a single positive electric charge, and that every atomic nucleus has at least one proton present. Rutherford proposed the existence of the neutron but he did not have enough factual information to prove its existence. He also proposed that all of the positive charge and all of the mass of the atom occupied a small space in the center of an atom and that most of the atom is empty space occupied by electrons (Historical Outline).
Henry Moseley was raised in an educational environment. His father was a biologist and a professor of anatomy and physiology, his grandfather was also a biologist and conchologist. Moseley was awarded a scholarship to attend Eton College. After Eton College, he went to Trinity College at the University of Oxford. He then went to the University of Manchester to study under Sir Ernest Rutherford (Moseley Wikipedia). Moseley was a young scientist, who was mostly known for his work with atomic numbers and the periodic table. He experimented and found that the x-rays elements emit increase in a linear manner with each following element on the periodic table. He thought this could be directly related to the positive charge on the nucleus (Henry Moseley Wikipedia). Moseley then proved that the atomic numbers of elements were not random, but “had a physical basis that could be measured”. This is known as Moseley’s law, and it justified many ideas in chemistry by placing the elements on the periodic table into the correct order based on atomic number, not atomic mass (Royal Society of Chemistry). Moseley’s law also justified Rutherford’s view of the atom. It justified this model of the atom because the law states that the atomic number of an element is also the exact number of positive charges in the central atomic nucleus, and also Rutherford’s proposal that the atom is a positively charged nucleus surrounded by negatively charged electrons (Moseley Wikipedia).
Niels Bohr also grew up in a very educational atmosphere. His father was a distinguished physiologist, and his mother came from a family in the field of education. Bohr attended Copenhagen University where he went on to study physics. After receiving his Doctor’s degree, Bohr studied at Cambridge, where he studied under Thomson and Rutherford. Bohr started to study the structure of atoms based on Rutherford’s findings (Bohr Nobel Prize). Ernest Rutherford found that an atom is a nucleus with electrons in orbit around it, Bohr realized that his model was not entirely correct; according to physics, it should be very unstable (PBS). Bohr came up with two rules that he applied to the movement of electrons. First, he said that electrons can only orbit a certain distance away from the nucleus. Secondly, he proposed that electrons only moved in restricted, consecutive orbital shells, and that the outer, high energy orbits determined the chemical properties of different elements (History of Atoms). Bohr also suggested that electrons “jump” from one orbital ring to another without ever being in an in-between state. Meaning that when atoms give off energy, such as light or heat, the electron will jump to a higher or lower orbit. Bohr created a model of what his evidence would look like; the picture above is the model Bohr created, it is still used today. When Bohr published this new discovery in 1913, some people did not accept it because of the basis of his information. Bohr developed his idea off of simple atoms, but the solid evidence was that his model matched up with patterns of light emitted by real hydrogen atoms (PBS).
Sir James Chadwick was born on October 20, 1891. He attended Manchester High school before entering Manchester University. Chadwick graduated in 1911 from the Honors school of Physics. He actually studied under Professor Rutherford, and he would continue to work with Rutherford for some time and in the future. Chadwick then moved on to Cambridge, where he would discover the neutron. While at Cambridge, Chadwick took a look at the current atomic model and found there was something missing (Chadwick Nobel Prize). In the previous atomic model, there were no neutrons, which made the atomic model seem incomplete. Ernest Rutherford had proposed the existence of an electrically neutral particle, but was never able to prove it existed. In 1932, Sir James Chadwick discovered the neutron. He found it to be slightly heavier than the proton with a mass of 1840 electrons and have a neutral charge (Discovery of Neutron).
The incredible effort and great amount of time these scientists put in to discover more about atoms and their potential is what led us to the knowledge we have today. All of their research combined resulted in the birth of the modern atomic theory.