The Evolution of the Atomic Theory
The five atomic theorys of the past two centuries represent the sudden
advancement of science in modern times. Begining with a basic theory on the
behavior of atoms to the current model, some changes have been made, and
some ideas are still the same. Ancient Greek philosophers belived that
everything was made up of invisible particles called atmos. Since then the
theory of atoms did not progress untill 1803.
John Dalton was the first scientist to compose a theory of matter based
on atoms. Dalton's atomic theory is based on four concepts. He stated:
"1. All elements are composed of atoms, which are indivisable and
indestructable particles.
2. All atoms of the same element are exactly alike; in particular, they
have the same mass.
3. Atoms of different elements are different; in particular, they have
different masses.
4. Compounds are formed by the joining of atoms of two or more
elements." 1
All of Dalton's ideas account for the laws of definate and multiple
proportions and the law of conservation of mass. Some of Dalton's points are
still thought to be true, but over time this origional theory has been modifyed.
The first of these modifications came in 1897 when J.J. Thomson discovered
the electron. Based on the work of William Crookes and his "Crookes tube"
(Cathode-ray tube), Thomson discovered a negative charged particle was the
cause of the light produced by the cathode-ray tube. He also discovered that
these particles are present in all elements. These cathode-ray particles are
now known as electrons. Soon after the discovery of electrons the proton
was discovered. This led Thomson to conclude that ther were an equal
number of both particles present in the atom.
Twelve years later Lord Ernest Rutherford was experimenting with
alpha particles. He shot a stream of them at a peice of gold foil surrounded
by zinc-sulfide. When an alpha particle strikes ZnS it produces a flash of
light. The particles mostly stayed in a constant stream through the foil, but a
few were deflected. This led Rutherford to belive that there must be a small,
dense cluster of protons in the middle of the atoms to deflect the small
number of particles.
Neils Bohr was the next physicist to advance the atomic theory. He
explained what Rutherford could not about how the electron could stay in
orbit around the nucleus. When the electron has little energy it is closer to the
nucleus, when it absorbs more energy it travels farther from the nucleus.
There is a definate number of electrons that can be in the same orbit. When
the orbits closest to the nucleus are filled the atom is at a ground state.
Although the atomic theory was developed in increments, George Johnston Stoney is most famous for contributing the term electron: fundamental unit quantity of electricity. Stoney would develop the concept fourteen years before he coined the term electron. He also made contributions to the theory of gasses, cosmic physics, and estimated the number of molecules in a cubic millimeter of gas.
This experiment was first done by J.J Thomson in 1897. The result from this was that he discovered that the atom was not a fundamental unit of matter and that it had charged constituents that could not be separated. Thomson investigated the nature of the cathode rays which then resulted in the conclusion that the cathode rays were negatively charged constituents of the atom; hence the discovery of the electron.
physics. The work of Ernest Rutherford, H. G. J. Moseley, and Niels Bohr on atomic
In 1911, Ernest Rutherford found a very spectacular in such a small thing. Rutherford found that an atom has a microscopic charged nucleus. Empty space surrounds the nucleus. The nucleus is also surrounded by electrons. By this discovery Rutherford made the model of the atom.
Many scientists helped with this. In 465 B.C. Democritus named the atom “can’t be divided” (OI). He proposed that matter was made of atoms (Doc. 1). Later, Lavoisier introduced the Law of Conservation of Matter. It stated that matter couldn’t be created or destroyed. After that, John Dalton published the Atomic Theory of Matter in 1803 (Doc. 2). It said that matter is made of atoms that are too small to be seen by the naked eye, and that each type of matter is made of only one kind of atom (OI). J. J. Thomson observed electrons using cathode rays. A few years later, Ernest Rutherford bombarded an extremely thin piece of gold foil with positively charged alpha particles. Most of the protons passed through the foil, but some bounced off. Rutherford concluded that the atom must be composed of mostly empty space. He also realized that the alpha particles must have bounced off something else that had positive charge. The positively charged objects were protons. Neutrons were discovered by James Chadwick. To conclude, the theories of the atom have been modified a lot since the
The Pauli exclusion principle is defined by Dr. Steven S. Zumdahl, "In a given atom no two electrons can have the same set of four quantum numbers." Due to this principle, only two electrons can inhabit a single energy level. The electrons that share the same energy level have opposite intrinsic angular momentums which is more commonly known as "spin". To determine the direction of the spin the angular momentum vector is analyzed.
Electron affinity is the amount of energy absorbed or released when an electron is added to a neutral atom in its gaseous state. Hence, it is a trend that shows the likelihood of an element to gain an electron in its valence shell. In the periodic table, some elements make bonds only with a group of other elements due to their electron affinities. Iodine and neon are two non-metals that may seem similar, but they react differently in bonds due to their affinity for electrons. To mention, neon is a noble gas and it does not have an affinity for electrons. In comparison, iodine has a greater affinity for electrons than neon due to the following factors: effective nuclear charge, atomic radius, and electronic arrangement.
To understand what a radioactive isotope is a basic understanding of the atom is necessary. Atoms are comprised of three subatomic particles : protons, neutrons and electrons. Protons and neutrons bind together to form the nucleus of the atom, while the electrons surround and orbit the nucleus. Protons and electrons have opposite charges and therefore attract one another (electrons are negative and protons are positive, and opposite charges attract), and in most cases the number of electrons and protons are the same for an atom (making the atom neutral in charge). The neutrons are neutral. Their purpose in the nucleus is to bind protons together. Because the protons all have the same charge and would naturally repel one another, the neutrons act as "glue" to hold the protons tightly together in the nucleus.
In source 1 what you are seeing is the first direct observation of an atoms electron orbital. To capture the image researchers utilised this incredible device. The orbital structure is the space in an atom that is occupied by an electron. But when describing super microscopic particles such as those of the atom, scientists have to rely on wave functions, which is the scientific way of describing the fuzzy states of quantum particles. Usually scientists use formulas such as the Schrodinger equation to describe these states, but this very often results in complex numbers and complicated
Alpha radiation/emission - Alpha particles are the nuclei of a Helium atom 42He. Consisting of two protons and two neutrons, positively charged.
about the nucleus of its parent atom and (2) its rotation about its own axis.
Then, in 1766 was born a man named John Dalton born in England. He is known as
of the atoms, so if there are more or larger atoms then there must be
To start off, learning the Bohr-Rutherford diagrams in grade nine was a very big confusion for me. I had never fully understood how many shells should go around the nucleus nor did I know how to do the many calculations. As I progressed into grade ten, the teachings became easier. The review shows an example such as in the the bohr diagram, a nucleus is in the center, which is a little circle, and following that there are shells surrounding it containing electrons. Each ring can only hold a certain amount of electrons, and so the first shell around the nucleus can hold a maximum of 2 electrons, the next shell is able to hold a maximum of 8 electro...
The idea of a world progressing, or evolving, in science hasn’t been around forever. In fact, the Enlightenment period in the seventeen hundreds with scientists such as Isaac Newton the man who discovered gravity, Louis Pasteur the chemist who invented the vaccine to prevent rabies, Charles Darwin the father of evolution, Benjamin Franklin the first scientist to toy with the dangers and possibilities of electricity, and so many more wonderful scientists was the start of the “progress” that revolutionized our world. Of the scientists who progressed our world, few shaped modern biology the way Charles Darwin managed to. Thomas Kuhn saw the progress people like Darwin made not as truth seeking, but simply as filling in another piece of the puzzle of science, challenging the very definition of the Scientific Revolution. After reviewing Kuhn’s idea of science, Darwin appears to play a substantial role in the paradigm shift from the science of old to new. Kuhn looked at Darwin and saw science evolve much as Darwin’s organisms appeared to evolve