Atomic Theory

What makes up an Atom? What subatomic particles make up an atom? How did chemistry evolve to have an understanding of what an atom looks like? Think-Pair-Share these two ideas before we continue the lesson.

Atom is made up of The three subatomic particles are : electrons, neutrons and protons. Protons have a positive charge. Neutrons have no (neutral) charge. Electrons have a negative charge. Protons and neutrons are found in the nucleus, the electrons surround the nucleus.

Atomic Theory Atomic Theory is – the theory that all mater is made up of tiny invisible particles called atoms. Atomic Theory is just a theory (and not a law) because it continually evolves. The more we understand what makes up an atom, the more we add to the theory. We use our understanding of the Atomic Theory to build and create the periodic table.

John Dalton John Dalton (1766 – 1844) All matter is made of atoms, which are particles that are too small to see. Each element has its own kind of atom, with its own particular mass. All atoms of one kind are identical. Compounds are created when atoms of different elements combine in a specific ratio. Atoms cannot be created, destroyed, or divided into smaller particles. Atoms are small, solid spheres.

Michael Faraday Michael Faraday (1791 – 1867) Revised Dalton’s theory to account for electric charges and static electricity. Matter must contain positive and negative charges. Opposite charges attract, and like charges repel. Atoms combine to form compounds because of the electrical attraction between charged atoms.

Electron’s? So what were these electron’s that Dalton and Faraday were talking about? J.J. Thompson, an English Scientist, hooked a power supply to a cathode ray tube.

Electron’s? Cathode ray tube has a fluorescent dye on the inside, and when an electron shots from one side to the other you can see it. Thompson noticed that it was very very very very very easy to manipulate the electron, and move it inside the tube.

J. J. Thompson J.J. Thomson (1856 – 1940) Thomson discovered that all atoms had very small negatively charged particles in them (later called electrons). Electrons have a negative charge and a very small mass, while the rest of the atom has a positive charge. Electrons are embedded randomly in the positive part of the atom. Electrons can be added to, or removed from, atoms to create charged atoms. Atoms look like a “raisin bun” or also called “plum pudding”

Mass of an electron American R.A. Millikan decided to find out how the mass of an electron. What he did was set up a box, spray oil into the box. He then had two horizontal plates that were charged. In the top plate he had a tiny hole for the charged electrons to drop through. He could then observe how rapidly the oil drops fell through the first charged plate (+) to the second charged plate (-). He was able to deduce that one electron’s mass was 9.109382 X 10-28. I.E. VERY VERY VERY VERY VERY light.

Millikan’s Experiment

Rutherford’s Experiment Rutherford wanted to know what would happen if he shot a positive particle (alpha particle) into metal (gold foil). Surrounding the gold foil was a screen to see where the alpha particles hit. Since alpha particles are positive charged, the original theory is that all shots would go straight through the gold foil and hit the back of the screen… but not all did.

Rutherford’s Experiment To the amazement of Rutherford, while about 80% or so did go straight though the gold foil, some of the alpha particles started to bounce all over the place. They would hit the gold foil and bounce left, and bounce right, and even bounce back towards the alpha particle emitter.

Rutherford’s Experiment British Physicist Ernest Rutherford famous gold foil experiment.

Rutherford Experiment Ernest Rutherford (1871 – 1937) Rutherford discovered that atoms have a positively charged nucleus (centre of the atom) that contains most of the atom’s mass. The nucleus has positively charged protons and uncharged neutrons. Neutrons have the same mass as protons. The nucleus is very small compared to the rest of the atom. The electrons orbit the nucleus, like satellites around a planet.

Rutherford Continued Ernest Rutherford (1871 – 1937) Continued: The mass of the electron is 1 1800 the mass of the proton. The size of the atom is determined by the size of the orbit of the electrons. There is only empty space between the electrons and the nucleus.

Niels Bohr Rutherford was very close to finalizing what the atom looked like. It was up to one of his students. One particular student, Niels Bohr, took Rutherford’s model and perfected it. Bohr examined the way light was emitted from different elements. He noticed that if he heated up an element it would produce a colour. Different elements produced different colours of light. He then concluded that the heat was giving electrons some energy, when the electron released the energy it released it as a form of colour. He hypothesized that the electrons were not just spinning around the nucleus, but that they were spinning around the nucleus in different orbits allowing an electron to jump from one orbit to the next, and then back down.

Light Emission + Electron Excitation n=1 n=2 n=3

Niels Bohr Niels Bohr (1885 – 1962) Electrons are located in defined shells, which are located certain distances from the nucleus. Electrons cannot exist between the defined shells. Electrons can gain energy to move up to a higher shell, or they can lose energy to move down to a lower shell. Electrons are more stable (have less energy) when they are closer to the nucleus.

Bohr’s Atomic Theory The first shell has 2 electrons. The second and third shell has 8 electrons. The fourth and fifth shell has 18 electrons. The sixth and seventh shell has 32 electrons. This isn’t quite accurate, but it was what Bohr concluded and he was VERY close to what we really have.

Assignment Page 208 – Q’s 1 – 12 Page 212 – Q’s 1, 2, 4, 7,