Atomic Theory

The Modern Periodic Table

How did we come up with the way the Periodic Table is set up?

Russian Scientist Mendelev started to weigh elements, calculate their density and he noticed a pattern.

He noticed that Hydrogen had a weight of 1 g, Lithium had a weight of 7 g, Sodium had a weight of 23 g and Potassium had a weight of 39 g.

He then noticed Beryllium had a weight of 9 g, Magnesium had a weight of 24 g, and Calcium had a weight of 40 g.

He was then able to see the following pattern.

Atomic Theory

Mendeleev’s Periodic Table

But what was causing the elements to have the weights that they had and the density that they had? Why was this pattern so noticeable?

John Dalton, in the 1800’s was able to put some context into what was happening.

He envisioned an atom – and he said that the atom was was the smallest piece of any element, it was s smooth round solid sphere and did not have an electrical charge.

Atomic Theory

Dalton stated: 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 any one element are identical. Compounds are created when atoms of different

elements are combined in a specific ratio. Atoms cannot be created, destroyed, or subdivided

during chemical changes.

Dalton’s First Theory

Basically, Dalton was able to indicate what an atom looks like, and how elements interact with other elements to create a chemical change and a chemical compound.

BUT…………. What about electric charges, how do they come into play?

Dalton’s first theroy

In 1830, scientist Michael Faraday said that electrons must be present in an atom, hence this is why we get an electric shock when we have a built up charge and then touch a metal object.

Dalton agreed and added three more parts to his theory: Matter must contain positive and negative charges Opposite charges attract, like charges repel. Atoms combine to form the particles of a compound

because of the electrical attraction between charged atoms.

Dalton’s revised theory

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.

Electron’s?

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.

So what is the mass of an electron?

Millikan’s Experiment

Because of Thompson and Millikan, we are able to add to the Atomic Theory: Atom’s contain electrons. The electrons have a negative charge and a very

small mass. The rest of the atom has a positive charge. The electrons are embedded randomly into the

positive parts of the atom. Electrons can be removed from, or added to,

atoms to create a charged atom.

Atomic Theory: The electron

So far we have deduced why chemical reactions occur based on Dalton’s theory, and we have been able understand more about electron’s, but if electrons are so light, what was it that Mendeleev was weighing then, and how did electrons and protons make up the arrangements of an atom?

Atoms – what do they look like

What Rutherford did was shoot an alpha particle from an alpha particle emitter.

He shot the alpha particle into a piece of 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’s Experiment

What Rutherford was observing was the positively charged alpha particle was hitting another positively charged particle and hence the alpha particle bounced around. Every once in a while an alpha particle would hit the gold foil and not bounce anywhere.

Rutherford deduced that there were three subatomic particles in an atom: an electron, a proton, and neutron.

So what happened?

Rutherford’s Atomic Theory: The nucleus contains all of the positive charge and most of

the mass of the atom. The nucleus contains positively charged protons and

uncharged neutrons. Neutrons have the same mass as protons. The nucleus is very small, compared with the size of the

atom. The electrons orbit the nucleus, like satellites orbit the planet. The mass of an electron is 1/1800 of a 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.

Rutherford’s Atomic Theory