ATOMIC STRUCTURE

Atomic Theory

Atoms were thought to be the smallest particles of matter (Democritus)

Every type of matter was made of a different atom

This idea that all matter is made up of fundamental particles is called Atomic Theory

Conservation of Matter

In 1782, Antoine Lavoisier observed that the mass of particles before and after a chemical reaction were the same.

He concluded that mass is neither created or destroyed. This is called the Law of Conservation of Matter.

Conservation of Matter

John Dalton then added that the atoms were only rearranged.

What this means is that the chemical composition before and after a chemical reaction are different, because the arrangement of the atoms is different.

Hypothesis, Theory, Law

Hypothesis – a testable prediction to explain an observation

Theory – an explanation based on many observations, that has been supported by many experiements

Law – a fact of nature that is observed so often that it becomes accepted as truth

Atomic Structure

Dalton thought that the atom was the smallest piece of matter, and it could not be simplified any more.

He was ALMOST right.

There are three subatomic particles.sub – meaning under, below, or beneath

Electron The first subatomic particle discovered was

the electron.

It was discovered by J.J. Thomson in 1897 by the cathode ray tube experiment.

Through this experiment it was determined that electrons have a negative charge.

Electron Cathode Ray Tube Experiment Video

Subatomic Particles

Thanks to Thomson’s discovery it was now known that atoms were not just neutral

So the search for more subatomic particles began

Subatomic Particles

Why did scientists know that there must be more subatomic particles after the discovery of the electron?

Atom are neutral because of a balance of positive and negative charges

Protons

Protons have a positive charge.

The amount of charge on a proton is equal to that on an electron.

Protons were discovered when it was observed that the rays of a cathode ray tube comes from both sides, positive and negative

Neutrons

Neutrons have a neutral charge, or no charge.

Once again we can thank Thomson for the neutron. While he did not discover the first neutron he found that Neon had two different weights in 1910.

Neutrons

This observation of an element having two different masses contradicted the current atomic theory.

Isotopes are two, or more, atoms of the same element that have a different mass.

Neutrons

Because it was known that the mass came from the protons there had to be another subatomic particle adding to the mass, but not effecting the charge.

The neutron was confirmed in the early 1930s.

Atomic Structure

While the discoveries of the different subatomic particles was happening discoveries of the structure were also occurring.

In 1909, Ernest Rutherford carried out his gold foil experiment.

Atomic Structure

Gold Foil Experiment Video

Atomic Structure

The gold foil experiment proved that the protons and neutrons, almost all the atom’s mass, was in a very small nucleus in the center of the atom.

Therefore, almost all of the atom is empty space.

Atomic Structure

But where are the electrons?

Electrons are located in what is called the electron cloud that surrounds the nucleus.

Atomic Structure

The electron cloud is broken into different layers called energy levels.

The first level holds two electrons and the rest hold eight electrons.

This holds true until larger

masses are reached

Numbers

Atomic number is the number of protons an atom has.

Mass number is the number of protons and neutrons that gives an atom its mass.

Numbers

Each isotope has a different mass number.

Atomic mass is an average of the mass of each isotope and how common it is.

Numbers

Chlorine has two major isotopesCl - 35 and Cl – 37Cl – 35 has an abundancy of 75%Cl – 37 has an abundancy of 25%

So what is the atomic mass of Chlorine?

Numbers

Chlorine has two major isotopesCl - 35 and Cl – 37Cl – 35 has an abundancy of 75%Cl – 37 has an abundancy of 25%

So what is the atomic mass of Chlorine?

A: 35.5 amu (atomic mass units)