The History of the Atom

AristotleAristotle was the first scientist that we

have record of questioning what stuff was made of.

What did he think?

The search for finding out about matter began with the Greek philosopher Democritus more than 2000 years ago.

THE QUESTIONCould matter be divided into

smaller and smaller pieces forever, or was there a limit to the number of times a piece of

matter could be divided?

The Answer:Democritus concluded that matter could not be divided into smaller and smaller pieces forever. Eventually the smallest piece would be obtained that could not be divided. This piece is called the “atom” coming from the Greek word “atomos” which means not to be cut, or indivisible.

The hypothesis:The people that shared this thought with Democritus hypothesized that atoms were all small, hard particles that were all made of the same material, but were of different sizes and shapes.

Is that hypothesis correct?

In 1803, John Dalton from England combined his and other observations

to propose the atomic theory.

The Atomic Theory

** All elements are composed of atoms. Atoms are indivisible and indestructible particles.

**Atoms of the same element are exactly the alike.

**Atoms of different elements are different.

**Compounds are formed by joining atoms of 2 or more elements.

John Dalton’s model Billiard Ball Model

atoms are neither created or destroyed in a chemical reaction

Law of conservation of Mass

Laws of ProportionsDefinite: water is always 2:1

Multiple: rearrange and the same elements in a 2:2 ration= a new substance!!

At the end of the 19th century cathode rays were being experimented with. These are produced in glass tubes under a vacuum and the tube it is in is called a cathode ray tube (CRT). In 1897 J.J. Thompson observed that the cathode rays are made up of very small, negatively charged particles that are a fundamental part of an atom.

The Ancestor of the TV Tube.

**What did J.J. Thompson just discover?

Thompson discovered the electron. He knew that something had to be in the atom to balance the negative

charge, but was unable to find it.

With his information he created a new model of the atom and called it the plum pudding model.

Millican Discovered the MASS of the electron to be

1/2000 (1/1837) of a Hydrogen atom. All atoms have electrons (acted the same in

the cathode ray) 2 inferences made from here:

Since atoms are electrically neutral-ther are = #s of protons and electrons

Since electrons mass is so small, most atom’s mass must be in the neucleus.

The Next New Find

In 1908 Ernest Rutherford fired a stream of positively charged particles at gold foil and found that most of them passed right

through while others hit something and bounced away.

What would cause something to repel a positive charge?

Another positive charge.

The Rutherford Model

From his information, Rutherford proposed that an atom had a small, dense, positively charged center that he named the nucleus.

In 1913 an improvement to the Rutherford model

was made by Niels Bohr. According to this model,

the electrons move in definite orbits around the

nucleus, much like planets in the solar

system. These orbits, or energy levels, are located at certain distances from the

nucleus.

Niels Bohr

The current atomic model has electrons in a probable location based on how much energy the electron has. An electron cloud is the space in which the electrons are likely to be found. This cloud is broken into energy levels. Electrons with lower energy levels are found close to the nucleus.