History of the AtomHistory of the Atom

What do you know about the What do you know about the atom?atom?

Put simply, the atom is the smallest Put simply, the atom is the smallest particle of pure essence. For example, particle of pure essence. For example, helium gas is made up of helium atoms. helium gas is made up of helium atoms. Each helium atom is identical to each Each helium atom is identical to each other. All of them have the same other. All of them have the same properties.properties.

What about water? Is water an atom? What about water? Is water an atom? No, the smallest particle of water is No, the smallest particle of water is called a molecule. Water is made from a called a molecule. Water is made from a group of atoms that are connected group of atoms that are connected chemically. Two hydrogen atoms and chemically. Two hydrogen atoms and one oxygen atom will form a water one oxygen atom will form a water molecule.molecule.

The History of the AtomThe History of the Atom

1). The Greeks first came up with the idea of 1). The Greeks first came up with the idea of the atom. (Atomos means not divisible). the atom. (Atomos means not divisible). Greek philosophers differed in their views of Greek philosophers differed in their views of what building blocks of life were. Aristotle what building blocks of life were. Aristotle thought it was the four original elements: thought it was the four original elements: earth, fire, water, and air. Democritus thought earth, fire, water, and air. Democritus thought it was atoms and these atoms had different it was atoms and these atoms had different sizes and properties. However, the views of sizes and properties. However, the views of Aristotle were held in such high regard Aristotle were held in such high regard nothing was studied of thought for hundreds nothing was studied of thought for hundreds of years.of years.

2). John Dalton (1766) is the father of modern 2). John Dalton (1766) is the father of modern chemistry and he drafted the Atomic Theory chemistry and he drafted the Atomic Theory of Matter.of Matter. This theory states: This theory states:

All matter is made of atomsAll matter is made of atoms

All atoms of a given element are identicalAll atoms of a given element are identical

Compounds are formed by combining different atomsCompounds are formed by combining different atoms

A chemical reaction involves rearranging different A chemical reaction involves rearranging different elements (but not creating new elements).elements (but not creating new elements).

In the early 1800’s scientists only knew the In the early 1800’s scientists only knew the atom existed, but what makes up an atom?atom existed, but what makes up an atom?

3). J.J. Thompson (1856) discovered 3). J.J. Thompson (1856) discovered the electron, a negatively charged the electron, a negatively charged particle. He suggested the atom looked particle. He suggested the atom looked like this.like this.

The “plum pudding” or chocolate chip model. It showed a positive atom (cookie) with negative electrons (chips) throughout. This was a short lived model.

4). Ernest Rutherford (1891) discovered that 4). Ernest Rutherford (1891) discovered that atoms must have a central part that holds a atoms must have a central part that holds a positive charge. He coined the term nucleus. positive charge. He coined the term nucleus. He thought the electron (e-) must have been He thought the electron (e-) must have been orbiting the nucleus.orbiting the nucleus.

In 1910 an experiment in Rutherford’s lab In 1910 an experiment in Rutherford’s lab caused the downfall of Thompson’s model. caused the downfall of Thompson’s model. They were looking at how alpha particles They were looking at how alpha particles were scattered as they hit a thin piece of gold were scattered as they hit a thin piece of gold foil. Rutherford had only seen a slight scatter foil. Rutherford had only seen a slight scatter angle that was consistent with Thompson’s angle that was consistent with Thompson’s model. One day he asked one of the students model. One day he asked one of the students to see if there were electrons being scatter at to see if there were electrons being scatter at larger angles. They observed that most of the larger angles. They observed that most of the particles passed directly though the gold foil particles passed directly though the gold foil but a few came straight back at them. but a few came straight back at them.

In 1911 Rutherford was finally about to In 1911 Rutherford was finally about to explain these results. He thought most explain these results. He thought most of the mass, and all the positive charge, of the mass, and all the positive charge, was in a very small and dense space he was in a very small and dense space he called the nucleus. So, most of the called the nucleus. So, most of the atom was empty space where the atom was empty space where the electrons are. Most of the alpha passed electrons are. Most of the alpha passed through because they went through the through because they went through the empty electron space. Sometimes an empty electron space. Sometimes an alpha particle either hits or passed very alpha particle either hits or passed very close to the nucleus and these ones close to the nucleus and these ones were deflected.were deflected.

5). Neils Bohr (1910’s) shows how e- 5). Neils Bohr (1910’s) shows how e- can jump from orbit to orbit and that can jump from orbit to orbit and that the orbits are energy levels. He was the orbits are energy levels. He was awarded the 1922 Nobel Prize and is awarded the 1922 Nobel Prize and is said to be the father of Quantum said to be the father of Quantum Physics.Physics.

6). James Chadwick (1932) discovered 6). James Chadwick (1932) discovered the neutron. It is a neutral particle the neutron. It is a neutral particle inside the nucleus. For this discovery inside the nucleus. For this discovery he was awarded the 1935 Nobel Prize.he was awarded the 1935 Nobel Prize.