What is an Atom?

The basic unit of a chemical element. Atoms are made out of protons, neutrons and electrons. Protons have positive charges, neutrons have zero charge and electrons have a negative charge. There different types of atoms called elements, for example The element helium, which contains atoms with two protons in the nucleus and The element iron contains atoms with 26 protons. Each element has their own unique number of protons. The protons and neutrons are found in the nucleus of an atom.

Why is it important to know about atoms?

It is important to know about Atoms because atoms explain all the properties of matter. We would not understand chemistry or physics without atoms. Modern technology has become powerful enough that we can see and use atoms. Examples to prove this are, atoms that are split for energy in nuclear power. Scientist can even move single atoms around and make structures out of atoms.

Who was Democritus and what was his belief?

Democritus was a pre-Socratic Greek philosopher, he based much of his theories from his teacher, Leucippus. His belief was that all matter is made up of various imperishable indivisible elements which he called atomos. (This is where we get the english word atom.) According to Democritus, the atoms had shape, mass, and motion but no other qualities such as color/flavor. 

Democritus wrote, " all things were composed of atoms - small, minute, and indestructible particles of pure matter, with a void between each and every atom. Atoms were solid, with no internal structure, and that they were different in size, shape, and weight."

Who was Lavoisier?

Antoine Lavoisier was a french chemist who founded modern chemistry. He named the elements: hydrogen and oxygen, and showed how together these elements can make water. Lavoisier had many contributions to chemistry in the 18th century. For example he assisted in creating the metric system. In 1777 he was the first person to discover sulfur is an element, not a compound. All three elements he discovered are all in the periodic table. Lavoisier is mostly known for his discovery of how oxygen has a role in combustion. By doing this, he disapproved the phlogiston theory, which is the theory that phlogiston is in flammable bodies and that it is released during combustion. He was also responsible for writing the first list of elements.

"what is observed in the combustion of sulfur and phosphorus may well take place in the case of all substances that gain in weight by combustion and calcination: and I am persuaded that the increase in weight of metallic calces is due to the same cause."

Who was Dalton?

John Dalton was an English chemist physicist, and meteorologist. He proposed the atomic theory. While Dalton was working on his atomic theory, he came across the law of definite proportions. This law explains that when given chemical compound it will always contain its elements in a fixed ratio that doesn't depend on its source. The law of multiple proportions is used to establish Dalton's atomic theory. It states that when two elements form more than just one compound, many masses of one element combined with a specific mass of a different element are related by whole-number ratios. Dalton spent a lot of his time experimenting in order to create his atomic theory. Because his theory was mainly based off of experimental data, his theory was widely accepted. Democritus had many beliefs however, he was unable to give enough experiment proof of his ideas. Although his theory was proposed in 1808, it is still used today in modern chemistry.

Dalton's Atomic Theory

* Elements are made of atoms

* Atoms can't be further divided, created or destroyed

* Chemical reactions --> atoms are combined, separated or rearranged

* Atoms of a specific element are the same such as in size, mass etc

* Atoms of a different element are different in the size mass and properties etc

* The atoms joins together in order to form new substances and chemical compounds

"Billard Ball" model

What is JJ Thomson's contribution to the atomic model?

In 1897, J.J. Thomson discovered the electron by experimenting with a Cathode ray tube. He demonstrated that cathode rays were negatively charged. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. Dalton thought that atoms were indivisible particles, and Thomson's discovery of the electron proved the existence of subatomic particles.

How did he make the discovery?

Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of “bodies much smaller than atoms” (electrons) that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself.

What was Millikan's Contribution?

An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'.

What Millikan did was put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have.

How were protons and neutrons discovered?

In 1911 Ernest Rutherford who performed many experiments to explore radioactivity did an investigation in which he discovered that the atom must have a strong positive center charge that contains most of the atom's mass. He suggested that the nucleus contained a particle with a positive charge the proton.

In 1932, James Chadwick, an English physicist who had worked with Rutherford, detected neutrons and measured their mass in an invisible game of billiards. He fired the neutrons in a block of paraffin wax, which has a high concentration of hydrogen and is therefore rich in protons. Some of the neutrons collided with protons in the wax and knocked them out. Chadwick could then detect these protons and measure their energy. Using his knowledge of energy and momentum, he was able to work out the mass of the neutrons from the range of energies of the protons that they knocked out. He found that its mass was slightly more than a proton.

What are protons and neutrons?

 Protons and neutrons are in the center of the atom, making up the nucleus. Protons have a positive charge. Neutrons are neutral particles.

Who was rutherford? And what was wrong with his experiments?

Ernest Rutherford discovered the nucleus of the atom in 1911. He was the first to discover that atoms have a small charged nucleus surrounded by largely empty space, and are circled by tiny electrons, which became known as the Rutherford model (or planetary model) of the atom. He is also credited with the discovery of the proton in 1919, and hypothesized the existence of the neutron. Rutherford's model did not account for the properties of electrons. atoms seemed too stable, they were expected to collapse when electron absorbed or released energy.