Democritus

1)Was the first person to come up with the idea of atom2)Believed that all matter was composed of indivisible particles he called “ATOMS”3)Which is derived from the Greek word

“Atomos” – meaning indivisible4)He also believed that different atoms:

Are different sizes Have different properties

5)Other philosophers of that time did not agree with his theories.

Dalton’s Atomic Theory 5 Postulates

 

1. All matter is composed of extremely small particles called atoms

2. All atoms of a given element are identical, having the same:

- size- mass- chemical properties.

3. All atoms of different elements are different.

4. Atoms cannot be created, divided into smaller particles, or destroyed.**In a chemical reaction, atoms of different elements are separated, joined or rearranged. They are never changed into the atoms of another element. We will learn more later**5. Atoms combine in definite whole number ratios to make compounds (you can’t have a ½ of a Carbon bonding with Oxygen; it’s a whole atom or no atom)

Dalton’s Atomic Theory 5 Postulates

Dalton’s Atomic Laws

1. Law of Conservation of MassMatter cannot be created or destroyed in any physical or chemical process, just transferred.2.     Law of Constant CompositionWhen atoms combine to form molecules, the ratio of atoms is constant.Example – H2O will always have 2 times as many Hydrogen atoms as Oxygen.

Dalton’s Atomic Laws

3.     Law of Multiple Proportions – if two elements can combine to form more than one compound, then the ratio of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.

Formula

Ratio of N:O

Example:CO vs. CO2

Thomson’s Plum Pudding Model

•Thomson’s Plum Pudding model is a + charge sphere that has (- )charged electrons scattered inside, like “raisins” in “plum pudding”.•Overall, the atom is neutral atom because the atom had the same number of positive and negative charges.•From Thomson’s experiments, scientists concluded

that atoms were not just neutral spheres, but somehow were composed of electrically charged particles.•The balance of positive and negative charge supports the neutral atom.

Rutherford Model

Took Thomson’s Plum Pudding Model and added to it.

Used the “Gold Foil Experiment” to discover the existence of:

An atomic Nucleus

Protons

Gold Foil Experiment Rutherford directed a

narrow beam of alpha particles (+ charges) at a thin piece of gold foil.

Based on observations from other experiments involving alpha particles, he predicted that the (+) charges would go through the foil

Results from Gold Foil Experiment

•Rutherford found that every once and a while, a + particle was deflected bounced back. (about 1% of the time)•Why? •Because the + charge hit a central mass of positive charge and was repelled.

The Gold Foil Experiment

Conclusions from Rutherford’s Gold Foil Experiment:

• The atom contains a positively charged “nucleus” •This nucleus contains almost all of the mass of the atom, but occupies a very small volume of the atom.•The negatively charged electrons occupied most of the volume of the atom.• The atom is mostly empty space.

Rutherford’s Planetary Model

• To explain his observations, Rutherford developed a new model•The electrons orbit the nucleus like the planets revolve around the sun.

Bohr Model

Worked in Rutherford’s lab

Wondered why – electrons are not attracted to the + nucleus and cluster around it

Disproved Rutherford’s Planetary Model

Experimented with light and its interaction with matter to develop a new model.

Bohr’s Energy Level Model

4. The electrons must absorb or emit a fixed amount of energy to travel between these energy levels

3. The electrons orbit the nucleus at fixed energy levels.

2. Electrons can only be certain distances from the nucleus.

1. Protons and neutrons are in the nucleus

Review

Q1)What were Rutherford’s conclusions from the Gold Foil Experiment?Answer:1)Atom has a positively charged nucleus2)electrons are outside, 3)atoms are mostly empty4)Nucleus contains most of the mass.