History of the AtomDemocritus

Dalton (JJ Thompson)

RutherfordBohr

Electron Cloud

Warm Up 2.02

Draw electron diagrams for the following elements:

1. Carbon

2. Silicon

3. Titanium

4. Lithium

5. According to the 2n2 law, what is the maximum number of electrons that will fit in the 1,2,3 and 4 electron orbits (respectively) around an atom?

Warm Up1. A silicon atom weighs 34 amu’s. How many protons,

neutrons and electrons does this atom contain?

2. A tritium atom ( a form of hydrogen) weighs 3 amu’s. How many protons, neutrons and electrons does this atom contain?

3. A uranium atom weighs 238 amu’s. How many protons, neutrons and electrons does this atom contain?

4. A thorium atom weighs 232 amu’s. How many protons, neutrons and electrons does this atom contain?

5. A plutonium atom weighs 244 amu’s. How many protons, neutrons and electrons does this atom contain?

Warm Up 090413

1. Person’s model that says atoms of the same element are all the same mass.

2. Person’s model that says electrons only travel in discrete orbits.

3. Person’s model that says atoms are made of negative “plums” stuck inside a “pudding” matrix.

4. Person’s model that says atoms are mostly empty space, but have a nucleus that contains neutrons.

5. Person that performed the “gold foil” test.

6. Person (or persons) that said that atoms are not dividable.

7. Person who said that electrons can jump to higher orbits but eventually fall back and give off energy in the form of light

Democritus (460BC-370BC)

Known as the laughing

philosopher.

Democritus (460BC-370BC)The theory of Democritus held that everything is composed of "atoms", which are physically indivisible; that between atoms, there lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.

John Dalton (1766-1844)

Dalton’s Model

(1803) John Dalton (England) formulated the modern version of the atomic theory. In his model all atoms in a given chemical element are exactly alike, while the atoms of different elements differ by atomic weight.

1. Chemical Elements are made up of atoms.

2. The atoms of an element are identical in their masses.

3. Atoms of different elements have different masses.

4. Atoms only combine in small whole number ratios – 1:1, 1:2, 2:3…

Warm Up

1. List two conclusions that John Dalton made about atoms!

2. What is the “nickname” given to J.J. Thompson’s model of the atom? Why do we use that nickname?

3. What did Rutherford conclude from his gold foil experiment?

4. What two rules did Neils Bohr say all atoms followed?

J.J. Thompson and the Discovery of the Electron

• 1906 Nobel Prize Winner-conduction of electricity through gases.

• 1856-1940.

• Joseph John.

• First to suggest the existence of sub-atomic particles.

• Builds the first CRT

The Cathode Ray Tube

J.J. Thompson’s Model

• (1898) J.J. Thompson (England) discovered the electron, the component of the atom with negative charge.

• His model of the atom had the negatively charged electron evenly distributed throughout a sphere of positively charged material.

• This is known as the “plum pudding” model of the atom. (electrons are like raisins in pudding.

Ernest Rutherford and the Nucleus

Ernest Rutherford (1871-1937)

• Knighted in 1914

• In 1921, while working with Niels Bohr, Rutherford theorized about the existence of neutrons, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons

Rutherford’s Model• Gold Foil and Alpha Particle experiment.• Rutherford could experimentally identify particles with positive

charge that he called protons. • Although he could explain the charge of atomic nuclei with the right

number of protons, the mass of the nucleus for large atoms was always larger than the sum of its protons.

• Therefore he postulated the existence of a neutral particle with a mass nearly the same as the proton which, when added to the protons in the nucleus, would give the right mass.

• Rutherford called this hypothetical particle the neutron. • Later (1930) Rutherford’s colleague James Chadwick was able to

detect the neutron experimentally.

Niels Bohr (1885-1962)

Niels Bohr-Planetary Model

• Born in Denmark.

• Bohr developed his Bohr Model of the atom with the atomic nucleus at the centre and electrons in orbit around it, which he compared to the planets orbiting the Sun.

• Nobel Prize in 1922.

Bohr’s Model

• Bohr said, "Here's some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them." Bohr came up with two rules which agreed with experiment:

• RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.

• RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.

Summary

Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today).

John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century.

In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.

In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.

In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.

We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).

Reference http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htmhttp://www.nobeliefs.com/atom.htm