History of Atomic Theory

Democritus - Bohr

Standards

3221.1.1 Compare and Contrast historical models of the atom

3221.1.3 Describe an atom in terms of its composition and electron characteristic http://www.learn360.com/ShowVideo.aspx?ID=130226s

Bettyellen

**Democritus Worksheet

Leucippas

5th century BCTeacher to Democritus

Father of Modern Science

Coined the term “atomos” which

means “indivisible”

Contemporary of Socrates

(475 BC)

Five Major Points1. All matter is composed of atoms, which

cannot be divided to further portions.2. There is a void, which is an empty space

between atoms.3. Atoms are completely solid.4. Atoms are homogeneous with no internal

structure.5. Atoms are different in their weights, sizes,

shapes.

Antoine Lavoisier

Father of Modern Chemistry

1790s

• Quantitative analysis of chemical reactions– Law of conservation of mass– Law of definite proportions– Law of multiple proportions

Handout and questions

Law of Conservation of Mass

Mass can be changed from one form to another but can be neither created or destroyed in a chemical reaction

In other words: the amount of stuff after a chemical reaction takes place is the same as the amount of stuff you started with

Joseph ProustLaw of Definite Proportions (“Proust’s Law”)

1799

Law stating that a pure substance will always have the same composition by mass (or percent by weight) as that substance everywhereEx. Water

*considered to be a radical law at the time and contested by some

Law of Multiple ProportionsDalton’s Law

If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.

Examples:CO CO2 CH3 CH4

H2O H2O2

*handout

English schoolteacher who proposed atomic theory in

18081. All matter is composed of

extremely small particles called atoms.

2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

3. Atoms cannot be subdivided, created, or destroyed.

4. Atoms of different elements combine in simple whole number ratios to form chemical compounds.

5. In chemical reactions, atoms are combined, separated, or rearranged.

Why was Dalton’s idea important?

It turned Democritus’s idea into a scientific theory that could be tested by experiment.

NOT wholly correct. Has been modified:1. all matter is composed of atoms2. atoms of any one element differ in properties from atoms of another element remain unchanged

Some more things Dalton did

• Developed the concept of the mol• Devised a system of symbols for known

elements (not the ones we use today)

More People

• Joseph Gay-Lussac (Law of Combining Volumes) 1808

• Amadeus Avogadro (Avogadro’s number)1811– Widely controversial idea that did not gain

acceptance for over 50 years

Discovery of the Electron

***Handout***

Cathode-ray tube experimentJ.J. Thomson (1897)

Experiments revealed following observations:

1. Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric current, which was known to have a negative charge

2. The rays were deflected away from a negatively charged object

Charge and Mass of the ElectronRobert A Millikan (1909)

Oil Drop Experiment

Mass of electron: 9.109 x 10-31 kg, or 1/1837 the mass of the simplest hydrogen atom

Based on what was learned about electrons, two other inferences were made:1. Because atoms are electrically neutral,

they must contain a positive charge to balance the negative electrons

2. Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass

Plum Pudding ModelJ.J. Thomson

Discovery of Atomic Nucleus

Ernest Rutherford (1911)

Gold-Foil Experiment

Rutherford’s experiment

• Scientists bombarded a thin piece of gold foil with fast-moving alpha particles, which are positively charged particles with about four times the mass of a hydrogen atom

• Rather than going straight through, some particles were deflected

What caused the deflection?

• The deflected alpha particles must have experienced some powerful force within the atom

• Force occupied very small space (so few electrons deflected)—densely packed bundle of matter with a positive electric charge (nucleus)

How big is the nucleus??

If the nucleus were the size of a marble, the atom would be the size of a football field…

During this same era…

• Before 1900, scientists thought light behaved solely as wave. However, by end of 20th century, scientists began to understand that light has a dual wave-particle nature

Wave Description of LightElectromagnetic radiation: form of energy that exhibits wavelike behavior as it travels

through space

(Gamma, X rays, Ultraviolet, Visible Light, Infrared, Microwave, Radio waves, Long waves)

Comprises electromagnetic spectrum

Wavelength (λ)

the distance between corresponding points on adjacent wavesCan be expressed in meters (m)

Wavelike Properties (cont’d)

Frequency (v): The number of waves that pass a given point in a specific time, usually one second

Expressed in waves/second1 wave/second = 1 hertz (Hz)

c = λvc is the speed of light (in m/s), λ is the wavelength (in

m), and v is the frequency (in s-1)

Photoelectric Effect

The emission of electrons from a metal when light shines on the metal

Max Plank: quantum: minimum quantity of energy that can be gained or lost by an atom

E = hvh: Plank’s constant6.626 x 10-34 J•s

Albert Einstein (1905)Introduced the idea that electromagnetic radiation has a dual wave-particle nature

Photon: particle of electromagnetic radiation having zero mass and carrying a quantum of energy

Ephoton = hv

Electromagnetic radiation is absorbed by matter only in whole numbers of photons. In order for an electron to be ejected from a metal surface, the electron must be struck by a single photon possessing at least the minimum energy required to knock the electron loose.

The Hydrogen-Atom Emission-Line Spectrum

ground state: lowest energy state of an atomexcited state: atom has a higher potential energy than it has in the ground

state

there are many possible excited states, each with a unique energy, but only one ground state energy for atoms of a given element.

when an excited atom returns to its ground state, it gives off the energy it gained in the form of electromagnetic radiation

When an electric current is passed through a glass tube that contains hydrogen gas at low pressure the tube gives off blue light. When this light is passed through a prism four narrow bands of bright light are observed against a black background.

Hydrogen-atom emission spectrum

Quantum Theory

New atomic theory that arose due to attempts to explain the hydrogen-atom emission spectrum

Bohr Model: compared atom to solar system in that electrons rotate around the nucleus in circular orbits

Bohr Model of the Hydrogen Atom (1913)

Spectral Series

The energy-state diagram for a hydrogen atom shows some of the energy transitions for the Lyman, Balmer, and Paschen spectral series.

Bohr’s model accounted mathematically for these changes.

PROBLEM only accounted for the energy transitions of the hydrogen atom

Bohr’s Model (cont’d)

IMPORTANT: electrons can only be located in orbits, NOT in between

Only true for hydrogenBohr’s theory did not explain the chemical

behavior of atoms