1

History of Atomic Theory

Alchemy ~ Before 400 B.C.

Experiment:

Beliefs:

Pseudoscience concerned with:

• Changing metal to gold• Finding an eternal life elixir

• All matter was made up of a combination of the four elements• Four elements: fire, wind, earth, water

Aristotle

2

Experiment: None. He had beliefs that were disregarded.

Beliefs:

Democritus ~ 400 B.C.

History of Atomic Theory

• Named the atom "Atomos." It means indivisible• Matter is composed of atoms too small to be seen.• Empty space between atoms• Atoms are solid and homogeneous• Different atoms have different sizes and shapes

Model: Sphere

Democritus

3

Sphere-Billiard balls

Experiment:

Beliefs:

History of Atomic Theory

Dalton ~ early 1803 English School Teacher

Studied chemical reactions, making observations and measurements

Five Principles-Dalton's Atomic Theory

1. All matter is made of indestructible atoms.

2. Atoms of the same element are identical in their physical and chemical properties.

3. Atoms of different elements have different physical and chemical properties.

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

5. In chemical reactions, atoms cannot be subdivided, created, or destroyed. They are combined, separated, or rearranged.

Model: Which principles remain true today?

John Dalton

4

Experiment:

Discoveries:

Model:

J.J. Thomson ~ 1897 English Physicist

History of Atomic Theory

Cathode Ray TubeElectricity is passed through a gas tube. The gas beam can be bent with a magnet.

• Atoms consist of charged particles• The negatively charged particles are called electrons (1897)• The positively charged particles are called protons (1920)

Plum Pudding or Chocolate Chip CookiesThe chocolate chips are electrons stuck in positive dough

Joseph John Thomson

5

History of Atomic Theory

Experiment:

Model:

Discoveries:

Rutherford ~ 1911 English Physicist (a student of Thomson)

Gold Foil Experiment

Positively charged alpha particles are shot at a piece of thin gold foil. Most alpha particles had little deflection. Some were deflected at large angles.

• A positively charged core of an atom called the nucleus• Electrons surround the nucleus• The rest of the atom is empty space

Nuclear atom

Quote: "It was about as believable as if you had fired a 15 inch shell at a piece of tissue paper, and it came back and hit you."- Rutherford

Ernest Rutherford

6

History of Atomic Theory

Experiment:

Discoveries:

Model:

James Chadwick ~ 1932 English Physicist (Student of Rutherford)

• A neutrally charged subatomic particle• The particle was called a neutron

Sir James Chadwick• Beryllium foil was bombarded with alpha particles• A neutral radiation was emitted• Emitted radiation would then knock protons out of the nuclei of other substances• The radiation was a stream of neutral particles having the same mass as a proton

Same as Rutherford

7

http://glencoe.mcgraw­hill.com/sites/dl/free/0078759864/164155/00044672.html

Review History of Atom

8

History of Atomic Theory

Experiment:

Discoveries:

Model:

Bohr ~ 1913 Danish Physicist (a student of Rutherford)

Problem: According to the laws of physics, charged particles will radiate energy when orbiting and spiral into the nucleus. Rutherford's atoms would all collapse.

Niels Bohr

Light SpectrumsElectrons give off energy in the form of colored light by falling from an excited state to a ground state.

Photo courtesy NASA

Hydrogen Spectrum

Helium Spectrum

• Electrons can only be found at certain energy levels• Each energy level requires a certain amount of energy• Lower (closer) levels have lower energy• Higher (farther) levels have higher energy

Vocabulary:ground state- all electrons in their lowest possible energy levelsexcited state-electrons absorbed energy & jump to a higher energy level

Ladder Rungs-Cannot stand in between rungs

9

10

11

History of Atomic Theory

Experiment:

Discoveries:

Model:

Quantum Mechanics

Erwin Schrodinger ~ 1926 Austrian Physicist

NONE! It is a mathematical model. It cannot be represented by anything that exists in the real world.

Erwin Schrodinger

Mathematical Equation:

• Mathematical model that deals with the probability of finding an electron within a given space• The probability is 90%• The given space are called orbitals (or electron clouds)• There are four orbitals with different shapes s p d f• These orbitals can be related to the periodic table• Electrons have wave properties

Problem: Bohr's model works well for Hydrogen, but fails for every other element

12

Electromagnetic Radiation: waves that are produced by electrically charged particles

Ex: sunlight, X rays, microwaves

Electromagnetic Spectrum

13

14

All electromagnetic radiation exhibits wave like behavior (wavelength, frequency, and speed)

15

Quantum Numbers: Specify the properties of atomic orbitals and the properties of electrons in those orbitals. Each e- has a set of four numbers; no two electrons in the same atom can have the same four numbers.

Four Quantum Numbers: Principal Angular Magnetic Spin

1. Principal Quantum Number (n)-

2. Angular Quantum Number (l)-

Refers to the distance of the orbital from the nucleus (says which of the main energy levels an e- is in)

• When n=1 is closest to the nucleus and has the least energy• n=1,2,3,4,5, etc.

Refers to the shape of the orbital (also associated with angular momentum)

a. Possible Shapes• s, p, d, f (in order of increasing energy)• l=integer values from 0 to n-1

b. Number of possible shapes is limited by the principal quantum number

16

3. Magnetic Quantum Number (m)-

4. Spin Quantum Number (s)-

Orientation of orbital(s) (direction of angular momentum)

- m=any integer from -l to +l

State of the electron that occupies an orbital

-Electrons are assigned one of the two possible directions it can be spinning

- + spin or - spin

17

18

19

20

21

Heisenberg's Uncertainty Principle: you can never know how fast an electron is moving and where an electron is at the same time. In other words, you can find out where the electron started and you can see where the electron ended up but how it got there WE DON'T KNOW!

22

Electron Configuration: The arrangement of electrons in an atom is known as the atoms electron configuration.

Rules:

1. Aufbau Principle: An electron occupies the lowest energy level that can receive it.

2. Pauli Exculsion Principle: No two electrons in the same atom can have the same set of four quantum numbers.

• If two electrons have the same n, m, and l values they have to have different spins• Two electrons fit into each orbital

3. Hund's Rule: Orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in singly occupied orbitals must have the same spin

23

24

25

4th energy level

3rd energy level

2nd energy level

1st energy level

1s

2s2p

3s

3p

4s

3d

4p

26

27

28

29

30

Attachments

Pictures of atom

Chadwick apparatus

Time line of scientists

Schrodinger equation

Conversation with science

orbitals