Atomic models 2011.notebook

1

September 16, 2011

Sep 1­12:00 AM

It all started in Greece about 2500 years ago

• Democritus • matter is discontinuous• indivisible• like grains of sand• differences in arrangement account for all elements

Sep 1­12:03 AM

Along came Aristotle about 100 years later...

• matter is continuous• made of earth, wind, water, fire• a magic knife could cut it forever• particles divisible

Sep 1­12:08 AM

John Dalton ­ 1800's

• all matter made of atoms (indivisible)

• atoms of same element are identical

• atoms of different elements are different

• during chemical reactions atoms combine in definite proportions to form products

Sep 1­12:12 AM

J.J. Thompson ­ 1906

• discovered the electron, a negative (­) particle that makes up part of the atom.

• They can detach themselves easily (static electricity)

• The atom is a positively (+) charged ball with electrons (­) imbedded

• Atoms are neutral + = ­

• Atoms are divisible

Sep 1­12:15 AM

Rutherford and the Gold Foil Experiment(McGill 1911)

• Radioactive substances can give off alpha (+), beta (­) and gamma (neutral) radiation

• Rutherford bombarded gold foil with alpha (+) particles

Sep 1­12:17 AM

Rutherford and the Gold Foil Experiment (Con’t)

Atomic models 2011.notebook

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September 16, 2011

Sep 1­12:18 AM

Rutherford and the Gold Foil Experiment (Con’t)

Results:• Most alpha (+) particle passed through

• A few hit the gold foil and bounced back

Conclusions:

• The atom is mostly empty

• The (+) alpha particles hit a concentrated (+) area called a nucleus

• The nucleus is Positively (+) charged

Sep 1­12:22 AM

Rutherford and the Gold Foil Experiment (Con’t)

Sep 1­12:24 AM

Rutherford’s Atomic Model

• The nucleus contains positively (+) charged particles called Protons

• There has to be an equal number of negatively (­) charged Electrons circling the nucleus to cancel each other out

Sep 1­12:25 AM

The Problem with Rutherford’s Model

• Opposite charges attract each other; the atom should collapse.

• What prevents the (­) electrons from crashing into the (+) protons ?

Sep 1­12:27 AM

The Rutherford­Bohr Model

Bohr’s Contribution • White light is broken into the electromagnetic spectrum

Sep 1­12:28 AM

• Bohr concluded electrons could occupy specific orbits (energy levels, shells)

• Electrons could jump orbits when energy (heat or electricity) is supplied

• When an electron falls back to it’s original orbit it gives off the energy in the form of colored light

Atomic models 2011.notebook

3

September 16, 2011

Sep 1­12:30 AM

Rutherford­Bohr Model

• Drawing the first 20 elements

• Look at the atomic #

• Atomic # = the # of protons (in nucleus)

• # of protons = the # of electrons (in orbit)

• Carbon: 6 protons = 6 electrons

• Electrons fill specific orbits or Energy Levels

Sep 1­12:33 AM

Orbits or Energy Levels

• 1st level: 2 e­

• 2nd level: 8 e­• 3rd level: 8 e­ • 4th level: 2 e­

Sep 11­8:58 PM

Bohr­Rutherford Diagrams

1. Protons go in nucleus2. Fill each shell before moving to the next

Ex: Nitrogen (N) Atomic # 7 7 protons, 7 electrons

Sep 11­9:08 PM

Ex: CalciumAtomic number 2020 protons, 20 electrons

Sep 1­12:39 AM

The Simplified Model(Similar to Rutherford­Bohr except...)

• Chadwick discovered the Neutron• It is in the nucleus• It holds protons together• It is neutral• The number of Neutrons is determined by,

Atomic Mass ­ Atomic # = # neutrons• The Atomic Mass can also be determined by # Protons + # neutrons = Atomic Mass

Sep 11­9:10 PM

Ex: Magnesium (Mg)Atomic number 12Atomic mass 2412 protons, 12 electronsmass ­ protons = neutrons24 ­ 12 protons = 12 neutrons

Atomic models 2011.notebook

4

September 16, 2011

Sep 11­9:17 PM

Ex: Aluminum (Al)Atomic number 13Atomic mass 2713 protons, 13 electronsmass ­ protons = neutrons27 ­ 13 = 14 neutrons

Sep 11­9:21 PM

Match the protons, electrons and neutrons with the elements in the table

Element Protons Electrons Neutrons

Chlorine (Cl)

Potassium (K)

Carbon (C)

8

6

12

184019 6

194018

12 21

3517 18

17

6

9

14

Sep 1­12:39 AM