The Model of the Atom

Question 1

John Dalton

Answer 1

• atoms make up all matter
• solid, indestructable
• cannot be created/destroyed/divided
• cannot be converted to atoms of another element
• atoms of one elements have same properties (mass, size)
• atoms combine to form compounds

Question 2

Revising Dalton’s Ideas

Answer 2

• atoms are divisible (though smallest particle retaining properties)
• nuclear reaction can convert elements into one another
• isotopes: one element, different # of neutrons

Question 3

JJ Thomson

Answer 3

• positive (protons): cloud of charge
• negative (electrons): in the cloud
• raisin bun model

Question 4

Ernest Rutherford

Answer 4

Gold Foil Experiment
- thin sheet of gold foil and special equipment to shoot Alpha Particles (+) at the gold foil
- most through foil
- some straight back or other direction
Conclusion:
- small positive nuceus
- mostly empty space

Question 5

Problem with Rutherford’s Model

Answer 5

Electrons should continously emit electromagnetic radiation, lose energy, and collapse the atom, BUT this is not what happens

Question 6

Niels Bohr

Answer 6

Solar System Model
- electrons go around nucleus in orbits
- electrons exist in energy levels at a certain distance from the nucleus (not between)
- to move up energy levels: absorb energy equal to E diff between levels

Explains Medeleev’s Periodic Law:
- periods result from filling of energy levels
- max # of electrons = number of elements in each period

Explains line spectrum of Hydrogen

Question 7

James Chadwick

Answer 7

• neutron in the nucleus

Question 8

Absorbtion Spectra

Answer 8

• white light is passed through gaseous sample of element and prism
• black lines observed on continuous spectrum
• represent frequencies (colours/energies) of light absorbed by atom

Question 9

Emission Spectra

Answer 9

• gaseous sample energized until emits light, passed through prism
• distinct light bands of specific colour on black
• represents frequencies (colours/energies) emitted

Question 10

Bohr’s Explanation of Atomic Spectra

Answer 10

• evidence that electron energy is quantized
• electrons are confined to energy levels
• when energized, electrons can be excited to higher energy levels
• missing frequencies correspond to energy difference between the two levels
• when excited electrons lose energy and fall back to ground state, it is released back as a photon of light
• electrons cannot exist between energy levels, so transition is instantaneous = distinct bands

Question 11

Bohr’s Mathematical Model of Hydrogen

Answer 11

Energy of electron at level n:
E(n) = -R(h) x (1/n^2)

R(h) = 2.179*10^-18 J

Energy difference b/w any levels:
E = E(n final) - E(n initial)

Predict frequencies of light:
f = R(h)/h x [1/n(final)^2 - 1/n(initial)^2]

h = Planck’s constant

Question 12

Balmer Series
Paschen Series
Lyman Series

Answer 12

Visible
Infra-red
Ultra-violet

Question 13

Convergence Limits

Answer 13

The higher the energy level, the smaller the energy difference gets (it converges)

This is proportionate to the energy needed to remove an electron

Question 14

Problem with Bohr’s Model

Answer 14

Works only for Hydrogen, since it does not account for electron-electron repulsion

Question 15

Hydrogen Emission Spectrum

Answer 15

410 nm - Violet
434nm - Blue
486nm - Blue-Green
656nm - Red