3.1 and 3.2 - Atom & Quantum Theory, Bohr's Model

Question 1

What is matter made of?

Answer 1

Democritus speculated that matter is composed of atoms, and scientists were later able to state with CERTAINTY that matter is in fact composed of TINY particles called ATOMS.

Question 2

Who proposed the first modern atomic theory?

Answer 2

John Dalton

Question 3

Dalton’s atomic theory

Answer 3

1. Matter consists of small particles called atoms
2. Each element is made up of its own type of atom
3. Atoms of different elements have different
   properties
4. Atoms combine in whole number
   ratios (e.g. H2O)
5. Atoms cannot be created,
   destroyed, or subdivided in a chemical change

Question 4

JJ Thomson

Answer 4

Used a Cathode Ray Tube to discover the presence
of electrons

Electrons are negatively charged

The overall charge on the atom is neutral

Electrons balance out with positive parts

Question 5

Ernest Rutherford

Answer 5

Concluded that an atoms contains a positively
charged “nucleus” surrounded by mostly
empty space

Some of this space contained negative
electrons

Question 6

Rutherford’s nuclear model

Answer 6

A model of the
atom in which
electrons move
around an
extremely small,
positively
charged nucleus

Question 7

Limitations of rutherford’s model

Answer 7

A nucleus made of entirely positive charges should
break apart due to electrostatic forces of repulsion

The model did not account for all of the atom’s mass –
(the neutron would not be discovered for another 20
years)

Electrons are accelerating bodies (their speed and/or
direction is changing) so they should be emitting
radiation (light) as they move, according to 20th
century scientific laws

This would mean the electrons would lose energy and should spiral in toward the nucleus and obliterate the atom (which, thankfully, doesn’t happen!)

Question 8

James Chadwick

Answer 8

Suggested the presence of neutrons in the nucleus
(have no charge)

ISOTOPES: atoms of an element that contain the same number of protons BUT different number of neutrons

E.g. Carbon-12 vs. Carbon-13

Question 9

Radioisotopes

Answer 9

Isotope with an unstable nucleus, meaning that the nucleus decays and emits radioactive gamma rays.

Question 10

Robert Millikan

Answer 10

Determined the charge of an electron.

Discovered that the fall of charged oil droplets
due to gravity could be stopped by adjusting the
voltage across two charged plates.

He was able to calculate the charge on the oil
drop from the voltage and the mass of the oil
drop. Using this value, and the charge-to-mass
ratio determined by Thomson, Millikan calculated
the mass of an electron to be….

9.1 x 10-31 kg.

Question 11

What did Bohr discover

Answer 11

Electrons have lowest energy
in energy level #1

Electrons fill the 1st level first
then 2nd, 3rd..

Electrons are in orbits
around nucleus

Electrons can JUMP from one
level to the next

Most of mass is in the
nucleus

Question 12

How did Bohr develop his model of the atom?

Answer 12

He observed the line spectra of elements
when they were heated up or subjected to
electricity

The electrons would jump to a new energy
level and then fall back down and give off
colour

Ground state (where e- starts) 🡪 excited state
(where e- jumps to)

Question 13

Emission spectrum of Hydrogen

Answer 13

The wavelength (colour)
of light depends on
where the electrons are
jumping from and to!

Question 14

Limitations of Bohr’s Model

Answer 14

The model could only explain behaviour in
one-electron atoms or ions

It did not explain the behaviour of ions or
atoms with more than one electron

The model did lay the groundwork for the
next generation of scientists developing the
quantum mechanical model of the atom

Question 15

Planck’s Quantum Hypothesis

Answer 15

-energy is not radiated or emitted continually
-it comes in packets known
as quanta