 The modern atom has gone through a few stages of
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development
Dalton’s Atomic Therory – idea of an atom
JJ Thompson – 1890 – negative charge (electrons)
Earnest Rutherford – 1911 - positive nucleus (protons)
Niels Bohr – 1913 – orbital shells
Chadwick – 1932 – neutrons
 This is a VERY simplified idea of the atom
 Nucleus
 Protons – positive charge – 1.6 x 10-19C
 Neutrons – no charge
 Diameter order of 10-15m
 Electron “cloud”
 Electrons – negative charge – 1.6 x 10-19C
 Diameter order of 10-10m
 The nucleus is about 100,000 times smaller than the
electron orbits.
 Imagine a pea in the center of a football field with the track
being the orbits.
 Protons and Neutrons have very similar mass.
 Protons and Neutrons are about 1800 times bigger than
electrons.
 ****IB DATA***
 See the data book for actual values.
Dalton’s Atomic Theory
1. All matter is composed of extremely small particles
called atoms.
2. All atoms of a given element are identical.
3. Atoms cannot be created, divided into smaller
particles, or destroyed.
4. Different atoms combine in simple whole number
ratios to form compounds.
5. In a chemical reaction, atoms are separated,
combined or rearranged.
Deomcritus
Atoms
Differences in atoms
1. All matter is composed of extremely small particles
2.
3.
4.
5.
called atoms.
All atoms of a given element are identical.
Atoms cannot be created, divided into smaller
particles, or destroyed. (This part proven wrong)
Different atoms combine in simple whole number
ratios to form compounds.
In a chemical reaction, atoms are separated,
combined or rearranged.
Deomcritus
Atoms
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
 J. J. Thomson – 1890-1900
 Used cathode ray tube to
prove existence of
electron.
 Proposed “Plum Pudding
Model”
 Cathode ray tube
 Stream of charged
particles (electrons).
 Plum Pudding
 J. J. Thompson
 Plum Pudding Model
Deomcritus
Thompson
Atoms
•Atoms composed
of electrons
Differences in atoms
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
 Gold Foil experiment
 Used to prove the existence
of a positively charged core
(Nucleus)
 Fired alpha
particles(2protons and 2
neutrons) into very thin gold
foil.
 The results were “like
firing a large artillery shell
at a sheet of paper and
having the shell come back
and hit you!”
 What should have
happened
• What DID happened
 After performing hundreds of tests and calculations,
Rutherford was able to show that the diameter of the
nucleus is about 105 times smaller than the diameter of the
atom
Deomcritus
Thompson
Atoms
•Atoms composed
of electrons
Differences in atoms
Rutherford
Dalton
•Atoms
•Sameness
•Created/destroyed
•Combination
•Rearragement
•Positively Charged
Nucleus
 Chadwick
 Worked with Rutherford.
 Noted there was energy in the nucleus, but wasn’t the
protons.
 Concluded that neutral particles must aslo exist in nucleus.
 James Chadwick – 1932
 Bombarded a beryllium target with alpha particles
 Alpha particles are helium nucleus
 Discovered that , carbon was produced with another
particle.
 **** Write reaction on board****
 Concluded this particle had almost identical mass to
proton but no charge.
 Called it a neutron
Deomcritus
Thompson
Atoms
•Atoms composed
of electrons
Differences in atoms
Rutherford
Dalton
•Atoms
•Positively Charged
Nucleus
•Sameness
Chadwick
•Created/destroyed
•Neutrons
exist in
Nucleus
•Combination
•Rearragement
 Three main particles:
 Proton
 Positive
 In nucleus
 Neutrons
 Neutral
 In nucleus
 Electrons
 Negative
 Orbiting the nucleus (not inside)
 If low-pressure gases are heated or current is passed through
them they glow.
 Different colors correspond to their wavelengths.
 Visible spectrum 400nm(violet) to 750nm(red)
 When single element gases such as hydrogen and helium
are excited only specific wave lengths were emitted.
 These are called emission line spectra
 If white light is pass through the gas the emerging light will
show dark bands called absorption lines.
 They correspond to the emission lines.
 Rutherford’s model didn’t explain why atoms emitted or
absorbed only light at certain wavelengths.
 1885 JJ Balmer showed that hydrogen’s four emission lines
fit a mathematical formula.
 This “Balmer series” also show the pattern continued into
non-visible ultra-violet and infra-red.
 Bohr called these “energy levels”
 Reasoned that the electrons do not lose energy
continuously but instead, lose energy in discrete amounts
called “quanta”.
 He agreed with Rutherford that electrons orbit the nucleus
but only certain orbits were allowed.
 Bohr explained the emission and absorption line spectra
with the idea that electrons absorbed only certain quantity
of energy that allowed it to move to a higher orbit or energy
level.
 Each element has its own “finger print”.
 IB Definitions
 Nucleon – any of the constituents of a nucleus. Protons
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and neutrons.
Atomic Number – The number of protons in the nucleus.
Nucleon Number – The number of nucleons in the
nucleus. AKA the mass number. (protons + neutrons)
Isotope – Nuclei which contain the same number of
protons but different numbers of neutrons.
Nuclide – the nucleus of an atom. The nuclides of
isotopes are different, even though they are the same
element.
 Atomic Number (proton number), Z
 How many protons there are.
 This is what defines the element.
 Ex. Hydrogen Z =1, Oxygen Z = 8 Carbon Z = 6
 Nucleon Number (mass number), A
 How many nucleons there are.
 Protons + neutrons
 Number of neutrons, N
 Mass number = atomic number + number of neutrons
 A=Z+N
 Standard notation is: A over Z in front of element(X)
*****Draw on board*****
Isotopes
 More evidence for neutrons is the existence of isotopes.
 When nuclei of the same element have different numbers
of neutrons.
 Carbon has 6 isotopes: Carbon-11, Carbon-12, Carbon-13,
Carbon-14, Carbon-15, Carbon-16.
 All have 6 protons but each has different number of
neutrons.
 The different isotopes don’t exist in nature in equal
amounts.
 Carbon:
 C – 12 is most abundant (98.9%)
 C – 13 is next (1.1%)
 This is where atomic mass comes from. It’s the weighted
average mass of all the different isotopes.
 Nuclei of different atoms are known as nuclides.
 Ex. C – 12, C – 14
 Both are carbon but different isotopes
 Their nuclei have different numbers of neutrons.
 These are different nuclides.
 How do like charge (protons), stay stuck together?
 We already know that like charges repel each other.
 We have also seen that they are stronger than gravitational
forces.
 Strong Force – The force that binds the nucleus together.
 It is an attractive force that acts between all nucleons.
 Short – range interactions only (up to 10-15m)