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Syllabus
Syllabus
What does this mean?
What does this mean?
History
History
You probably don’t need to know much about Democritus.
He was the Ancient Greek philosopher who imagined the world made up of un-cuttable particles.
Atomos means un-cuttable.
You may need to know that John Dalton revived the idea, this time backed up by evidence.
He decided :
The world consisted of atoms that can’t be created or destroyed
That each element has only one type of atom
Compounds contain more than one type of atom in whole-number ratios.
His idea was that atoms were like tiny pool balls of different sizes and masses but that nothing smaller could exist. A pretty good idea, just not correct
You should know that J.J. Thomson discovered that “cathode rays” consisted of tiny negatively charged particles he called electrons.
Sometimes electrons came out of atoms.
He decided that this meant atoms must be positive with negative electrons scattered throughout them like “plums in a plum pudding”.
Rutherford.
If you only learn about one scientist, learn about Rutherford’s contribution.
He applied a beam of positive Alpha particles to very thin Gold foil.
Alpha particles are smaller than atoms and positive.
He expected most to go through with almost no change in direction.
He would detect the Alpha particles with a fluorescent screen.
Questions.
Questions.
1. How did the results of his experiment agree/disagree with predictions based on the plum pudding model?
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2. What evidence is there for a positively charged nucleus?
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3. How do you think Rutherford estimated the size of the nucleus?
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Sub-atomic particles
Sub-atomic particles
Following the discovery of the proton and the neutron Physics had as almost much knowledge of atoms as required for IB Chemistry!
v You must be able to fill in this table of properties for the 3 sub-atomic particles.
Video
Video
Atomic and Mass Numbers
Atomic and Mass Numbers
On most (but not all) Periodic Tables the Atomic Number is at the bottom.
It is always smaller than the mass number.
v Learn: Atomic Number = Number of Protons in nucleus
v Learn: Mass Number = Number of Protons + Number of Neutrons
v Learn: Atoms contain equal numbers of protons and electrons but ions don’t.
Questions
Questions
1. There are 3 isotopes of Carbon. What is similar/different about them?
2. How many subatomic particles in the following?
3. What is the difference between the numbers of subatomic particles in an atom of Iron and in an Iron (III) ion?
4. Complete the table right.
Note some isotopes have charges.
Radioisotopes
Radioisotopes
Not all isotopes are stable – stable isotopes do not suffer radioactive decay (or have extremely long half-lives). This means they do not change and do not give out nuclear radiation.
Isotopes with short half-lives change quickly. They give out nuclear radiation which can be dangerous. It can also be useful.
Radioactive Tracers.
Radioactive Tracers.
Explain how 131I is used to help image the Thyroid gland. Why is it important that 131I gives out Gamma radiation rather than Alpha or Beta?
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14C dating
14C dating
Carbon 14 is found in all living things. Its half-life is 5730 years. Explain how a scientist might estimate the age of a skeleton from 15,000 years ago.
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What is the half-life of this radioisotope?
How old is a sample with an activity of 25 counts per min?
Mass Spectrometer
Mass Spectrometer
You’re unlikely to ever need to draw a mass spectrometer.
But you should know how one operates as well as what they are used for.
Learn the sequence of events below:
I. Vaporisation (samples must be heated and made into gases if they are not already gas)
II. Ionisation (atoms/molecules are bombarded by high energy electrons which causes them to become (positive) cations
X(g) + e- --> X+(g) + 2e-
III. Acceleration (samples are attracted through negatively charged plates)
IV. Deflection (ions are deflected around a bend by a variable electromagnet controlled by a computer)
V. Detection (ions are detected when they induce an electric current which can be measured)
Deflection: It is easier to deflect lighter atoms/molecules
It is easier to deflect atoms/molecules with bigger charges.
So, the deflection process separates ions according to their mass-charge ratio, the m/z ratio.
VIDEO
VIDEO
Using a Mass Spectrum
Using a Mass Spectrum
This is the simplified mass spectrum of Chlorine.
a Why are there two peaks?
b Why is one peak larger than the other?
c Why should the Relative Atomic Mass of Chlorine be closer to 35 than to 37?
d Calculate the Relative Atomic Mass.
VIDEO
VIDEO
Questions
Questions
1. Calculate the Relative Atomic Mass of Krypton
2. Calculate the Relative Atomic Mass of Strontium
3. Indium has two isotopes 113In and 115In.
The Relative Atomic Mass is 114.82.
Find the abundances of the two isotopes.
4. Nitrogen has two isotopes 14N and 15N.
The Relative Atomic Mass is 14.001
Find the abundances of the two isotopes.
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