1. What are the different parts of an atom ?
2. If an atom is the size of the earth, how big would the nucleus be ?
3. What is an ion ? What are they called ?
5. What is the atomic number ?
7. How do we write the symbol for an atom ?
10. What are the different characteristics of different isotope for the same substance ?
11. What about isotopes and radioactivity ?
12. What are radioactive isotopes used for ?
13. What is an atomic mass unit ?
13. What is the mass and charge of electrons, neutrons and protons ? ?
14. What is a mass spectrometer used for ?
15. Draw a sketch of a mass spectrometer.
16. What is five basic operations of a mass spectrometer ?
17. How is the atomic mass of an atom calculated ?
20. What is the difference between a line spectrum and a continuous spectrum ?
22. Make a drawing of the main energy levels in the hydrogen atom.
23. What does it mean that an atom is excited ?
24. How many electrons can one put in every main energy shell ?
25. What is Planck's formula for the energy of a photon ?
26. What is the difference between the Bohr model and the Wave model of an atom ?
Download notes on electron shells.
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Nucleus: Protons and Neutrons form the nucleus of the atom.
Electrons Electrons orbit the nucleus and form a cloud around it. The electrons can only be in certain
energy levels called electron shells which defines what the electron cloud look like.
If we let the earth represent an atom, size-wise, a nucleus is roughly a soccer ball. So electrons occupy the majority of an atom.
An ion is an atom which has gained or lost one or a couple of electrons so that it becomes
positive or negative.
Cation: Positive ion (has lost electrons).
Anion: Negative ion (has gained electrons).
Isotopes are atoms with the same number of protons but different number of neutrons.
Z: The atomic number is the number of protons in the nucleus
A: The mass number is the number of protons + neutrons in the nucleus
X = atomic symbol (H, He, Cl, O, Na ....).
Note! The mass number (the large number) on top left, atomic number
(number of protons) at the bottom.
number of protons = Z
number of electrons = Z (unless the atom is an ion because then it is Z - charge number)
number of neutrons = A - Z
Two isotopes of the same substance have the same chemical properties but different mass. The different mass means that they have different density, and they can then have different melting and boiling points.
Some isotopes have too few neutrons or too many neutrons. They then want to fall apart (decay) because they are not stable. They are then said to be radioactive. If there are too few neutrons, the isotope likes to send out alpha radiation (helium nuclei), If there are too many neutrons, the isotope likes to send out beta radiation (electrons).
The radioactive isotopes are used to determine the date of a certain object (ex.14C carbon dating), perform radiotherapy (ex.60Co), and tracing molecules for medical or laboratory needs.
Since the mass of electrons are negligible and the mass of a proton is
roughly the same as a neutron, the mass of an atom can be relatively
represented as the number of protons and neutrons combined. And such
standard - the mass of a proton/neutron ( 1/12 of the mass of a
carbon-12 atom in practice) is called 'amu', atomic mass unit.
1 amu = 1.66053886 × 10-24 grams
| Particle | Mass (amu) | Charge |
|---|---|---|
| Protons | 1 | +1 |
| Neutrons | 1 | 0 |
| Electron | 1/1840 | -1 |
A mass spectrometer is exploiting the difference
in mass among different isotopes to separate them. Imagine you throw stones of different
weight away with roughly the same strength. The lighter one would travel
further while the heavier one would not fly for long. The
separation of isotopes is the same idea.
The stages of operation are: vaporisation, ionisation, acceleration, deflection and detection.
Substance to be tested is vaporised (by heat, in the absence of
oxygen) then ionised by electric current. Ions are accelerated through
an electric field, then deflected by a magnetic field. Ions are then
detected. The angle of deflection due to the magnetic field reflects
their mass to charge ratio.
The angle of deflection of each fragment is proportional to its mass,
and so it is possible to find the relative atomic mass of each 'spike'.
The height of the spike represents the frequency, therefore, the
abundance can be calculated. The relative atomic mass is the weighted
average of the isotope masses times their percentage abundance
(frequencies).
Atomic masses are the average of the actual atomic mass of each isotope (isotopic mass) times the isotope’s relative abundance.
Bromine has 2 stable isotopes: 79Br & 81Br.
Their natural abundances are 50.69% and 49.31% respectively. Then the
average atomic mass of bromine can be approximated (using the isotopic
mass numbers) as 79.99. A = (79•0.5069 + 81•0.4931 = 79.9862 ≈ 79.99).
Boron exists as two isotopes: 10B and 11B. The atomic mass is 10.81.
What is the abundance of the two isotopes ?
A = 10x + 11(1-x) = 10.81 where x is the fraction of 10B isotopes and 1-x is the fraction of 11B isotopes.
x = 11 - 10.81 = 0.19 = 19%. This is the abundance of 10B.
1-x= 1-0.19 = 0.81 = 81%. This is the abundance of 11B.
Continuous spectrum goes continuously through red, orange, yellow,
green, blue, indigo, violet. A line spectrum contains only some
individual lines from this spectrum.
Atoms can be excited if they absorb energy. This causes electrons to 'jump' into higher electron shells ( X
-> X* ). This state is only temporary, however, and the
electron falls back to it's ground state. This change decreases the
energy of the electron, and this energy is emitted in the form of a
photon. If this photon falls into the visible spectrum of light, then it
produces a visible spectrum. As electrons move further away from the
nucleus, the electron shells become closer together in terms of space
and energy, and so lines converge towards the end of the spectrum.
The main electron levels have the following number of electrons before they are full:
2, 8, 18 etc…2n2 for n = 1 , 2, 3 .......
The energy of a photon emitted by an atom is given by its frequency (f) because E = h•f where h is a constant.
Bohr model:In this model it is assumed that electrons move in circular orbits around the nucleus. And that
each orbit corresponds to an energy level.
The Bohr model is not correct but can still be used to calculate many things.
Wave model:In this model the electrons are instead moving everywhere but they are more likely to be in some regions than others.
The most probable regions to find the electrons in are called orbitals. These can have a spherical shape but also other very strange shapes.
