atomic structure topic 2 and 12
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8/12/2019 Atomic Structure Topic 2 and 12
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Standard Level
2.1 The mole and Avogadro's constant
2.1.1 State the position of protons, neutrons and electrons in an atom
The protons and neutrons are in the nucleus, the electrons go around nucleus.
2.1.2 State the relative masses and relative charges of protons, neutrons and electrons
Relative mass Relative charge
p 1 +1
n 1 0A.
e 5 x 10-4 0 -1
2.1.3 Define the terms mass number (A), atomic number (Z) and isotopes of an element
Mass number (A): Total number of neutrons and protons in its nucleus
Atomic number (Z): number of protons
Isotopes: Atoms of the same element (species with the same Z) but not the same A
2.1.4 Deduce the symbol for an isotope given its mass number and atomic number
Atomic Number stays the same - mass number changes
X: Element Symbol
A: Mass Number
Z: Atomic Number
2.1.5 Calculate the number of protons, neutrons and electrons in atoms and ions from the mass number,
atomic number and charge.
Electrons: The number of electrons is equal to the atomic number. An ion will have it's superscript number
added.
Protons: The number of protons is equal to the atomic number.
Neutrons: The number of neutrons is equal to the atomic number subtracted from the mass number.
For a given element, the atomic number doesn't change.
2.1.6 Compare the properties of the isotopes of an element
Isotopes don't vary in their chemical properties because chemical reactions only deal with the movement of
electrons. However, there can be various physical differences between the isotopes of one element. Some
differences include density, rate of diffusion if gas, mass, radioactivity etc.. These difference help in identifying the
isotope.
The reason why elements have fractional relative masses and not whole numbers is because of the existence of
isotopes.
To find relative atomic mass, use the following formula:
Where Ar = Relative atomic mass of the element X of any given element
2.1.7 Discuss the use of radioisotopes
The stability of a nucleus depends on the balance between protons and neutrons. When a nucleus contains either too
many or too few neutrons, it is radioactive and changes to a more stable nucleus by giving out radiation.
Radioactive substance: unstable nuclei (change in number of neutrons) are not in the band of stability (graph) for
example, the radiation of alpha particles, beta particles or gamma rays. Radioactive isotopes can be used in society
for various tasks, however it must be internationally monitored so that no one is exposed to serious radiation.
This isotope contains 8 neutrons which is too many to be stable
However when an organism dies, carbon-14 decays
Helpful for finding dates of archaeological objects through the knowledge of the half life (5730 years).
The relative abundance of this isotope in living plants is constant,
Carbon-14 Dating
This isotope can be used in the treatment of cancer
Damages good cells too, however it can be controlled with proper radiotherapy and normal cells can recover
Cobalt-60:
Atomic Structure: Topic 2 and 12Friday, 25 March 2011
10:03 PM
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This is due to its protons and neutrons change their positions ins the nucleus
Cobalt-60 is used because it emits strong gamma radiation
This isotope can be used as a medical tracer
however due to radioactivity, it can be monitored
Radioisotopes have same chemical properties of the same element
Can diagnose and treat thyroid cancer
Used as a medical tracer because of radioactivity
Quickly eliminated from body because of its short half-life
Iodine-131:
2.2 The Mass Spectrometer
2.2.1 Describe and explain the operation of a mass spectrometer
A mass spectrometer is used to determine the masses of elements and their isotopes
Vaporisation: Allows individual analysis of atoms
Ionisation: Atoms get an electron kicked out by being bombarded with
high energy electrons:
Acceleration: Positive ions attracted to negatively charged plate -
accelerated through a hole in the plate (electric field)
Deflection: The ions are deflected by the magnetic field. The amount
of deflection is proportional to m/z ratio.
Ions with smaller mass/higher charge are deflected more
Detection: (+) ions detected. The strength of the signal is the measure
of the number of ions with a particular m/z ratio.
2.2.2 State the relative masses and relative charges of protons, neutrons and electrons
Mass spectrometer can be used to find the mass of atoms range from 10 -14g to 10-22g.
All relative masses relative to 12C, taking into account the relative abundances of all the naturally occurring isotopes of the
element. Because it's a relative term, it has no units.
2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.
Results of mass spectrometer is given as mass spectra. First find total mass, then divide by 100. The following graph is
example of the mass spectrum of the element Rubidium.
Deduce the relative atomic mass of the element Rubidium
from the data given:
77% have a mass of 85, and 23% have a mass of 87
Consider a sample of 100 atoms.
= 8546
Total mass = (8577) + (8723)
Therefore the relative atomic mass
(average mass of atom) is = 8546/100=85.46
2.3 Electron Arrangement
2.3.1 Describe the electromagnetic spectrum
All electromagnetic radiation travel at the speed of light. For this reason they can be distinguished by their varying wavelength ().
For example, red light has longer wavelength than blue light, however blue has a higher frequency (f).
When white light such as sunlight shines through a prism, a continuous spectrum is produced, meaning all colours.
Here is a diagram of the electromagnetic spectrum:
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2.3.2 Distinguish between a continuous spectrum and a line spectrum
There are two types of spectra that can show absorption and emission of electromagnetic radiation.
Continuous
spectrum
The continuous spectrum shows unbroken sequence of frequencies such as the spectrum of visible light. This is an example of the
spectrum of white light through a prism:
Line Spectrum Only has certain frequencies of light. It is produced by excited atoms/ions as they fall back to a lower energy level. Line spectrum is an
emission spectrum of an element. This is the visible emission spectrum of hydrogen:
These lines form the Balmer series and they converge at higher energies. The colours present in the emission spectrum are
the same as those that are missing from the absorption spectra. Different elements have different line spectra, so they are
used to identify elements and find e- arrangement.
2.3.3 Explain how the lines in the emission spectrum of hydrogen are related to electron energy levels
According to the Bohr model of the atom, electrons move around the nucleus in fixed energy levels called shells. The shells close to the
nucleus are of low energy and those further out are of higher energy. The maximum number of electrons is 2n2. The electrons in the outer
shell are called valence electrons. The electrons in shell n=1, are the lowest energy state (ground state).
If atoms are subjected to large amounts of energy, the electrons jump to higher energy levels further from the nucleus and when the
electrons return to the ground state the extra energy is released as light. Because there are fixed energy levels, only a specific wavelength is
emitted. This can be seen in the emission spectrum of an element, however some of the emissions may be radiation of a wavelength that is
not visible to the naked eye.
The movement of electrons in different energy levels. Each line in the hydrogen emission spectrum corresponds to a transition between two
energy levels of the hydrogen atom. The energy levels of an atom become close together the further they are from the nucleus. The lines in
each series of the emission spectrum become closer together as the energy increases (and the wavelength decreases). The lines converge at
higher energies because the energy levels inside the atoms are closer together. When the electron is at n = , it is no longer in the atom. The
atom is said to be ionisedand the energy needed for an electron to escape is called ionisation energy.
2.3.4 Deduce the electron arrangement for atoms and ions up to Z = 20
For atoms with an atomic number ranging up to Z=20, the electron arrangement starts at 2 electrons in the first shell, then 8 in subsequent shells.
E.g. for Z(Al) = 13, the electron arrangement is 2.8.3. If it is an ion, you take into account the number of electrons either gained or lost (Al3+ 2.8)
Higher Level12.1 Electron Configuration
12.1.1 Explain how evidence from first ionisation energies across periods accounts for the existence of main energy levels
and sub-levels in atoms
Consider the graph below showing the ionisation energies across for elements Hydrogen to Xenon.
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The peaks correspond to the nobles elements in group 0 of the periodic table of elements. These elements have high first ionization
energies because they have a full electron shell and an associated high degree of energetic stability. The drops are the elements in group 1
which one have one electron in the outer shell - little energy is needed for these electrons to leave the atom.
12.1.2 Explain how successive ionisation energy data is related to the electron configuration of an atom
The pattern in successive ionisation energies reflects the electron
arrangement in the atom. Jumps in the ionisations energies within an
energy level indicate the presence of sub levels.
For example the jump between the ninth and tenth ionisation energies shows
that the 11th electron in Aluminium is more difficult to remove.
Therefore, it is seem that the energy level has been divided into sub-shells.
Successive ionisation energy show it's easier to remove 2p shell than the 2s sub-shell.
12.1.3 State the relative energies of s, p, d and f orbitals in a single energy level
12.1.4 State the maximum number of orbitals in a given energy level
12.1.5 Draw the shape of an orbital and the shapes of the px, py, pzorbitals
s p d
12.1.6 Apply the Aufbau principle, Hund's rule and the Pauli exclusion principle to write electron configurations for atoms
and ions up to Z = 54
The Aufbau Rule considers the electrons behaviour around each other. It states that electrons are placed into orbitals of lowest energy first.
Boxes can be used to present the atomic orbitals with single-headed arrows to represent the spinning electrons.
Heisenberg's uncertainty principle says that because of electrons dual nature and it's
wave-like properties, it is impossible to determine where the electron is. Therefore, with
the invention of quantum mechanics, electrons were placed ins orbitals.
Orbitals are regions of space in which electrons may be found. Schrdinger's wave
equation gives the probability of the whereabouts of an electron within an orbital. When
this probability is plotted on a 3 dimensional axes, the shapes of orbitals can be seen.
The atomic orbital represents the 90% probability of there being an electron.
A group of orbitals with the same energy is called a subshell. The number of
orbitals that make a subshell increases as the energy increases.
An orbital, which is a wave description of the electron, shows the volume of
space in which the electron is likely to be found.
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The Pauli Exclusion principle states that no two electrons can occupy any one orbital, and if two electrons are in the same orbital they must
spin in opposite directions. For this reason, within a box the arrows point in different directions.
Hund's rule states that if more than one orbital in a sub-level is available, electrons occupy different orbitals with parallel spins.
However there are exceptions to these rules:
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