atomic structure topic 2 and 12

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

Important Notes Page 1


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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:

atomic structure topic 2 and 12

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