Flashcards in Atomic Structure and the Periodic Table Deck (45)
What is the mass of an electron?
1/1836 (the mass of a proton)
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons
What is the mass number?
The sum of the number of protons and neutrons in the nucleus of an atom of that isotope
Properties of isotopes
Chemical properties of isotopes are similar, but physical properties (e.g rate of diffusion, boiling point etc) may differ, especially if their relative atomic masses are very different
What is relative isotopic mass?
The mass of an atom of an isotope on the scale on which an atom of the isotope C-12 weighs 12 units exactly
What is relative atomic mass?
The weighted average mass of an atom of a element on the scale on which the isotope C-12 is taken to weigh 12 units exactly
What is relative molecular mass?
The weighted average mass of a molecule of that element or compound on the scale on which the isotope C-12 is taken to weigh 12 units exactly
Main steps of a mass spectrometer
1) Vaporise the sample (if it is not a gas) so it can move through the machine
2) The sample enters the ionisation chamber where it is ionised
3) Positive ions are then accelerated using an electric field of over 1000V and they may travel at up to 100,000 mph
4) Ions pass through a velocity selector, meaning the ions that pass into the rest of the spectrometer are all travelling at the same speed
5) Ions are passed into a magnetic field where they are deflected. The amount they are deflected depends on their charge and mass. The lighter and higher charged the ions, the more they will be deflected. Amount of deflection is proportional to the square root of charge/mass
6) Ions are detected. If the electric and magnetic field remain constant, then ions of 1 particular mass charge ratio will hit the ion detector (ions with a smaller mass charge ration will be deflected too much, and bigger ones too little.
Why is the sample ionised? (mass spectrometer)
So it can be accelerated in the electric field and deflected in the magnetic field
How is the sample ionised? (mass spectrometer)
Usually by the electron impact method - a stream of high energy electrons bombards the sample of vapour atoms/molecules to remove electron(s). Usually only 1 electron is removed
e^- + M - > M^+ + e^- + e^-
high energy electron + atom - > positive ion + electron knocked out of M + high energy electron retreating
2 electrons may also be knocked out of M
High energy electrons are produced by a heated filament
How does the detector in a mass spectrometer work and how does it produce a trace?
Positive ions strike the detector and give a current. The detector is linked through an amplifier to a recorder and the current is measured. The magnetic field is slowly increased and ions of increasing mass are detected. A trace is printed out, giving the masses of the ions and their relative abundances (found from the height of the peaks)
How do we know a mass spectrometer is accurate?
It is calibrated by using standard compounds with very accurately known relative atomic masses
Why is the mass spectrometer apparatus maintained under high vacuum?
To prevent collisions of ions with air particles
Fragmenting (mass spectrometer) (Butane as an example)
Butane has a molecular ion peak/parent ion peak at m/z = 58 - its Mr, but that is not the largest peak. The butane molecule has been fragmented, some of which have a positive charge and so produce a peak in the mass spectrometer. This happens because the beam of electrons used to generate the positive ions may break chemical bonds, causing positively charged fragments to be formed
Notes about fragmentation (using butane as an example)
The most common and therefore most stable ion is at the highest peak and is known as the base peak (at m/z = 43 in Butane).
A small peak at m/z = 15 indicates a methyl group (CH3). This peak is smaller because of the lower stability of the ion. CH3 is a free radical, meaning it has unpaired electrons
Uses of mass spectrometry
1) Detection of drugs in urine samples
2) In the pharmaceutical industry
3) Radioactive dating
What is spectroscopy?
The study of radiation emitted or absorbed by bodies is called spectroscopy. A substance is excited by the absorption of energy and it emits energy as it returns to its normal or ground state
Emission spectra are produced by excited bodies emitting energy and returning to their ground state, Gas atoms/ions produce line spectra, which is a series of lines of different wavelengths. Every element has a specific spectrum and so substances can be analysed to find what substances are present and at what concentration
How are emission spectra produced?
1) It can be vaporised in a Bunsen burner flame. This happens in flame tests of alkali metals and alkaline earth metals in particular
2) Gases may be excited by applying a high voltage to gases at a low pressure in a discharge tube
3) If 2 metal rods in contact are connected to a DC supply and are then drawn a few mm apart then a continuous spark is obtained across the gap, called an electric arc. If a sample of solid is placed in the gap it vaporises and gives an emission spectrum
What is first ionisation energy?
This is the energy required to remove one mole of electrons from one mole of free gaseous atoms if an element, giving 1 mole of unipositive ions
Ne (g) - e^- -> Ne^+ (g)
What is second ionisation energy?
This is the energy required to remove 1 mole of electrons from 1 mole of free gaseous unipositive ions of an element, giving 1 mole of dipositive ions
Why are logarithms of ionisation energies used on graph axis rather than just the ionisation energy?
There is a large range of values, and so plotting log values allows for a meaningful scale
Which group of elements have the highest ionisation energies?
Which group of elements have the lowest ionisation energies?
What happens to the ionisation energies as you go down the group?
Ionisation energy decreases. Going down the group, nuclear charge increases, but the distance between the outer electron and the nucleus increases as the number of shells increases.
There are also more shells of electrons between the nucleus and the outer electron, resulting in an increased shielding effect, which shield the outer electron from the effect of the nucleus, making it easier to escape. More complete shells = more shielding, so a lower ionisation energy
Going down the group, the increase in the nuclear charge is outweighed by the increased distance of the outer electron and increased shielding effect, so the 1st ionisation energy decreases
What happens to the ionisation energy as you move across a period? (GENERALLY)
Going across the period, an extra electron is added to the outer shell and an extra proton to the nucleus. The shielding effect hardly changes, but the extra proton can pull the electrons in more closely and make it more difficult for an electron to be lost, therefore ionisation energy increases
Distance - slight decrease
Shielding effect - stays the same
Nuclear charge - stays the same
What are the subshells?
1s, 2s, 2p, 3s, 3p, 3d,
(Then continues: 4s, 4p, 4d, 4f etc, but don't need to know)
What is the order of energy levels in the subshells?
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 4d,4f
What is an orbital?
An orbital is a 3-dimensional volume of space surrounding the nucleus in which there is a high probability of finding an electron. Each orbital has a fixed energy level and can hold up to 2 electrons