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Relative molecular mass
The weighted average mass of a molecule, in a given sample of that molecule, relative to 1/12 of the mass of a carbon-12 atom
relative formula mass
The weighted average mass of one formula unit, relative to 1/12 of the mass of a carbon-12 atom
Unified atomic mass unit
Measured relative to 1/12 of the mass of a carbon-12 atom
Relative atomic mass
the average mass of an ATOM (considering the relative abundance of isotopes), relative to 1/12 of the mass of a carbon-12 atom
How to calculate RAM from a mass spectrum
The mass/charge ratio (x-axis) will tell you the mass of the isotope (since charge is usually +1). You can convert the relative abundances into percentage abundances by summing the abundances and then calculating the fractions that individual abundances are of the total abundance.
How does a Time of Flight (ToF) mass spectrometer work?
Sample is vapourised
Atoms/molecules are ionised by firing electrons at them, which will knock an electron out of the outer shell, so the atom/molecule has a charge of +1
Ions are accelerated using an electric field
Deflect particles using a magnetic field
Deflected ions induce a current on a detector and this can be used to calculate abundance
What does the mass spectrum of organic molecules tell you about the molecule?
The number of carbon atoms
Which peak to you look at on a mass spectrum of an organic compound to determine the number of carbon atoms? What’s the exception?
The one with the greatest mass/charge ratio, i.e. the one furthest to the right. Exception: when the M+ +2 peak is equal to or a third of the M+ peak: equal means that there is one bromine atom in the molecule; a third means that there is one chlorine atom.
M+ peak / molecular ion peak
The peak on the mass spectrum with the highest mass/charge ratio that tells you the mass of the species.
Beam of neutrons moving in an electric field
Will continue straight and not deflect, as neutrons are uncharged particles
Beam of protons moving in an electric field
Will deflect towards the cathode (negatively charged plate) as protons are positively charged
Beam of electrons moving in an electric field
Will deflect towards the anode (positively charged plate) as electrons are negatively charged
Comparing a beam of protons and a beam of electrons, moving at the same velocity, g in the same electric field
The beam of electrons will deflect more than the beam of protons, as they have a significantly lower mass (relative mass of 1/1836 compared to 1)
Atomic radius across a period
Decreases, as the nuclear charge increases and the number of electrons increases, so the electrostatic forces of attraction between the nucleus and electrons grow stronger so the electrons are pulled closer to the nucleus
Atomic radius down a group
Increases, as (even though the nuclear charge increases), the number of quantum shells increases, so there is more shielding, so the electrostatic forces of attraction between the nucleus and valence electrons is weaker, so the valence electrons are further away.
Ionic radius changes for positive ions
In an isoelectronic series, as the positive nuclear charge increases across a period, the ionic radius decreases, as the effective nuclear charge has increased, so the electrons are more strongly electrostatically attracted to the nucleus
Ionic radius changes for negative ions
In an isoelectronic series, as the negative charge increases (backwards across a period) the ionic radius increases as the electrons repel each other.
Principal quantum level
The overall energy of an electron, i.e. which quantum shell it is in
Ground state (of an atom)
When the electrons in the atom are in the lowest energy levels available
Free radical
A species with one or more unpaired electron
How do electrons fill up orbitals
They fill up the lowest energy orbitals first. They fill up orbitals of the same energy singly where possible, to minimise spin-pair repulsion
Why is the 4s subshell filled before 3d
Because 4s has lower energy than 3d, as it can get closer to the nucleus
Why is the 4s subshell ionised (it loses electrons) before the 3d subshell
The electrostatic forces of attraction between the nucleus and the electrons in the 4s orbital are weaker when the 3d orbital is filled, due to shielding, and the electrons in the 4s orbital are further away from the nucleus.
First ionisation energy
The heat energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive ions
Why does the first ionisation energy decrease down a group
Going down a group, there are more quantum shells and the atomic radius increases. This means that the valence electrons are further from the nucleus, and they experience more shielding, WHICH OUTWEIGHS THE INCREASE IN NUCLEAR CHARGE, so the electrostatic forces of attraction between the nucleus and valence electrons are weaker, so less HEAT energy is needed to overcome them and remove the electron.
Why does helium have the greatest 1st ionisation energy
It has the smallest atomic radius and there is no shielding. It has 2 protons compared to 1 in hydrogen, so the electrostatic forces of attraction between the nucleus and the valence electrons are stronger, so more heat energy is required to remove an electron.
Why does first ionisation energy overall increase across a period
The nuclear charge increases, and the atomic radius decreases, so the valence electrons are closer to the nucleus (the shielding is the same). This means that the electrostatic forces of attraction between the nucleus and valence electrons are stronger, so more heat energy is needed to overcome them and remove the electron.
Oxidation number
A number given to an atom or ion in a compound that describes how oxidised or reduced it is
Reasons for the electronic configuration of chromium
The energy gap between the 4s and 3d subshells is less than that associated with spin-pair repulsion, which would occur if there were two electrons in the 4s orbital. This means that it is more energetically favourable to have only one electron in the 4s orbital, then fill the 3d orbital.
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