Atomic structure Flashcards
Proton
Relative mass: 1
Relative charge: +1
Neutron
Relative mass: 1
Relative charge:0
Electron
Relative mass: 1/1836
Relative charge: -1
Structure of an atom:
An atom has a positively charged nucleus ( containing protons and neutrons) and is surrounded by shells containing electrons
Atomic Number (Z)
Number of protons in the nucleus of an atom
Mass Number (A) (Nucleon Number)
Sum of the protons and neutron in the nucleus
Relative atomic mass
Relative atomic mass is the average mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12
Relative molecular mass
Relative molecular mass is the average mass of a molecule relative to one twelfth of the mass of an atom of carbon-12
Relative isotopic mass
Relative isotopic mass is the mass of one atom of an isotope compared to one twelfth of the mass of one atom of carbon-12
Isotopes
Atoms of the same element that have the same number of protons but different number of neutrons
Analysing mass spectrum
The abundance of the isotopes is always shown on the Y axis. It can be shown as a % or as a nominal value. If it is in %, all your isotopes must add to give 100%
M/z is always shown on the X as just the mass of the isotope divided by the charge. As most have just a +1 charge, this is the same as their isotopic mass
The highest peak is taken as the “base peak” and in the relative abundance method, it is assigned as 100% and all other peaks are assigned as a percentage of that.
No of peaks = number of isotopes
How to work out relative atomic mass?
To work out the relative atomic mass:
R.A.M = (isotopic mass of A x relative abundance of A) + (isotopic mass of B x relative abundance of B) / total relative abundance (usually 100)
How to predict mass spectra
To predict mass spectra:
- Write the percentages as decimals
- Create a table showing the isotope combinations
- Any molecules that are the same, add the abundances up
- Divide all the relative abundances worked out before by the smallest value.
This will give you a whole number ratio which can be used to predict your spectra
How can mass spectrometry be used to determine the relative molecular mass of a molecule:
In a mass spectrum for molecules the peaks show fragments of the original molecule
The last peak is the m+1 peak or the molecular ion peak
This is the same as the relative molecular mass of the molecule
What is ionisation energy?
Ionisation energy is the amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous state
Successive ionisation energy
The energy required to remove than 1 electron (one by one) from the same atom is called successive ionisation
How does the number of protons affect ionisation energies?
Size of the positive nuclear charge:
As the nuclear charge increases, its attraction for the outermost electron increases and more energy is required to remove an electron.
This means that the ionisation energy increases.
How does the size of the atom affect ionisation energies?
Size of atom (distance of outermost electron from the nucleus):
As atomic size increases, the attraction of the positive nucleus for the negative electron decreases and less energy is required to remove an electron.
This means that the ionisation energy decreases.
How does shielding affect ionisation energies?
Shielding (screening) effect of inner shell electrons:
The outermost electron is screened (shielded) from the attraction of the nucleus by the repelling effect of the inner electrons.
As shielding increases, the attraction of the positive nucleus for the negative electron decreases and less energy is required to remove an electron.
This means that the ionisation energy decreases.
How does the fact that an electron is on its own in an orbital or paired with another electron (Spin to spin repulsion) affect ionisation energies?
Two electrons in the same orbital experience a bit of repulsion from each other. This offsets the attraction of the nucleus, so that paired electrons are removed rather more easily than you might expect.
How does the electron sub-shell from which the electron is removed affect the ionisation energy?
If an electron already has a high energy, then the energy it needs to gain in order to be removed will not be very large.
If, however, the electron is in an orbital of a low energy quantum shell, for example the 1s orbital, then it will need to gain considerably more energy to be removed
Why are successive ionisation energies always larger?
The second ionisation energy of an element is always bigger than the first ionisation energy.
When the first electron is removed a positive ion is formed.
The ion increases the attraction on the remaining electrons and so the energy required to remove the next electron is larger.
Trend in 1st ionisation energy across period
The 1st ionisation energy increases across a period
Nuclear charge increases as you go across a period and there is no change in the amount of shielding or the distance of the outermost electron from the nucleus
This leads to the attraction between the outer electron and the nucleus increasing
Therefore, more energy is required to remove an electron ie; a higher ionisation energy is needed
Trends in ionisation energy across period 3:
In period 3 there is a general increase in the ionisation energies but there is a small drop from Mg to Al and from P to S