Atomic Structure Flashcards
Relative Atomic Mass (Ar)
The average mass of an atom of an element on a scale where an atom of carbon-12 is 12
Relative Isotopic Mass
The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12
Relative Molecular Mass (Mr)
The average mass of a molecule on a scale where an atom of carbon-12 is 12
Electrospray Ionisation
The sample is dissolved and pushed through a small nozzle at high pressure.
A high voltage is applied to it, causing each particle to gain an H+ ion.
The sample is turned into a gas made up of positive ions
Electron Impact Ionisation
The sample is vaporised and an electron gun is used to fire high energy electrons at it.
This knocks one electron off each particle, so they become +1 ions
Describe acceleration of TOFMS
The positively charged ions are accelerated by an electric field so that they all have the same kinetic energy.
This means that the lighter ions will end up moving faster than the heavier ions
Describe ion drift of TOFMS
The ions enter a region with no electric field, so they just drift through it.
Lighter ions will drift through faster than heavier ions
Describe detection of TOFMS
Because lighter ions travel at higher speeds in the drift region, they reach the detector in less time than heavier ions.
The detectors used in mass spectrometers detect charged particles.
An electrical current is produced in the detector when a charged particle hits it, and this produces a mass spectrometer
Relative Formula Mass
How to calculate
Used instead for compounds that are ionic or giant covalent.
To find, just add up the relative atomic masses (Ar) of all the atoms in the formula unit
First Ionisation Energy
The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
Nuclear charge and ionisation energy
The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons
Distance from the nucleus and ionisation energy
Attraction falls off very rapidly with distance.
An electron close to the nucleus will be much more strongly attracted than one further away
Shielding and ionisation energy
As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge.
This lessening of the pull of the nucleus by inner shells of electrons is called shielding (or screening)
Second Ionisation Energy
The energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
High Ionisation Energy
Means there’s a high attraction between the electron and the nucleus.
Therefore more energy is needed to remove the electron
Trend in Ionisation Energy down Group 2
Explain why
Ionisation energy decreases down group 2
Each element down group 2 has an extra electron shell compared to the one above.
The extra inner shells will shield the outer electrons from the attraction of the nucleus.
The extra shell means that the outer electrons are further away from the nucleus, so the nucleus’s attraction will be greatly reduced.
Both of these factors makes it easier to remove outer electrons resulting in lower ionisation energy
Trend in Ionisation Energy across Period (3)
Explain why
Ionisation energy increases across a period
The number of protons is increasing across a period, which means a stronger nuclear attraction.
All the extra electrons are at roughly the same energy level, even if the outer electrons are in different orbital types.
This means there’s generally little extra shielding effect or extra distance to lessen the attraction from the nucleus
Explain why aluminium has a lower ionisation than magnesium?
Where is this drop in the periodic table?
Aluminium’s outer electron is in a 3p orbital rather than a 3s.
The 3p orbital has a slightly higher energy than the 3s orbital, so the electron, is on average, to be found further from the nucleus.
The 3p orbital has additional shielding provided by the 3s2 electrons.
Both these factors together are strong enough to override the effect of increased nuclear charge, resulting in the ionisation energy dropping slightly
Between groups 2 and 3, in period 3 (showing sub-shell structure)
Explain why sulfur has a lower ionisation energy than phosphorous?
Where is this drop in the periodic table
The shielding is identical in the phosphorous and sulfur atoms, and the electron is being removed from an identical orbital.
In phosphorous, the electron is being removed from a singly-occupied orbital.
But in sulfur, the electron is being removed from an orbital containing two electrons.
The repulsion between two electrons in an orbital means that electrons are easier to remove from shared orbitals.
Between groups 5 and 6, in period 3 (evidence also for the electronic structure model)
