(physical) 3.1.1 atomic structure Flashcards
What can differ in a isotope ?
physical properties (such as density )
what is mass spectrometry ?
instrumental method of analysis used to :
- to find the mass of each isotope and percentage (abundance) of each isotope in a sample to determine relative atomic mass
- find the relative molecular mass of substances made of molecules
What are the 4 main stages of ToF mass spectrometry ?
- ionisation
- acceleration
- flight tube
- detection (ion)
What are the 2 types of ionisation ?
- electron impact ionisation (electron ionisation )
- electrospray ionisation
What is the method for electron impact ionisation ( electron ionisation ) ?
- sample is vapourised
- high energy electrons are fired at it with the use of an electron gun
- This allows an electron to be knocked off from each particle forming 1+ ions
- 1+ ions attracted to a negative electric plate where they are accelerated
What is the general equation for electron impact ionisation ?
X (g) —–> X+ (g) + e-
e.g
Mg (g) —–> Mg+ (g) + e-
(g) means gas
What is the method for electrospray ionisation ?
- sample is dissolved in a volatile solvent
- injected through hypodermic needle at high pressure to produce a fine mist (aerosol)
- high voltage applied to needle
- particles ionised by gaining a proton (H+ ion ) from the solvent as they leave the needle producing XH+ ions
- solvent evaporates and XH+ ions are attracted towards a negative plate where they are accelerated
What is the general equation for electrospray ionisation ?
X(g) + H+ —–> XH+ (g)
Stage 2 - acceleration (what is the method? )
- positive ions are accelerated by electric field
- all ions have the same kinetic energy
- ions with lower mass/charge ratio experience greater acceleration (lighter so accelerate more quickly )
in stage 2 - acceleration of ToF mass spectrometry why are the ions accelerated using a electric field ?
so that they have the same kinetic energy
What does the velocity of each particle depend on
The mass
What velocity do lighter particles have ?
faster velocity as they have less mass
What velocity do heavier particles have ?
slower velocity as they have more mass
What equation would be used for acceleration ?
KE = 1/2 mv^2
(rearrange to find velocity, given in exam )
What is kinetic energy (KE) measured in ?
J (joules)
What is mass (m) measured in ?
kg
What is velocity (v) measured in ?
m s ^-1
What is stage 3 in Tof mass spectrometry ?
Flight tube
What is the method for flight tube ?
- positive ions travel through a hole in the negatively charged plate into the tube .
- Time of flight depends on velocity
- The velocity depends on the mass
What is the equation for time of flight along the flight tube ?
t = d/v
then substitute v with the rearranged formula of KE = 1/2mv^2
what does t stand for and what is its unit ?
t = time of flight ( s )
What does d stand for and what is its unit ?
d = length of flight tube (m)
What does v stand for and what is its unit ?
v = velocity of particle (m s^-1)
What does m stand for and what is its unit ?
m = mass of particle (kg)
What does KE stand for and what is its unit ?
KE = kinetic energy of particle (J)
What does the ToF along flight tube equation show ?
It shows ToF is directly proportional to the square root of the mass of the ions
If ToF is directly proportional to the square root of the mass of the ions what does this mean ?
- lighter ions travel faster and reach detector in less time - heavier ions move slower and take longer to reach detector
What is stage 4 in ToF mass spectrometry ?
Detection
What is the method for detection ?
- positive ions hit the detector
-when they hit the plate the positive ions are discharged by gaining electrons from the plate - this generates a movement of electrons and an electric current gives a measure of the number of ions hitting the plate
- size of current (charge) is proportional to the abundance of the ions
What does the mass spectrum show ?
the mass to charge ratio (m/z) and the relative abundance of each ion
What does mass spectrometry tell us ?
mass of each isotope and percentage (abundance) of each isotope in a sample to determine relative atomic mass
how would you calculate relative atomic mass?
mass times abundance / 100
if there is a mass spectrum produced via electron impact ionisation what would a high m/z value mean ?
Its value is from the molecular ion and its m/z value gives the relative molecular mass
if there is a mass spectrum produced via electron impact ionisation why are there small peaks (low m/z ratio) ?
- Due to fragmentation (main answer)
- due to molecular ions that contain different isotopes
what does the position of the peak on a mass spectrum show ?
relative atomic mass
what does the peak height in a mass spectrum show ?
relative isotopic abundance (relative amount of each isotope )
What is an orbital ?
region in which an electron can be found
how many electrons can each orbital hold ?
up to 2 electrons as long as they have opposite spin
What is Pauls Exclusion principal ?
orbitals can hold up to 2 electrons as long as they have opposite spin .
What are the 4 different shapes of orbitals ?
s , p ,d ,f
What shape are S orbitals ?
spherical
How many sets do S orbitals come in ?
- it comes in sets of one (can hold 2 electrons )
- one occurs in every principal energy level
What shape are P orbitals ?
- dumbbell shaped
How many sets do P orbitals come in ?
- come in sets of 3 ( can hold up to 6 electrons)
- 3 occur in every principal energy level except the first
What shape are d orbitals ?
- various shapes
How many sets do d orbitals come in ?
- come in sets of 5 (can hold up to 10 electrons )
- five occur in energy levels except the first and second
How many sets do f orbitals come in ?
- come in sets of 7 (can hold up to 14 electrons)
- 7 occur in each energy level except first ,second and third
What orbital does the first energy level contain ?
s orbital (1s)
What orbitals does the second energy level contain ?
s orbital and 3 p orbitals labelled 2s and 2p
What orbitals does the third energy level contain ?
- s orbital , 3 p orbitals and 5 d orbitals
- labelled 3s , 3p and 3d
What orbitals does the fourth energy level contain ?
- s orbital , 3 p orbitals , 5 d orbitals and 7 f orbitals
- labelled 4s , 4p , 4d and 4f
What is the order in which orbitals are filled ?
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 4d, 4f
- note : 4s is before 3d
What kind of arrows are used to indicate electrons ?
half arrows
What are the 2 elements that don’t follow the orbital rule?
Cu (copper) and Cr (chromium )
What is the key fact about the electronic structures of Cu and Cr ?
The electronic structures are more stable than alternative structures that follow the pattern
What is different about the electronic configuration of Cr and Cu ?
As they fill the 3d shell before the 4s shell
e.g
Cr - 1s^2 , 2s^2 , 2p^6 , 3s^2 , 3p^6 , 4s^1 ,3d^5
- if it followed pattern it would have been 4s^2 and 3d^4
What happens to the electronic configuration of transition metals when they form ions ?
They lose the 4s electrons before the 3d electrons
What does measuring ionisation energies tell us ?
provides evidence that electrons are arranged in shells or energy levels
What is the meaning of ionisation energy ?
measure of the amount of energy needed to remove electrons form atoms
What is the meaning of first ionisation energy ?
energy needed to remove one mole of electrons from one mole of gaseous atoms.
(to form one mole of gaseous positive ions )
What is the equation that illustrates the process of first ionisation energy ?
X(g) –> X+ (g) + e-
What does the value of the first ionisation energy depend on ?
electronic structure
What is the general trend across period 3 ?
General increase in 1st ionisation energy
Why does nuclear attraction increase in period 3 ?
because successive elements have a bigger nuclear charge and the same shielding
In period 3 why is there a dip from Mg to AI ?
- because Mg is losing 3s electron , AI is losing 3p.
- 3p is higher in energy so easier to remove due to increase shielding and increase distance
- if they have more energy less energy is required to remove
Why is there a dip from P to S (period 3 ) ?
- because repulsive force between the two paired up electrons (opposite spin ) in S makes electrons easier to remove than P
note : S has opposite spin so more energy so less energy needed to remove
What do the dips provide evidence for ?
electrons being arranged in sub levels (sub- shells )
What happens when there are filled inner shells ?
exert a shielding effect which lowers the effective nuclear pull
What happens when the electrons are further away from the nucleus ?
lower nuclear attraction for an electron
- nuclear charge stays the same but due to shielding it is felt less
What is the trend down group 2 ?
- 1st ionisation energy decreases down group 2
Why does 1st ionisation energy decrease down group 2 ?
- with each successive element there is an extra electron shell
- so the outer electron is further from the nucleus and more shielded.
- increased nuclear charge outweighed by greater shielding and distance
What does the decrease down group 2 prove ?
evidence for electrons being arranged in energy levels (shells )
What is the trend down group 1 ?
value decreases down group 1
Why is the outer s electron easier to remove in group 1 ?
despite increased nuclear charge as you go down group the outer s electron is easier to remove due to increase shielding and greater distance from the nucleus - outer electron held less strongly so easier to remove
What factors cause a increase in ionisation energy ?
- similar distance
- similar shielding
- increase nuclear charge
What factors cause a decrease in ionisation energy ?
- starting a new orbital ( s to p ) - cause decrease because e.g. 2p has more energy than 2s2 so less energy is required to remove
- due to opposite spin - if they have opposite spin they have more energy so require less energy to remove
- new shell - increase shielding and distance
What is the meaning of successive ionisation energies (2nd I.E) ?
What is the equation that illustrates the process of successive I.E (2nd I.E)?
X+(g) –> X^2+ (g) + e-
Why do successive ionisation energies get bigger ?
- Because there are fewer electrons and the same number of protons
- The remaining electrons are held more tightly by the unchanged nuclear charge
When do large increases (jumps ) occur in successive I.E ?
When there is a change of shell
Why do large increases (jumps ) occur ?
when going to one energy level (shell ) to another The jump occurs because the new energy level is closer to the nucleus so there is a big decrease in shielding
What can large increases be used to predict in successive I.E ?
group of unknown element
Why are cations always smaller than corresponding atoms ?
- cations ( lose electrons ) contain fewer electrons than their parent atoms, while having same nuclear charge .
Meaning they have a stronger attraction that pulls the remaining electrons closer to the nucleus
Why are anions always larger than corresponding atoms ?
What is nuclear charge ?
Total charge of the nucleus which is equal to the number of protons in nucleus
- doesn’t decrease with increase of shells
How does the shielding and distance affect nuclear charge ?
- shielding prevents the outer electron shell to feel the full positive charge of nucleus (nuclear charge does not change but is felt less )
- the increase in distance means that nuclear charge is felt less as there is more shielding
When does nuclear charge increase ?
- across a period
- because there is an increase number of protons meaning an increase in effective nuclear charge of the atoms
