Atomic structure (3.1.1) Flashcards
protons
charge = +1
mass = 1
neutrons
charge = neutral
mass = 1
electrons
charge = -1
mass = 1/1850
what does A represent?
mass number
what does Z represent?
atomic (proton) number
how has knowledge and understanding of atomic structure changed over time?
evolved from plum pudding model (1904, JJ Thompson) to the Rutherford model after the gold foil experiment and eventually to today’s nuclear model
how does electron configuration work?
electrons occupy energy levels, but each energy level is made up of different numbers of sub-levels (sub-shells), and sub-levels are made up of different numbers of orbitals
how many electrons can each orbital hold?
maximum of 2
what are the sub-levels?
s, p, d, f
how many orbitals are in each sub-level?
s - 1 orbital (holds 2 electrons)
p - 3 orbitals (holds 6 electrons)
d - 5 orbitals (holds 10 electrons)
what are isotopes?
atoms of the same element containing the same number of protons but different numbers of neutrons (have same chemical properties as they have the same number of outer shell electrons)
how are electron configurations represented?
electrons in boxes - each box represents an orbital, and these make up sub-levels which appear with increasing distance from the nucleus
written configurations
what is the order of sub-levels for writing electron configurations?
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s
why is the 4s sub-level filled before 3d?
because it is lower in energy
how do you write shorthand electron configuration?
find the noble gas with the closest number of electrons below the element you are looking at, write the symbol of that noble gas and then finish of the configuration with the remaining sub-levels
what is the long and shorthand electron configuration for zinc?
1s2 2s2 2p6 3s2 3p6 4s2 3d10
[Ar] 4s2 3d10
what does the mass spectrometer do?
gives accurate information about relative isotopic mass and also about the relative abundance of isotopes
what can mass spectrometry be used to do?
identify elements and determine relative molecular mass
what is the first stage of TOF mass spectrometry?
ionisation
what are two techniques of ionising the sample?
electron impact and electrospray ionisation
how does electron impact work?
the sample being analysed is vaporised
high energy electrons are fired at the sample by an ‘electron gun’ (a hot wire filament with current running through it)
usually one electron is knocked off each particle meaning they form a 1+ ion (molecular ion)
the 1+ ions are attracted towards a negative electric plate where they ae accelerated
in what situation is electron impact used?
used for elements and substances which have a low formular mass
what would an equation for the ionisation of a methane molecule look like with electron impact?
CH4 (g) –> CH4^+ (g) + e-
how does electrospray ionisation work?
the sample is dissolved in a volatile solvent eg. water, methanol
it is then injected through a fine hypodermic needle to give a fine mist
the tip of the needle is attached to the positive terminal of a high voltage power supply
particles gain a proton (H^+ ion) from the solvent as they leave the needle which produces XH^+ ions
the solvent evaporates and the XH^+ ions attract towards a negative electric plate where they are accelerated
in what situation is electrospray ionisation used?
used for substances with a high molecular mass eg. biological molecules like proteins
what would an equation look like for the ionisation of molecule M with electrospray ionisation?
M (g) + H^+ –> MH^+ (g)
what is the second stage of TOF mass spectrometry?
acceleration
how are positive ions accelerated in the acceleration area?
using an electric field - there is an attraction between the positively charged ions and negatively charged plates
what is the rule for the kinetic energy of ions after passing through the acceleration area?
they all have the same kinetic energy
how do you work out the kinetic energy of an ion?
KE = 1/2 x m x v^2
KE in J
m in kg
v in ms^-1
how do you work out the velocity of each particle?
v = square root of 2KE/m
what does the velocity of each particle depend on?
its mass - lighter particles have faster velocity and heavier particles have slower velocity
what is the third stage of TOF mass spectrometry?
flight tube
what happens in the flight tube?
the positive ions travel through a hole in the negatively charged plate into a tube - the time of flight of each particle through the flight tube depends on is velocity which depends on its mass (lighter particles travel faster so will reach the detector in less time than heavier particles)
how do you calculate the time of flight along the flight tube?
t = d/v
d (length) in m
t in s
t = d square root m/2KE
(so TOF is proportional to the square root of the mass of the ions)
what is the fourth stage of TOF mass spectrometry?
detection
what happens in the detection stage?
the positive ions hit a negatively charged electric plate and the positive ions are discharged by gaining electrons from the plate - the movement of electrons generates an electric current which is measured
what is the size of the current proportional to?
the number of ions that hit the plate
what information do mass spectra show?
the m/z of ions in the sample (x axis) and their relative abundances (y axis)
in TOF mass spectrometry which ion will generate the largest current?
the one with the highest relative abundance
in TOF mass spectrometry which ion will travel the fastest?
the one with the lowest m/z ratio
A naturally occurring sample of the element boron has a relative atomic mass of 10.8. In this sample, boron exists as two isotopes, 10B and 11B. Calculate the percentage abundance of 10B in this naturally occurring sample of boron.
% of 10B = x
% of 11B = 100-x
10.8 = 10x + 11(100-x)/100
1080 = 10x + 1100 - 11x
11x - 10x = 1100 - 1080
x = 20
what does 1 mole/ 6.022x10^23 atoms or ions of an element weigh?
its mass number (in g)
how do you work out the mass of one atom or ion in g?
find the mass number for the element and then divide by 6.022x10^23
what is the simple equation for when you have two isotopic ions travelling down the same flight tube?
mass of first ion/TOF of first ion ^2 = mass of second ion/TOF of second ion ^2
what is the definition of first ionisation energy?
the energy required to remove one mole of electrons from one mole of gaseous atoms
what is shielding?
repulsion from inner electrons (more energy levels = more shielding for outer electrons)
what are the three factors that affect ionisation energies?
- distance between the nucleus and the electron being removed
- nuclear charge
- shielding between nucleus and outer electron by electrons in inner shells
what would the equations look like when measuring the first 5 ionisation energies for sodium?
1st: Na (g) –> Na^+ (g) + e^-
2nd: Na^+ (g) –> Na^2+ (g) + e^-
3rd: Na^2+ (g) –> Na^3+ (g) + e^-
4th: Na^3+ (g) –> Na^4+ (g) + e^-
5th: Na^4+ (g) –> Na^5+ (g) + e^-
when are there significant jumps in the energy needed to remove electrons?
when you are moving to the next energy level
how would you explain the significant jump in energy in terms of atomic structure eg. between electrons 1 and 2 for a sodium atom?
the first electron is removed from the 3rd energy level/3s sub-level which is further from the nucleus, more shielded by inner shells of electrons and therefore less tightly held than the second electron which is removed from the second energy level/2p sub-level
what are the steps for explaining the jump in energy for IEs?
- state which energy level and sub-shell the electron is removed from
- mention distance from nucleus
- mention shielding
- mention nuclear charge (less tightly held)
- compare to next electron with location
explain why the first IE of Mg is greater than the first IE of Na?
Mg has a greater nuclear charge, so Mg attracts its outer electron more strongly than Na meaning more energy is required to remove it
explain why the second IE of Na is greater than the second IE of Mg?
the second electron of a sodium atom is removed from the 2nd energy level/ 2p sub-level which is closer to the nucleus and therefore more tightly held than the second electron of a magnesium atom, which is removed from the 3rd energy level/3s sub-level so it is further from the nucleus
an Ne atom and an Mg^2+ ion both have 10 electrons - explain why the 1st IE of Ne is lower than the 3rd IE of Mg?
in both cases, the outer electron will have the same amount of shielding, but Mg has a higher nuclear charge so will therefore attract the outer electron more strongly
how should you go about answering questions comparing the IEs of different elements?
write the electron configuration for that element so you can identify where the electron is being removed from
identify which factor (distance, shielding, nuclear charge) is causing the difference in IE and explain this
what do ionisation energies provide evidence about?
trends in electron configuration on the periodic table going across periods and going down groups
what is the general trend in first ionisation energies going across period 3 (Na, Mg, Al, Si, P, S, Cl, Ar)?
the first IE increases across group 3 because the nuclear charge increases so the outer electrons are more strongly attracted to the nucleus
what are two anomalies in the trend across group 3?
Al and S (first IE for them decreases instead of increases)
why is the first IE of Al lower than Mg?
the outer electron of Al is in a higher energy 3p orbital whereas for Mg it is in a 3s orbital. This means the outer electron of Al is less attracted to the positive nucleus which means less energy is required to remove the electron
why is the first IE for S lower than P?
in S there are two electrons in a 3p orbital, and these two electrons repel each other. It takes less energy to remove one of the electrons from this pair than it does to remove the first electron of P where all the 3p orbitals are singly occupied
(write ec and then draw orbitals for 3p)
what is the general trend in first ionisation energy going down group 2?
the first IE decreases down group 2 because there are more energy levels so the atomic radius increases. This means the outer electron is further away and there is more shielding, so it is less tightly held and less energy is needed to remove it
