ATOMIC STRUCTURE Flashcards
what can mass spec be used for?
find the abundance and mass of each isotope in an element allowing us to determine its
relative atomic mass
find the relative molecular mass of substances made of molecules.
what happens in ToF mass spec?
particles of the substance are ionised to form 1+ ions which are accelerated so
that they all have the same kinetic energy.
The time taken to travel a fixed distance is then
used to find the mass of each ion in the sample.
what are the 2 ways of ionising a sample?
- electron spray
- electron gun
describe the steps in electron impact ionisation?
1- The sample being analysed is vaporised and then high energy electrons are fired at it.
2- The
high energy electrons come from an ‘electron gun’ which is a hot wire filament with a current
running through it that emits electrons.
3- This usually knocks off one electron from each
particle forming a 1+ ion.
4-The 1+ ions are then attracted towards a negative electric plate where they are accelerated.
what is the equation for an element being analysed via electron impact?
X(g) –> X+(g) + e–
why do we use electron impact ionisation to ionise some molecules?
This technique is used for elements and substances with low formula mass (that can be
inorganic or organic molecules).
what is the product formed from electron impact ionisation
When molecules are ionised in this way, the 1+ ion formed is
known as a molecular ion
what other than molecular ions appear on the mass spec?
The molecular ion often breaks down into smaller fragments some of which are also detected
in the mass spectrum.
describe the steps of electron spray ionisation
1- The sample X is dissolved in a volatile solvent (eg water or methanol) and injected through a
fine hypodermic needle to give a fine mist (aerosol).
2- The tip of the needle is attached to the
positive terminal of a high-voltage power supply.
3- The particles are ionised by gaining a proton
(ie an H+ ion which is simply one proton) from the solvent as they leave the needle producing XH+
ions (ions with a single positive charge and a mass of Mr + 1).
4- The solvent evaporates away while the XH+
ions are attracted towards a negative plate where
they are accelerated.
what is the equation for samples being analysed by electron spray?
X(g) + H+ –> XH+(g)
what types of substances do we use electron spray to analyse?
This technique is used for many substances with higher molecular mass including many
biological molecules such as proteins.
This is known as a ‘soft’ ionisation technique and
fragmentation rarely takes place.
what are the 4 stages of ToF?
1- ionisation
2-acceleration
3-flight tube
4-detection
how are positive ions accelerated?
using an electric field?
why are positive ions accelerated?
so that they all have the same kinetic
energy.
what does this equation stand for and what are the units:
KE = 1/2mv^2?
KE = kinetic energy of particle (J) m = mass of the particle (kg) 𝑣 = velocity of the particle (m s–1)
make velocity the subject of this equation KE = 1/2mv^2?
𝒗 = √(𝟐𝑲𝑬)/m
when particles have the same energy, what does velocity depend on?
its mass
how does mass of a particle effect its speed?
Lighter particles have a faster velocity, and heavier particles have a slower
velocity.
how do the positive ions travel through through the flight tube?
The positive ions travel through a hole in the negatively charged plate into a tube.
what does the ToF of each particle depend on?
The time of
flight of each particle through this flight tube depends on its velocity which in turn depends on
its mass.
what equations are used to calculate ToF along a flight tube?
t=d/v
t=𝒅√(𝒎/𝟐𝑲E)
t = time of flight (s) d = length of flight tube (m) 𝑣 = velocity of the particle (m s–1) m = mass of the particle (kg) KE = kinetic energy of particle (J)
what /does t=𝒅√(𝒎/𝟐𝑲E) show
This shows that the time of flight is proportional to the square root of the mass of the ions.
Therefore lighter ions travel faster and reach the detector in less time than the heavier
particles that move slower and take longer to reach the detector
in what order do ions of the isotopes of 24Mg+, 25Mg+,26Mg+ reach the detector and why?
1=24Mg+
2= 25Mg+
3=26Mg+
in order of mass, lighter= faster
describe what happens in the detection stage of ToF?
The positive ions hit a negatively charged electric plate. When they hit the detector plate, the
positive ions are discharged by gaining electrons from the plate.
This generates a movement
of electrons and hence an electric current that is measured.
The size of the current gives a
measure of the number of ions hitting the plate.
how is the mass spectrum generated?
A computer uses the data to produce a mass spectrum.
This shows the mass to charge (m/z)
ratio and abundance of each ion that reaches the detector.
Given that all ions produced by
electrospray ionisation and most of the ions by electron ionisation have a 1+ charge, the m/z
is effectively the mass of each ion.
how is Ar calculated from a mass spectrum?
relative atomic mass (Ar) = (combined mass of all isotopes / combined abundance of all isotopes)
for molecules that have been ionised by electron impact, what does the signal with the greatest m/z value represent?
m the molecular ion and its m/z value gives the relative molecular mass
what do the small peaks in a ToF from a substance analysed by ToF represent?
molecular ions
that contain different isotopes
what are the small peaks due to in electron impact?
peaks at lower m/z values due to fragments caused by the break up of molecular ion
what are the small peaks due to in electron spray?
2H or 13C
what is in the nucleus?
protons and neutrons
what is the mass of an electron?
1/2000
what does the mass number tell us?
the number of protons and neutrons
what does the atomic number tell us?
the number of protons which is equal to number of electrons
define neutrones
elements with the same number of protons but a different number of neutrons
what is the history of the atom?
1- John Dalton= atoms are spheres and each element is made from different spheres
2-discovered the electron. the atom wasn’t sold and was made up of other particles. The plumb pudding model was developed.
3- Discovered the nucleus. Also discovered that the nucleus was very small and positively charged. he also concluded the atom was mainly empty space made up a negative ‘cloud’.
GOLD LEAF EXPERIMENT- Positive alpha particles fired at thin gold leaf. Most went through the gold leaf (mainly empty space). small number deflected back (they hit a small positive nucleus)
4- Niels Bohr- discovered a problem with Ruherford’s model. the cloud could collapse into the positive nucleus! he proposed electrons were in fixed energy shells.
THE EXPERIMENT PROOF-
when EM radiation is absorbed, electrons move between shells. They emit this radiation when electrons move down to lower energy shells.
5- ATOMIC MODEL TODAY-
electrons don’t have the same energy in shells. We have subshells. This explains ionisation trends
define relative atomic mass
the average mass of an atom of an element when measured on scale on which the mass of an atom of C12 is exactly 12
define relative molecular mass
the average mass of a molecule when measured on scale of which the mass of an atom is C12 is exactly 12
define relative isotopic mass
the mass of an atom of an isotope of an element on scale on which the mass of an atom of C12 is exactly 12
what is the rule for transition metal ions?
you lose from 4s first them 3d
chromium and copper
define ionisation energy
ionisation energy is the minimum amount of energy required to remove 1 mole of electrons from 1 mole of atoms in a gaseous state
what determines the ionisation energy?
1) shielding= the more electrons shells between the positive nucleus and negative electron that is being removed the less energy is required. There is a weaker attraction.
2) Nuclear charge= the more protons in the nucleus the bigger the attraction between nucleus and outer electrons. This means more energy required to remove the electron.
3) Atomic Size= the bigger the atom the further away the outer electrons are from the nucleus. the attractive force between nucleus and outer electrons reduces- easier to remove electrons
define successive ioninsation energy
the removal of more than 1 electron from the same atom is called successive ionisation
what is the general formula for the first ionisation energy?
X–> X+ + e-
what is the trend in 1st ionisation energy down groups?
IONISATION ENERGY DECREASES AS WE GO DOWN A GROUP
- The atomic radius increases as we go down the group. outer electrons further from the nucleus. Attractive forces is weaker. Energy required to remove an electron decreases.
- Shielding increases as we go down the group. More shells between nucleus and outer shell. Attractive force is weaker. Energy required to remove an electron decreases.
what is the trend in 1st ionisation energy as we go down a group?
IONISATION ENERGY INCREASES AS WE GO ACROSS A PERIOD
- as we go across the period there is an increasing number of protons in the nucleus. this increases the nuclear attraction.
- Shielding is similar and distance from nuclear marginally decreases.
- More energy required to remove an outer electron. Ionisation energy increases
why is there a general increase in first ionisation energy across a period?
. As one goes across a period the electrons are being added to the same
shell which has the same distance from the nucleus and same shielding
effect. The number of protons increases, however, making the effective
attraction of the nucleus greater.
why has Na a much lower first ionisation energy?
This is because Na will have its outer electron in a 3s shell further from
the nucleus and is more shielded. So Na’s outer electron is easier to
remove and has a lower ionisation energy.
why is there a small drop from Mg to Al?
Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s
sub shell. The electrons in the 3p subshell are slightly easier to remove because
the 3p electrons are higher in energy and are also slightly shielded by the 3s
electrons
why is there a small drop from P to S?
With sulfur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly fill
the first 3p orbital.
When the second electron is added to a 3p orbital there is a slight repulsion between
the two negatively charged electrons which makes the second electron easier to
remove.
