Physical - Atomic Structure Flashcards
What is relative atomic mass?
The average mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12.
What is relative isotopic mass?
The mass of a single isotope of an element relative to 1/12th of the mass of an atom of carbon-12.
What is mass spectrometry?
An analytical technique that measures the mass to charge ratio of different ions.
What do electrons occupy?
energy levels
How are energy levels referred to?
Energy levels are referred to by their principal quantum number, n. The higher the principal quantum number/energy level, the more energy the electrons in it will have.
What are sub-levels?
Each energy level contains one or more sub-levels/sub-shells. These sub-levels have the names s, p, d and f.
How many electrons can the s sub-level hold?
2
How many electrons can the p sub-level hold?
6
How many electrons can the d sub-level hold?
10
How many electrons can the f sub-level hold?
14
What does isoelectronic mean?
When species (atoms/ions) have the same electron configuration.
What are orbitals?
An atomic orbital is a region around the nucleus that can hold up to 2 electrons with opposite spins. Each sub-shell is made up of orbitals.
Different atomic orbitals have different energies. Each orbital has a number that tells us the main energy level that it corresponds to. The atomic orbitals of each main level have different shapes, which in turn have slightly different energies. These are the sub-levels.
How many orbitals does the s sub-level have?
1 (2 electrons in total)
How many orbitals does the p sub-level have?
3 (6 electrons in total)
How many orbitals does the d sub-level have?
5 (10 electrons in total)
How many orbitals does the f sub-level have?
7 (14 electrons in total)
How are electrons shown in orbitals?
Electrons are shown in orbitals using arrows in box notation. Electrons in the same orbital have opposite spins.
What are the rules for determining electron configuration?
- Aufbau (construction) Principle
- Hund’s Rule
- Pauli Exclusion Principle
- Ion Formation
What is Aufbau’s (construction) Principle?
Electrons fill the lowest energy orbital available first.
What is Hund’s Rule?
If we have orbitals with the same energy, then we put electrons into individual orbitals before we pair them up. This is because electrons in the same orbital repel.
What is Pauli’s Exclusion Principle?
We can have up to 2 electrons in the same orbital but they must have opposite spins.
What is the Ion Formation rule?
Atoms lose electrons from the highest occupied orbital but transition metals lose 4s before 3d.
Which elements are the exception to the rules?
Chromium and Copper.
What is an electron?
A cloud of negative charge. The negative cloud charge has the shape of the orbital occupied by the electron.
What does the atomic orbital show us?
Scientists can never be certain of the exact location of an electron. The atomic orbital shows us a 95% probability of where an electron will exist.
What shape does an s orbital have?
spherical
What shape does a p orbital have?
dumb bell
How many electrons can the first main energy level have?
2
How many electrons can the second main energy level have?
8
How many electrons can the third main energy level have?
18
How many electrons can the fourth main energy level have?
32
What happens as we move away from the nucleus?
The energy of the sub-levels increases.
Why do we fill the 4s sub-level before the 3d sub-level?
The energy of the 4s sub-level is less than the energy of the 3d sub-level. When we have a positive ion, we take out the electrons in the 4s sub-level before the 3d sub-level.
How is the electron configuration always written?
In order of the electron shells, not the order of filling.
What is the expected and actual electron configuration of chromium?
Expected = 1s2 2s2 2p6 3s2 3p6 3d4 4s2
Actual = 1s2 2s2 2p6 3s2 3p6 3d5 4s1
What is the expected and actual electron configuration for copper?
Expected = 1s2 2s2 2p6 3s2 3p6 3d9 4s2
Actual = 1s2 2s2 2p6 3s2 3p6 3d10 4s1
Why do chromium and copper only have one electron in their 4s sub-level?
The 3d sub-level is more stable when it is either half full or completely full.
In the case of chromium, by having only one electron in the 4s sub-level, it can have a half full 3d sub-level.
In the case of copper, by only having one electron in the 4s sub-level, it can have a completely full 3d sub-level.
Why do period 5 elements have a maximum of 18 electrons in their fourth energy level?
Because elements are more stable when their sub-level is half full or completely full.
What is the ground state?
The lowest energy for the atom (electrons are not excited).
What are mass spectrometers used for?
- provide structural information
- finding the abundance and mass of each isotope in an element allowing us to determine its relative atomic mass
- finding the relative molecular mass of substances made of molecules
It detects individual ions, so different isotopes are detected separately because they have different masses.
What is the equation for time of flight mass spectrometry?
KE = (mv^2)/2
v = d/t
How do you calculate the mass of an ion?
mass of one ion (in g) = relative isotopic mass/6.022 x 10^23
What is Avogadro’s constant?
The number of ions in a sample with a total mass equal to its isotopic mass in grams.
What are the six stages to time of flight mass spectrometry?
- sample injection/vapourisation
- ionisation
- acceleration
- ion drift
- detection
- data analysis
What happens in stage one of TOF mass spectrometry?
Injection/Vapourisation
The sample is injected into the mass spectrometer (a vacuum chamber to prevent the ions that are produced colliding with molecules from the air) and is vapourised (turned into a gas).
What happens in stage two of TOF mass spectrometry?
Ionisation
In order to be accelerated and detected, the sample needs to be ionised (form a positively charged ion). There are 2 main methods of ionising samples, each have their advantages and disadvantages and are useful for different substances.
What are the two types of ionisation?
- electron impact ionisation
2. electrospray ionisation
What is electron impact ionisation?
It is used with elements and low Mr compounds. The sample is vaporised and high energy electrons are fired at it from an electron gun, which is a hot wire filament with a current running through it that emits beams of high energy electrons. This usually knocks off one electron from each particle, forming a 1+ ion.
What are the disadvantages of electron impact ionisation?
Can cause fragmentation - where the molecule is broken down into smaller parts.
What is electrospray ionisation?
It is used with high Mr compounds (e.g. proteins). The sample is dissolved in a polar, volatile solvent which vapourises easily, and acts as a source of protons to facilitate the ionisation process. The vapourised solvent is forced through a hypodermic needle that is connected to the positive terminal of a high voltage supply. The positive charge of the terminal produces tiny positively charged droplets. These tiny droplets have gained a proton. The solvent evaporates from the droplets into the vacuum and the droplets get smaller and smaller until they may contain no more than a single positively charged ion.
What happens in stage three of TOF mass spectrometry?
Acceleration
Ionisation has produced a positively charged sample. These ions are then accelerated to the same kinetic energy towards a negatively charged plate (by an electric field into a curved path), which they are attracted to. Lighter ions and more highly charged ions achieve a higher speed.
What happens in stage four of TOF mass spectrometry?
Ion Drift
The accelerated ions are allowed to drift, this separates the lighter and heavier ions. This stage only works because the ions are accelerated to the same kinetic energy. When ions with the same charge arrive at the detector, the lighter ions reach the detector sooner than the heavier ions as they have higher velocities. The ions pass through a hole in the negatively charged plate, forming a beam. The beam of ions travels along a tube, called the flight tube, to a detector. The flight times are recorded.
What happens in stage five of TOF mass spectrometry?
Detection
When the positive ions hit the negatively charged detection plate, they gain an electron producing a flow of charge (electric current). The greater the abundance of ions, the greater the current produced.
What happens in stage six of TOF mass spectrometry?
Data Analysis
The signal from the detector is passed to a computer. The mass spectrometer is coupled with a computer that generates a mass spectrum, (a type of graph) that shows the m/z (mass/charge) on the x axis and abundance on the y axis.
If the sample is a molecule, then you can easily determine the Mr. If the sample in an element, the information can be used to calculate the RAM of a sample of an element.
What is the velocity calculation?
Va = root ((mb x Vb^2)/ma)
What is the time of flight calculation?
ta = root ((tb^2 x ma)/mb)
What are nucleons?
protons and neutrons
Why don’t protons repel each other in the nucleus?
The protons and neutrons are in the centre of the atom, held together by a force called the strong nuclear force. This is much stronger than the electrostatic forces of attraction that hold electrons and protons together in the atom, so it overcomes the repulsion between the protons in the nucleus. It acts only over very short distances, within the nucleus.
What determines the chemical properties of an element?
the number of electrons in the outer shell of an atom
What defines the chemical identity of an element?
the atomic number
What are isotopes?
Atoms with the same number of protons but different numbers of neutrons are called isotopes. Different isotopes of the same element react chemically in exactly the same way as they have the same electron configuration. Atoms of different isotopes of the same element vary in mass number because of the different number of neutrons in their nuclei.
What is an electron?
a cloud of negative charge
What is the basic principle that all mass spectrometers work on?
There are several types of mass spectrometers but all work on the principle of forming ions from the sample and then separating the ions according to the ratio of their mass to their charge (smaller mass to charge ratio travels faster down the flight tube).
What happens in a time of flight mass spectrometer?
In outline, the substance(s) in the sample are converted to positive ions, accelerated to high speeds (which depend on their mass to charge ratio), and arrive at a detector.
What does the peak height on a mass spectra show?
The peak height gives the relative abundance of each isotope and the horizontal scale gives the m/z which for a singly charged ion is numerically the same as the mass number.
What is high and low resolution mass spectrometry?
Mass spectrometers can measure relative atomic masses to five decimal places of an atomic mass unit - this is called high resolution mass spectrometry. However most work is done to one decimal point - this is called low resolution mass spectrometry.
How do you work out the kinetic energy of the ions in a TOF mass spectrometer?
KE = 1/2(mv^2)
Which method is mosy likely to lead to the break up of the ions into fragments?
electron impact ionisation
Why are ions accelerated by an electric field?
so that they each have the same kinetic energy
Explain why different ions take different times to travel through the flight tube? Why are they detected separately?
The time of flight depends on the mass of the ions. When the ions are allowed to drift, they get separated into lighter and heavier ions - the lighter ions reach the detector sooner than the heavier ions.
What happens when a 2+ charged ion is produced?
During the ionisation process, a 2+ charged ion may be produced. This means it will be affected more by the magnetic field producing a curved path of smaller radius. As a result, its mass to charged ratio (m/z) is halved and this can be seen on spectra as a trace at half the expected m/z value.
What does the spectra produced by the mass spectrometry of chlorine display?
Spectra produced by the mass spectrometry of chlorine display a characteristic pattern in a 3:1 radio for Cl+ ions and a 3:6:9 ratio for Cl2+ ions. This is because one isotope is more common than the other and the chlorine molecule can form in different combinations.
Why is a vacuum chamber used in TOF mass spectrometry?
This excludes air and therefore stops the ions from colliding with the air particles.
Why is a negatively charged detection plate used?
The positive ions are attracted to the negative plate, causing them to accelerate.
How does the mass affect deflection?
If you have a larger mass, you’ll be deflected less. If you have a smaller mass, you’ll be deflected more.
What is the first ionisation energy?
The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. It is measured in kJ mol -1.
How does the ionisation energy affect an atoms ability to form an ion?
The lower the ionisation energy, the easier it is to form an ion.
What factors affect ionisation energy?
- nuclear charge
- atomic radius
- shielding
How does the nuclear charge affect ionisation energy?
The larger the nuclear charge, the more difficult it is to remove an electron.
How does the atomic radius affect ionisation energy?
The further the electron is from the nucleus, the easier it is to remove.
How does shielding affect ionisation energy?
The more shells between the nucleus and outer electron, the more easily the electron can be removed.
What are successive ionisation energies?
Successive ionisation energies involve removing additional electrons. Each electron that is removed has its own ionisation energy.
Why doesn’t hydrogen have a 2nd ionisation energy?
Because it only has one electron. After the 1st ionisation energy, there’s no more electrons to remove.
How do successive ionisation energies provide evidence for electronic structure?
Big jumps in ionisation energy show an electron being removed from a lower energy level or a shell closer to the nucleus.
What is ionisation energy?
Electrons can be removed from atoms and the energy it takes to remove them can be measured. This is called ionisation energy because as the electrons are removed, the atoms become positive ions.
How can you measure the energies required to remove the electrons?
You can measure the energies required to remove the electrons one by one from an atom, starting from the outer electrons and working inwards.
- The first electron needs the least energy to remove it because it is being removed from a neutral atom. This is the first ionisation energy.
- The second electron needs more energy than the first because it is being removed from a +1 ion. This is the second energy.
- The third needs yet more, and so on.
What is electron affinity?
The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.
The energy change for the formation of a negative ion is called the electron affinity. The term ionisation energy is used only for the formation of positive ions.
Why does the ionisation energy go down in period three, from magnesium to aluminium?
In going from magnesium (1s2, 2s2, 2p6, 3s2) to aluminium (1s2, 2s2, 2p6, 3s2, 3p1), the ionisation energy goes down, despite the increase in nuclear charge. This is because the outer electron in aluminium is in a 3p orbital which is of a slightly higher energy than the 3s orbital, so the electron is more easily lost. The outer electron is also slightly further away from the nucleus and slightly more shielded. It therefore needs less energy to remove it.
Why does the ionisation energy go down in period three, from phosphorus to sulfur?
In phosphorus (1s2, 2s2, 2p6, 3s2, 3p3), each of the three 3p orbitals contains just one electron. In sulphur (1s2, 2s2, 2p6, 3s2, 3p4), one of the 3p orbitals must contain two electrons. These electrons occupy the same area of space. The repulsion between these paired electrons which are both negatively charged makes it easier to remove one of them, despite the increase in nuclear charge.
What evidence is there that confirms the existence of s- and p-sub-levels?
The case of aluminium and sulfur, which go against the expected trend, are evidence that confirms the existence of s- and p-sub-levels. These were predicted by quantum theory and the Schrodinger equation.
Why is it that although the nuclear charge increases down a group, there is a general decrease in first ionisation energy?
Going down a group, the nuclear charge increases. You would expect this to make it more difficult to remove an electron. However, the actual positive charge ‘felt’ by an electron in the outer shell is less than the full nuclear charge. This is because of the effect of the inner electrons shielding the nuclear charge.
What is the M+1 peak due to?
carbon-13 isotopes
What is the trend in ionisation energy down a group?
decreases because more shielding and greater atomic radius
What is the trend in ionisation energy across a period?
increases because greater nuclear charge, small atomic radius and similar shielding so has no effect
What is relative molecular mass?
The average mass of all the atoms in a compound/molecule relative to 1/12th of the mass of one atom of carbon-12.
What is the valence bond theory?
Valence bond theory states that if two atoms have orbitals which contain unpaired electrons, then the orbitals will overlap to form a covalent bond.
What is the difference between a sigma and pi bond?
In a sigma bond, one of the atoms can rotate around the bond axis without affecting the amount of overlap between the two orbitals (orbitals overlap end on).
In a pi bond, if one of the atoms rotates around the bond axis, the amount of overlap would change (orbitals overlap side on).
