1. Atomic Structure Flashcards
What happened in 1662 to support the development of the atom?
Robert Boyle proposed that some substances could not be made simpler
What happened in 1803 to support the development of the atom?
John Dalton suggested elements were composed of atoms that could not be broken down
What happened in 1896 to support the development of the atom?
Henri Becquerel discovered radioactivity - showing particles could come from inside the atom
What happened in 1911 to support the development of the atom?
Ernest Rutherford found that most of the mass and charge of the atom is concentrated in a tiny central nucleus
What is the relative mass of an electron?
1/1840
What are protons and neutrons held in the centre of atoms by?
The strong nuclear force
What does the strong nuclear force, holding together the protons and neutrons, overcome?
Electrostatic forces of attraction that hold electrons and protons together in the atom
What letter is sometimes used to represent the mass number?
A
What letter is sometimes used to represent the atomic number?
Z
What is the mass number?
The total number of neutrons and protons in the nucleus
What is the atomic number?
The number of protons in the nucleus
What is the atomic number also called?
The proton number
What determines the chemical properties of an element?
The number of electrons in the outer shell
How many electrons does the first shell hold?
2
How many electrons does the second shell hold?
8
How many electrons does the third shell hold?
18
How many electrons does the fourth shell hold?
32
What expression shows the number of electrons in each shell? (where n is the number of the shell)
2n²
What will change for isotopes of the same element, in terms of chemical and physical properties?
Chemical properties will stay the same with each isotope, but physical properties will change due to the change in mass
How can relative atomic mass be worked out?
Average mass of one atom ÷ 1/12 mass of 1 atom of C-12
How can relative molecular mass be worked out?
Average mass of one molecule ÷ 1/12 mass of one atom of C-12
What is Mr?
Relative molecular mass
What is Ar?
Relative atomic mass
What does the mass spectrometer do?
Determines the relative isotopic mass and relative abundance for each isotope
An overview of how a time of flight mass spectrometer works?
The substances in the sample are converted to positive ions, accelerated and then arrive at a detector
What is relative isotopic mass?
Mass of each individual isotope relative to carbon-12
What an important initial condition required when using a time of flight spectrometer?
The apparatus is kept under high vacuum
Why is the time of flight apparatus kept under a high vacuum?
To prevent the ions that are produced from colliding with molecules from the air
What must happen if the sample isn’t a gas before entered into a mass spectrometer?
It must be vaporised
What are the two ways that a sample can be ionised in preparation for mass spectrometry?
- electron impact
* electrospray ionisation
What happens in electron impact?
- gaseous sample entered into mass spectrometer
- bombarded with high speed electrons from an electron gun
- knocks out an electron (or two) from the atom/molecule
What is the electron gun formed from for electron impact?
A hot wire filament with a current running through it that emits electrons
How many electrons are knocked out of the atom in electron impact?
Usually one, but sometimes two
Why is only one electron usually knocked out of an atom during electron impact?
It requires a lower energy than two, so is more likely
When is electron impact used?
For elements and substances with a low formula mass
When molecules are ionised using electron impact, what are the 1+ ions formed known as?
Molecular ions
Can molecules be ionised?
Yes
Equation for ionisation of methane using electron impact?
CH4 -> CH4+ + e-
Which method of ionisation for mass spectrometry often breaks down the molecular ion into smaller fragments?
Electron impact
What charge is required on ions during mass spectrometry?
Positive
What happens in electrospray ionisation?
- sample dissolved in solvent
- sample injected through fine needle to give fine mist
- tip of needle attached to +ve side of power supply
- particles gain a proton
In which method of ionisation for mass spectrometry do atoms gain a proton?
Electrospray ionisation
What is the technical name for the fine needle in electrospray ionisation?
Hypodermic
What is the mass of new ions in electrospray ionisation?
Mr + 1
Equation for ionisation using electrospray ionisation?
X + H+ -> XH+
What eventually happens to the solvent that was added in electrospray ionisation?
It is evaporated
What charge does the needle have in electrospray ionisation?
Postive
What kind of substance is electron impact used for?
Substances with low formula mass
What kind of substance is electrospray ionisation used for?
Substances with higher molecular mass (e.g. proteins)
In which type of ionisation does fragmentation rarely occur?
Electrospray ionisation
Where do substances gain a proton from in electrospray ionisation?
From the solvent
What happens after substances are ionised in mass spectrometry?
They are accelerated
How are the ions accelerated in mass spectrometry?
Using a negatively charged plate
Why are ions accelerated during mass spectrometry?
So that they all have the same kinetic energy
Which particles will have a higher velocity during mass spectrometry?
Lighter particles
What is the equation for the velocity of each particle during acceleration?
v = √2KE ÷ m
What does the velocity of each particle depend on in mass spectrometry? Why is this?
It depends on the mass as the kinetic energy is the same for all particles
What happens after ions are accelerated during mass spectrometry?
They pass through a hole in the negatively charged plate, forming a beam
When do ions move through the flight tube during mass spectrometry?
After they have passed through the hole in the negatively charged plate
What does the time of flight of a particle depend on?
Its velocity; which in turn depends on its mass (and charge)
What expression is used to give time of flight?
t = d/v
How is the equation for velocity incorporated into the expression for time of flight?
t = d/ √2KE ÷ m
What is time of flight proportional to?
The square root of the mass
What happens when the ions reach the detector in mass spectrometry?
They gain an electron from the detector, generating a movement of electrons and therefore an electric current
What does the size of the current in the detector show during mass spectrometry?
The size of current gives a measure of the number of ions hitting the plate
What happens to the signal from the detector during mass spectrometry?
It is passed to a computer which generates a mass spectrum
What does a mass spectrum show?
The mass to charge (m/z) ratio and abundance of each ion
Why is m/z effectively the mass of each ion?
All ions produced by electrospray ionisation and most by electron impact have a 1+ charge
Can ions formed by electrospray be 2+?
No
Which signal is from the molecular ion on a mass spectrum?
The peak furthest to the right (greatest m/z value)
For molecules ionised by electrospray, what is the relative molecular mass?
The signal with the greatest m/z value minus 1
Why are the relative molecular masses of molecules ionised by electrospray the greatest m/z value minus 1?
Due to the gaining of a proton during ionisation and then gaining an electron during detection
Why are different isotopes detected separately during mass spectrometry?
They have different masses
How many decimal places can mass spectrometers measure to?
5 - however most is done to 1 d.p.
What is low resolution mass spectrometry?
When relative atomic masses are only worked out to 1 d.p.
What is the definition of relative atomic mass?
The average mass of one atom of an element compared to 1/12 of the mass of one atom of carbon-12
How is Ar calculated?
(mass no. x abundance)+(mass no. x abundance) / total abundance
What happens in a mass spectrum if the ion has a 2+ charge?
The m/z value will be half the mass
What happens to the mass spectrum if the sample is a molecule?
The molecule can either split up or remain as a molecular ion
(molecular ions easily identified as it’s the last large peak along m/z axis)
What happens if a molecule does not split up during mass spectrometry in terms of its isotopes?
Each atom can have various combinations of isotopes (e.g. Cl-35 - Cl37+
Uses of mass spectrometers?
- investigate illegal drugs, explosives and chemical pills
* space probes - to identify elements in rock sample or gases in the atmosphere
How many decimal places are relative atomic masses usually to?
1
How are electrons arranged around the nucleus?
In energy levels
What are shells also called?
Main energy levels
How are main energy levels labelled?
1, 2, 3 etc.
What formula is used to give the maximum number of electrons in a main energy level?
2n²
Which main energy level cannot be divided into sub levels?
The first (s only)
What are main energy levels divided into?
Sub-levels
How are sub-levels labelled?
s, p, d, f
What happens to energy of electrons as distance from the nucleus increases?
It increases
What does each sub-level consist of?
Orbitals
How many electrons can orbitals hold?
2
How many orbitals are in the s sub-level?
1
How many orbitals are in the p sub-level?
3
How many orbitals are in the d sub-level?
5
How many orbitals are in the f sub-level?
7
How is the arrangement of electrons for any atom given?
using:
- main energy level
- sub-level
- number of electrons in each sub-level
After 1s, 2s, 2p, 3s, and 3p, what happens?
The next electon goes into 4s, skipping 3d
Why is 4s before 3d?
The 3d sub-level has a higher energy
How is the electron considered now (instead of a particle)?
As a cloud of negative charge
What is the volume of space that an electron fills up called?
Its atomic orbital
What does the first main energy level consist of?
A single s-orbital
What does the second main energy level consist of?
A single s-orbital and 3 p-orbitals
How many electrons can s sub-levels hold?
2
How many electrons can p sub-levels hold?
6
How many electrons can d sub-levels hold?
10
How many electrons can f sub-levels hold?
14
What order are sub-levels filled in?
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 4f, 5d
Where is the f block on the periodic table?
The (separate) bottom two rows
Where is the s block on the periodic table?
First two columns
Where is the d block on the periodic table?
Where the transition metals are
Where is the p block on the periodic table?
The right hand side
What does chlorine being in the p-block tell us?
The highest energy sub-shell is a p subshell
What does chlorine being in period 3 tell us?
The highest energy sub-shell will be the 3 sub-shell
What happens when writing the electronic configuration of ions?
Electrons are added or subtracted from the total
What happens when working out the ion configuration for transition metals?
Electrons are firstly removed from 4s and then from 3d
What spins do electrons in the same orbit have?
Opposite
Why do electrons in the same orbit have opposite spins?
Like charges will repel
How are the electrons represented in spin diagrams?
Arrows pointing up or down to show the different directions of spin
What are the three rules for allocating electrons to atomic orbitals?
- orbitals of lower energy are filled first
- orbitals of the same energy fill singly before pairing starts
- no orbital can hold <2 electrons
Why do orbitals fill singly before pairing starts?
The electrons repel each other
Why are some sub-levels half filled?
A half sub-level is quite stable
What is ionisation energy?
The energy required to remove a mole of electrons from a mole of atoms in the gaseous state
What is the first ionisation energy?
The energy required to remove one electron from a gaseous atom
What state must a substance be before it is ionised?
A gas
What is ionisation energy measured in?
kJ mol-1
Why does the first electron need the least energy to remove?
It is being removed from a neutral atom
What are successive ionisation energies?
The energy to required to remove further electrons one by one
What charge is the particle when 2nd, 3rd, 4th etc. ionisation energies are measured?
Positive
1+ for 2nd, 2+ for 3rd etc.
What is effective nuclear charge?
Attraction between the nucleus and outer electrons when shielding is taken into account
Which factors can affect ionisation energy?
- distance between outer electrons and nucleus
- how many electron shells there are - ‘shielding’
- nuclear charge - proton no.
How does shielding affect ionisation energy?
- More electron shells = more -ve charges to repel electrons
* So more shells means it’s easier to lose electrons
What is the general trend in first ionisation energies across period 3?
They increase
Does shielding affect first ionisation energy across period 3?
No, as there is the same number of shells
Why does first ionisation energy generally increase across period 3?
• proton number ↑ so nuclear charge ↑ (so ↑ charge
Why does the atomic radius decrease slightly across period 3?
Due to increase in positive charge on the nucleus
What are the exceptions to the trend in IE of period 3 elements?
Two drops in the graph - from Mg to Al and from P to S
Why is there a drop in the period 3 IE graph from Mg to Al?
- Mg electron removed from s sub-level whereas Al moved from P
- P higher in energy
- so Al is easier to remove
Why is there a drop in the period 3 IE graph from P to S?
- Both are removed from the p sub-level
- But S has a pair of electrons in the p orbital which repel
- This makes the electron from S easier to remove`
What is mutual repulsion?
When electrons in the same orbital repel each other
What is the general trend in first ionisation energies down group 2?
It decreases
Why does first ionisation energy decrease down group 2?
- increase in shells
- so increase in distance from nucleus
- so shielding increases
Why are outer electrons lost first during ion formation?
They are the easiest to remove
Why does ionisation energy down a group decrease, even though nuclear charge increases?
Increased shielding and distance outweigh this
Why do successive ionisation energies increase?
- same number of protons but fewer electrons
- so electrons are more strongly attracted to the nucleus
- and harder to remove
What does a large increase in successive ionisation energies suggest?
That the electron is being removed from an electron shell closer to the nucleus
Where does the peak furthest to the right on a mass spectrometer come from?
The molecular ion - and it’s m/z value gives the relative atomic mass
Aluminium’s first 3 electrons come from energy level 3, whereas the 4th is removed from energy level 2. What effect will this have on the 4th IE compared to the other 3?
4th IE will be much greater as it’s being removed from an energy level closer to the nucleus, so stronger nuclear attraction
Why does the first ionisation energy of atoms generally increase across a period?
Proton number increases so nuclear attraction increases
Why is the first ionisation energy of boron less than that of beryllium?
Boron’s outer electron is removed from the p sub-level which is further from the nucleus so easier to remove
Why do first ionisation energies decrease down a group?
Greater distance from nucleus, and more shielding, means there’s less nuclear attraction
Why is the second ionisation energy of sodium much greater than the first?
2nd electron removed from a more positive species so electrons are being attracted more - greater nuclear attraction
Examples of solvents used in electrospray ionisation?
- water
* methanol
