1.1 Atomic Structure Flashcards
What is the name given to each electron shell (Main Energy Levels) of an atom?
Principle Quantum Number (Or N)
Electrons are arranged around the nucleus in principal energy levels or principal quantum shells
How can we calculate the maximum number of electrons each shell can hold?
2N²
What is an Atomic Orbital?
- A region around the nucleus which can hold up to two electrons
- With opposite spins
- When we have two electrons in the same orbital, these two electrons must have opposite spins
You need to know that electrons can have either an up-spin or a down-spin
Relative mass & charge of subatomic particles table:
What is an Atomic Number?
The atomic number (or proton number) is the number of protons in the nucleus of an atom and has symbol Z
The atomic number is equal to the number of electrons present in a neutral atom of an element
What is a Mass Number?
(Nucleon)
The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom and has symbol A
The number of neutrons can be calculated by: Number of neutrons = mass number - atomic number
What are Isotopes?
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
* Isotopes have similar chemical properties because they have the same electronic structure.
* They may have slightly varying physical properties because they have different masses.
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number:
E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively
How to calculate the relative atomic mass of an isotope:
Time of Flight Mass Spectrometry Diagram:
Time of Flight Mass Spectrometry : Ionisation
Stage 1 : Ionisation
There are two key ways in which the sample could be ionised:
* Electron Impact (or electron ionisation)
* Electrospray Ionisation
Time of Flight Mass Spectrometry : Ionisation
What is Electron Impact Ionisation? (P1)
This method of ionisation is used for elements and substances which have a lower molecular mass
- This method of ionisation is used for elements and substances which have a lower molecular mass
- The sample is vaporised and then bombarded with high energy electrons
- The electrons are ‘fired’ from an electron gun
- The electron gun is a hot wire filament which emits electrons as a current runs through it
- As the sample is bombarded by these electrons, an electron is knocked off each particle, forming a 1+ ion
- X (g) → X+ (g) + e-
The mass of the original atom remains constant
Electron impact is used for elements and substances with low formula mass. Electron impact can cause larger organic molecules to fragment.
Time of Flight Mass Spectrometry : Ionisation
What is Electron Impact Ionisation? (P2)
- The 1+ ions which have been formed are called molecular ions, or M+ ions
- These are then attracted towards a negatively charged plate
- This accelerates them through the mass spectrometer
The molecular ion can be broken down further, or fragmented:
The fragments are also accelerated through the sample and hit the detector, causing different peaks to show on the mass spectrum which is produced
Time of Flight Mass Spectrometry : Ionisation
What is Electrospray Ionisation? (P1)
This method is used for substances which have a higher molecular mass
Unlike with electron impact ionisation, fragmentation is unlikely to happen
This is often called a soft ionisation technique
- For this method, the sample is dissolved in a volatile solvent
- he solvent is injected into the mass spectrometer using a hypodermic needle. This produces a fine mist or aerosol
- The tip of the needle is attached to the positive terminal of a high voltage power supply, so as the sample is injected, the particles are ionised by gaining a proton from the solvent
- X (g) + H+ → XH+ (g)
- The solvent evaporates and the XH+ ions are attracted towards a negatively charged plate
- This accelerates them through the mass spectrometer
- They gain a single positive charge
Electro spray ionisation is used preferably for larger organic molecules. The ‘softer’ conditions of this technique mean fragmentation does not occur.
Time of Flight Mass Spectrometry : Acceleration
Acceleration
- The 1+ ions formed from either ionisation method are accelerated using an electric field
- They are all accelerated to have the same kinetic energy
- This is important for you to remember when completing calculations
- Since all 1+ ions will have the same kinetic energy,their velocity will depend on their mass
- Lighter ions will move faster and heavier ions will move slower
Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity.
Time of Flight MS : Stage 3: Ion Drift (in the flight tube)
Ion Drift
- The 1+ ions will pass through a hole in the negatively charged plate and move into a flight tube
- This is where the name ‘Time of Flight’ comes from
- The time of flight of each 1+ ion in this tube depends on their velocity
- Again, this is important to remember when completing calculations
Time of Flight Mass Spectrometry : Detection
Detection
- Once they have pass through the mass spectrometer, the 1+ ions will hit a negatively charged ‘detector’ plate
- As they hit this electric plate, they gain an electron
- This gaining of an electron discharges the ion, and causes a current to be produced
- This size of the current is proportional to the abundance of those ions hitting the plate and gaining an electron
- The detector plate is connected to a computer, which produces the mass spectrum
Key Equations for the TOFMS:
KE = kinetic energy of the particles (J)
m = mass of the particles (kg)
v = velocity of the particles (ms-1)
t = time of flight of the particles (s)
d = the length of the flight tube (m)
Shells/Orbitals Diagram
Each shell can be divided further into subshells, labelled s, p, d and f
Each orbital can only hold 2 electrons
What is a Subshell/Sublevel?
All of the orbitals of the same type in the same shell
S Orbital Shape:
- The s orbitals are spherical
- The size of the s orbitals increases with increasing shell number
E.g. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)
S/P Orbital Shape Diagram:
Note that the shape of the d orbitals is not required
Representation of orbitals (the dot represents the nucleus of the atom) showing spherical s orbitals (a), p orbitals containing ‘lobes’ along the x, y and z axis
What is the Ground State?
- The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy
- This is achieved by filling the subshells of energy with the lowest energy first (1s)
- The order of the subshells in terms of increasing energy does not follow a regular pattern at n = 3 and higher
Deducing the Electron Configuration (Periodic Table)
s block elements:
* Have their valence electron(s) in an s orbital
p block elements:
* Have their valence electron(s) in a p orbital
d block elements:
* Have their valence electron(s) in a d orbital
f block elements:
* Have their valence electron(s) in an f orbital
How do Subshells vary in energy?
- The subshells increase in energy as follows: s ＜ p ＜ d ＜ f
- The only exception to these rules is the 3d orbital which has slightly higher energy than the 4s orbital
- Because of this, the 4s orbital is filled before the 3d orbital
- All the orbitals in the same subshell have the same energy and are said to be degenerate E.g. px, py and pz are all equal in energy
Why do Chromium and copper have electron configurations, which are different to what you may expect:
The configurations are energetically stable
Box Notation
The electrons in titanium are arranged in their orbitals as shown. Electrons occupy the lowest energy levels first before filling those with higher energy
What is the First Ionisation Energy (IE)?
- The Ionisation Energy (IE) of an element is the amount of energy
- Required to remove one mole of electrons
- From one mole of gaseous atoms of an element
- To form one mole of gaseous ions
What is the Second Ionisation Energy (IE)?
- This is the energy needed to remove one mole of electrons
- From one mole of 1+ Ions in their gaseous state to form
- One more of 2+ Ions also in their gaseous state
What is the size of the First Ionisation Energy affected by (4)?
First ionisation energy increases across a period and decreases down a group
- Size of the nuclear charge - (The more protons in the nucleus the greater the attraction)
-
Distance of outer electrons from the nucleus (Atomic Radius) - (The bigger the atom the further the outer electrons are from the nucleus and the
weaker the attraction to the nucleus) - Shielding effect of inner electrons - (An electron in an outer shell is repelled by electrons in complete inner shells, weakening the attraction of the nucleus)
- Spin-pair repulsion
Why does the Ionisation Energy Increase Across a Period?
- Across a period the nuclear charge increases (The Number of Protons Increase)
- This increases the attraction between the nucleus and the electrons
- This causes the atomic radius of the atoms to decrease, as the outer shell is pulled closer to the nucleus, so the distance between the nucleus and the outer electrons decreases
- This means that the outer electrons are now more attracted to the nucleus
- The shielding by inner shell electrons remain reasonably constant as electrons are being added to the same shell
- It becomes harder to remove an electron as you move across a period; more energy is needed
- So, the ionisation energy increases
Why does the First Ionisation Energy Decrease Down a Group? (2)
- Moving down a group - The Atomic Radius Increases - The outer electron shell is further away from the nucleus
- The number of internal energy levels increase - There is more shielding between the nucleus and the outer electrons
Both of these factors mean that going down a group, the attraction between the nucleus and the outer electrons decrease - causing the first ionisation energy to fall
Ionisation Energy Trends across a Period & going down a Group Table
Successive Ionisation Energies of Beryllium Table
How does the size of the Atomic Radius affect ionisation energy?
- As the Atomic Radius increases in size
- The force of attraction between the positive nucleus and negative outer electrons decreases
How does the Charge of the nucleus affect ionisation energy?
- The Negative electrons are attracted to the Positively Charged protons in the nucleus
- The greater the number of protons - The greater the force of attraction between the outer electrons and the positive nucleus
How does the Effect of Shielding affect ionisation energy?
- Electrons in the outer shells are repelled by electrons in the inner shells
- This shielding effect reduces the attraction between the outer electrons and the nucleus
What can Successive Ionisation Energies be used to show?
- Predict or confirm the simple electronic configuration of elements
- Confirm the number of electrons in the outer shell of an element
- Deduce the Group an element belongs to in the Periodic Table
Why is there a vaccum in the TOFMS?
- So ions do not collide with molecules in the air
Describe how ions are formed in a time of flight (TOF) mass spectrometer (2)
- (For Electrospray Ionisation) A high voltage is applied to a sample in a polar solvent
- Then the sample molecule M, gains a proton forming MH+
- OR (Fr electron impact ionisation)
- The sample is bombarded by high energy electrons
- The sample molecule loses an electron forming M+
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer (2)
- Ions, not molecules, will interact with and be accelerated by an electric field
- Only ions will create a current when hitting the detector
Explain how ions are accelerated, detected and have their abundance determined in a time of flight (TOF) mass spectrometer (3)
- (Ions accelerated by) attraction to negatively charged plate/electric field
- Ions detected by gaining electrons
- Abundance determined by (size) of current flowing in the detector
Deduce the formula of the compound that contains 2+ ions and 3− ions that both have the same electron configuration as argon.
Ca3P2
State which of the elements magnesium and aluminium (Same Period) has the lower first ionisation energy.
Explain your answer.
- Al
- (Outer) electron in (3)p sublevel / orbital
- Higher in energy / further from the nucleus
- so easier to remove
State and explain the general trend in first ionisation energy across Period 3 (4)
- General Increase
- As there is a greater nuclear charge / more protons
- Same shielding
- Therefore a stronger Attraction (from nucleus) for outer electron (s)
Give one example of an element which deviates from the general trend in first ionisation energy across Period 3.
Explain why this deviation occurs (3)
- Aluminium / Al (lower than Mg)
- (Outer) electron in (3)p orbital / sub-shell (level)
- (3p) higher in energy
Identify the Period 3 element that has the highest melting point. Explain your answer by reference to structure and bonding (4)
- Silicon
- As it is a Giant Covalent Structure
- Containing Covalent Bonds
- And there are Many strong Covalent bonds which require lots of energy to break
Electron Sub-Levels Table:
Electron Capacity of Sub-Levels Table:
Electron Configuration Notation:
Electron Configuration Exceptions Include:
Copper
Chromium and Copper
Electron Configuration Exceptions Include:
Chromium
Transition elements (d-block) will always … when forming transition metal ions (i.e in compounds)
Orbital Notation (d Group)
What is an Orbital?
A region of space in which there is a high probability (95%) of finding an electron
How many Orbitals are in each Sub-Level?
Details of the three Sub-Atomic (fundamental) particles Table:
Why are successive ionisation energies always larger?
- When the first electron is removed a positive ion is formed.
- The ion increases the attraction on the remaining electrons and so the energy required to remove the next electron is larger.
How are ionisation energies linked to electronic structure? (Diagram)
The fifth electron is in a inner shell closer to the nucleus and therefore attracted much more strongly by the nucleus than the fourth electron.
It also does not have any shielding by inner complete shells of electron
How are ionisation energies linked to electronic structure? (Table)
The First Ionisation Energy of the elements Graph :
You need to carefully learn the patterns
Why does helium have the largest first ionisation energy?
Its first electron is in the first shell closest to the nucleus and has no shielding effects from inner shells. Helium has a bigger first ionisation energy than H as it has one more proton
Why do first ionisation energies decrease down a group?
As one goes down a group, the outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller
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 than Neon?
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? (First Ionisation Graph)
- Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3ssub 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? (First Ionisation Graph)
- 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.
Define the mass number of an atom (1)
Number of protons + neutrons (in the nucleus of the atom)
Define the term relative atomic mass (2)
- The average mass of an atom of an element
- Compared to 1/12th the mass of an atom of carbon-12
The sample of chromium is analysed in a (TOF) mass spectrometer.
(d) Give two reasons why it is necessary to ionise the isotopes of chromium
before they can be analysed in a TOF mass spectrometer (2)
- (Ions will interact with and) be accelerated (by an electric field)
- Ions create a current when hitting the detector OR ions create a current in the detector/electron multiplier.
Describe how molecules are ionised using electrospray ionisation (3)
- (Sample is) dissolved (in a volatile solvent)
- (Injected through) needle/nozzle/capillary at high voltage/positively charged
- Each molecule/particle gains a proton/H+
Explain the pattern in the first ionisation energies of the elements from lithium to neon (6)
The first ionisation energies of the elements in Period 2 change as the atomic
number increases.
State and explain the trend in the first ionisation energies of the elements
in Group 2 from magnesium to barium (3)
- Decrease
- Ions get bigger / more (energy) shells
- Weaker attraction of ion to lost electron
Describe how ions are formed in a time of flight (TOF) mass spectrometer for electron impact ionisation (2)
- the sample is bombarded by high energy electrons
- the sample molecule loses an electron forming M+
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer (2)
- Ions, not molecules, will interact with and be accelerated by an electric field
- Only ions will create a current when hitting the detector
Suggest why electrons which occupy the 2p sub-levels have a higher energy than electrons in the 2s sub-level (1)
Further from nucleus (1)
Explain why the first ionisation energy of helium is very large (1)
Electron is not shielded from nucleus (1)
State how, and explain why, the first ionisation energy of aluminium does not follow this general trend (3) (First ionisation - Increasing)
- lower than expected / lower than Mg /
- less energy needed to ionise; e– removed from (3)p sub-level;
- of higher energy / further away from nucleus
Explain why atomic radius decreases across Period 3 from sodium to chlorine (2)
- Number of protons increases (1)
- Electrons in same shell (1) (or same shielding)
Values for the covalent radii of the elements in Period 3 are given in the next table. Explain the decrease in the values shown in the table (3)
- increased nuclear charge / nuclear attraction number of protons (1)
- same shielding / electrons added to same or outer shell / increase in number of electrons in outer shell (1)
- therefore (outer) electrons attracted / pulled in more strongly or more closely (1)
State any differences and similarities in the atomic structure of the isotopes of an element. State the difference, if any, in the chemistry of these isotopes. Explain your answer (4)
- Differ in mass number or number of neutrons (1)
- Same proton / atomic number (1)
- Isotopes have the same chemical properties (1)
- because all have the same electron configuration or number of electrons or same number of valence electrons (so no chemical difference) (1)
Describe and explain the variation in first ionisation energy of the elements across Period 3 from sodium to argon (6)
- general increase across period (1)
- because number of protons (or nuclear charge) increases (1)
- but electrons in same shell (or similar shielding) (1)
- fall from Mg to Al (1)
- Al’s outer electron is in a p orbital (1)
- higher in energy than s electron in Mg (1)
- fall from P to S (1)
- two of the p electrons in S are paired (or in same orbital) (1)
- Repulsion - Less energy needed
Give the relative masses and charges of the three particles in an atom (3)
In terms of the number of their fundamental particles, what do two isotopes of an element have in common and how do they differ? (2)
- Same number of protons (and electrons) [1]
- Different number of neutrons [1]
Define the term relative atomic mass of an element. (2)
- Weighted average mass of a (naturally occurring sample of an) atom of an element (1)
- Relative to 1/12 the mass of an atom of 12C in its ground state
Chromium
b) From the species mentioned in part (a), identify one species that does not appear to follow the usual pattern of electronic structures, and explain why this is so.
Half-filled 3d is more a more stable (lower energy) arrangement than 3d44s2 [1]
Symbol equation for First Ionisation Energy:
- A mole of electrons is removed
- From a mole of atoms in the gas state/ gaseous atoms
- Leaving (a mole of) unipositive/1+ ions (in the gas state)
Explain the variation in ionisation energy illustrated by this graph, with specific reference to the s and p orbitals (6)
- Generally increasing due to increasing nuclear charge and no increase in shielding/outer electron in same principle energy level (2)
- Decrease from Mg to Al due to outer Al electron removed from (3)p orbital rather than (3)s for Mg (1)
- p orbital higher energy orbital than s (1)
- Decrease from P to S due to electron removed being paired in a 3p orbital (1)
- Paired electrons experience more/extra repulsion (which lowers 1st i.e.) (1)
State and explain the variation in first ionisation energies of the elements in a group such as Group 1 (3 Marks)
- Decreases down the group (1)
- Outer electron removed from a higher energy level/further from nucleus (1)
- Increase in shielding of nuclear charge outweighs increase in nuclear charge (1)
A method of ionising samples in time of flight mass spectrometry is by electrospray ionisation. How is this ionisation done? (4 Marks)
- Sample dissolved in a volatile solvent (water/methanol) (1)
- injected through a fine hypodermic needle to give a fine mist (aerosol) (1)
- Needle is attached to the positive terminal of a high-voltage power supply (1)
- Particles/sample ionised by gaining a proton/ H+ from the solvent as it leaves the needle (1)
(Solvent evaporates away, ions are attracted towards a negative plate where they are accelerated)
Explain why different ions in a TOF mass spectrometer take different times to travel through the flight tube (2 Marks)
- Same kinetic energy for all ions (1)
- Higher velocity for smaller mass (1)
Must have same kinetic energy to score second mark
Explain how the ions are detected and their abundancies determined in the mass spectrometer (3 Marks)
- Positive ions gain an electron from the detector (1)
- Causing a current to flow (1)
- Magnitude of current is proportional to relative abundance (1)
Show how these expressions can be used to show the time of flight
Rearranging Expressions to show TOF:
KE = 1⁄2 m 𝑣2 | the time to travel through the flight tube as: t=d / 𝑣
Explain why the second ionisation energy of beryllium is greater than the first ionisation energy (2)
- Electron removed from positive ion (1)
- which attracts the electron more (1)
Equation for electron impact ionisation (1)
Ge(g) + e– → Ge+(g) + 2 e–
How to calculate mass in TOFMS calculatons:
(Mass number / 1000) x 6.02x10^23
