[3.1.1] Atomic Structure Flashcards
Fundamental Particles, Mass Number and Isotopes & Electron Configuration
What is the relative charge and relative mass of the three sub-atomic particles?
PROTON
- Relative charge = +1
- Relative mass = 1
NEUTRON
- Relative charge = 0
- Relative mass = 1
ELECTRON
- Relative charge = -1
- Relative mass = 1/1840
What is the mass number and which letter represents it?
- The mass number is the total number of protons and neutrons in the atom.
- It is represented by the letter A.
What is the atomic number and which letter represents it?
- The atomic number is the total number of protons in the nucleus.
- It is represented by the letter Z.
How would you calculate the number of neutrons in an atom?
Number of neutrons = mass number - atomic number
What are isotopes?
- Isotopes are atoms with the same number of protons, but different numbers of neutrons.
Describe the principles of time of flight mass spectrometry.
- The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements.
- It needs to be under a vacuum otherwise air particles would ionise and register on the detector.
IONISATION
Sample can be ionised in two ways:
- ELECTRON IMPACT
- Vaporised sample is injected at low pressure.
- An electron gun fires high-energy electrons at the sample.
- This knocks out an outer electron.
- Forming positive ions with different charges
- e.g. Ti (g) -> Ti+ (g) + e⁻
- This method is used for elements and substances with low formula mass, electron impact can cause larger organic molecules to fragment.
- ELECTROSPRAY IONISATION
- The sample is dissolved in a volatile, polar solvent.
- It is injected through a fine needle giving a fine mist or aerosol.
- The tip of the needle has a high voltage.
- At the tip of the needle the sample molecule, M, gains a proton, H⁺, from the solvent forming MH⁺.
- This method is used for larger organic substances because the ‘softer’ conditions mean fragmentation does not occur.
ACCELERATION
- Positive ions are accelerated by an electric field to a constant kinetic energy.
- Given that all the particles have the same KE, the velocity of each particle depends on its mass.
- Lighter particles have a faster velocity and heavier particles have a slower velocity.
ION DRIFT
- The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z but will move faster.
- Heavier ions will take longer to move through the drift area as they’re moving at a lower speed.
- The ions are distinguished by different flight times.
DETECTION
- The ions reach the detector and generate a small current, which is fed to a computer for analysis.
- The current is produced by electrons transferring from the detector to the positive ions.
- The size of current is proportional to the abundance of the species.
- For each isotope, the mass spectrometer can measure an m/z (mass/charge ratio) and an abundance.
How do you calculate relative atomic mass?
- R.A.M = ∑(isotopic mass x % abundance) / 100
If relative abundance is used instead of percentage abundance, use this equation:
- R.A.M = ∑(isotopic mass x abundance) / total relative abundance
How would you determine the Mr of a molecule that’s undergone electron impact ionisation from a mass spectrum?
- If a molecule is put through a mass spectrometer with an electron impact ionisation stage, it will often break up and give a series of peaks caused by fragments.
- The peak with the largest m/z, however, will be due to the complete molecule and will be equal to the relative molecular mass, Mr, of the molecule.
- This peak is called the parent ion or molecular ion.
How would you determine the Mr of a molecule that’s undergone electro-spay ionisation from a mass spectrum?
- If a molecule is put through a mass spectrometer with electro-spray ionisation then fragmentation will not occur.
- There will be one peak that will equal the mass of the MH⁺ ion.
- It will therefore be necessary to subtract 1 to get the Mr of the molecule.
- So if a peak at 521.1 is for MH⁺, the relative molecular mass of the molecule is 520.1.
Describe how electrons are arranged in an atom.
ELECTRONS ARE ARRANGED ON…
-
Principle energy levels numbered 1, 2, 3, 4…
- 1 is closest to the nucleus.
SPLIT INTO ->
-
Sub energy levels labelled s, p, d & f.
- s holds up to 2 electrons.
- p holds up to 6 electrons.
- d holds up to 10 electrons.
- f holds up to 14 electrons.
SPLIT INTO ->
- Orbitals which hold up to 2 electrons of opposite spin.
- This means that:
- s sub energy levels have 1 orbital.
- p sub energy levels have 3 orbitals.
- d sub energy levels have 5 orbitals.
- f sub energy levels have 7 orbitals.
Describe the order in which an atom fills its subshell.
IN ORDER OF INCREASING ENERGY
1s -> 2s -> 2p -> 3s -> 3p -> 4s -> 3d -> 4p -> 5s -> 4d -> 5p
- 3d is higher in energy than 4s and so gets filled after the 4s.
What are orbitals? What shape are the s and p orbitals?
- Orbitals represent the mathematical probabilities of finding an electron at any point within certain spatial distributions around the nucleus.
- Each orbital has its own approximate, 3D shape.
- S orbitals are shaped like spherical.
- P orbitals are shaped like dumbbells.
Describe how you would write the electronic structure for oxygen using letters and numbers.
Describe how you would use a spin diagram to represent fluorine’s electronic structure.
What are s, p and d block elements?
THE PERIODIC TABLE IS SPLIT INTO BLOCKS…
- s block elements are ones whose outer electron is filling a s-sub shell.
- p block elements are ones whose outer electron is filling a p-sub shell.
- d block elements are ones whose outer electron is filling a d-sub shell.
What is the electronic structure of chromium (Cr)?
Cr = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵
What is the electronic structure of copper (Cu)?
Cu = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
What happens when d-block elements form ions?
- They lose their 4s electrons first.
Define first ionisation energy.
The first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.
What is the equation for first ionisation energy?
H (g) -> H⁺ (g) + e⁻
- The equation for 1st ionisation energy allows follows the same pattern.
- It does not matter if the atom does not normally form a +1 ion or is not gaseous.
Define second ionisation energy.
The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge.
What is the equation for second ionisation energy?
Ti⁺ (g) -> Ti²⁺ (g) + e⁻
What factors affect ionisation energy?
THERE ARE THREE MAIN FACTORS…
-
THE ATTRACTION OF THE NUCLEUS
- The more protons in the nucleus, the greater the attraction.
-
THE DISTANCE OF THE ELECTRONS FROM THE NUCLEUS
- The bigger the atom, the further the outer electrons from the nucleus and the weaker the attraction to the nucleus.
-
SHIELDING OF THE ATTRACTION OF THE NUCLEUS
- An electron in an outer shell is repelled by electrons in complete inner shells, weakening the attraction of the nucleus.
Why are successive ionisation energies always larger?
- The second ionisation energy is always bigger than the first ionisation energy.
- 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.
Explain the big jump between 4 & 5.
Why has helium got 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 hydrogen as it has one more proton
Why do first ionisation energies decrease down a group?
- As one goes down the 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.
- However, the number of protons increases making the effective attraction of the nucleus greater.
Why has Na have a much lower first ionisation energy than Ne?
- This is because sodium will have its outer electron in a 3s shell further from the nucleus is more shielded.
- So sodium’s outer electron is easier to remove and has a lower ionisation energy.
Why is there a small drop in ionisation energy from Mg to Al?
- Al is starting to fill a 3p subshell, whereas Mg has its outer electrons in the 3s subshell.
- The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and also slightly shielded by the 3s electrons.
Why is there a small drop in ionisation energy from P to S?
- With sulfur there are 4 electrons in the 3p subshell and the 4th is starting to doubly fill the first 3p oribital.
- When the second electron is added to the 3p orbital there is a slight repulsion between two negatively charged electrons which makes the second electron easier to remove.
