Atomic structure Flashcards
How do you calculate relative atomic mass?
(mass number of isotope 1 x abundance) + (mass number of isotope 2 x abundance) / 100 (or total abundance, usually 100)
How did John Dalton describe the atom?
Tiny solid spheres that couldn’t be divided
How did JJ Thompson describe the atom?
The plum pudding model
A ball of positive charge with negatively charged electrons studded throughout
How did Ernest Rutherford describe the atom?
The nuclear model
The atom is mostly empty space in which negative electrons surround the positive nucleus (where most of the mass is concentrated) in a ‘cloud’
What was the alpha particle scattering experiment and what did it determine?
Alpha particles were fired at a thin sheet of gold foil - most of the particles passed through or were deflected from their path, proving that the atom is mostly empty space and has a nucleus where most of the mass is concentrated
How did Neils Bohr describe the atom?
Bohr’s nuclear model
The electrons orbit the nucleus at a fixed distance in shells / energy levels
What did James Chadwick discover?
The neutron - the nucleus is made up of both protons and neutrons
What kind of samples is electron impact used for?
Samples of a smaller mass
What kind of samples is electrospray used for?
Samples of a larger mass
Why does a mass spectrometer use a vacuum pump?
To remove air from the apparatus so the ions don’t collide with air particles
What is mass spectrometry used for?
To find the abundance and mass of each isotope (therefore determining relative atomic mass) and finding the relative molecular mass of substances
What is the process of electron impact (also called electronic ionisation) ?
The sample is vaporised and bombarded by a stream of high energy electrons from an electron gun which can knock off an electron from an atom, producing a positively charged 1+ ion to be attracted towards the negatively charged plate
What is the general formula of electron impact ionisation?
X(g) –> X+(g) + e-
What is the process of electrosray ionisation?
The sample is dissolved in a volatile solvent (e.g. water / methanol) and injected through a fine hypodermic needle to give a fine mist (aerosol)
The tip of the needle is then attached to a high voltage power supply and the particles are ionised by gaining a proton (usually from a H+ ion) from the solvent as they leave the needle so they are then attracted towards the negatively charged plate
What is the general equation for electrospray ionisation?
X(g) + H+ –> XH+(g)
What are the four stages in a time of flight mass spectrometer?
Ionisation
Acceleration
Ion drift
Detection
What happens during acceleration?
The positive ions are accelerated using an electric field strength so that they all have the same kinetic energy
What does the velocity of each particle depend on?
It’s mass because all the particles have the same kinetic energy; lighter particles have a faster velocity, heavier particles have a slower velocity
What happens during ion drift?
The ions travel down a long tube at varying velocity due to their masses
What happens during detection?
The ions are distinguished by the time it takes for them to hit the detector
How is the mass (and therefore Mr) of the ions determined by the detector?
When the positive ions are discharged by gaining electrons from the plate, it generates an electric current that is measured; the more ions that hit the plate the bigger the current
A mass to charge ratio (m/z) is detected, but given the charge of each ion is +1, the m/z is effectively the mass of each ion
Why might some other higher peaks be present on the mass spectrometer?
Due to the presence of ²H or ¹³C atoms
Why is it necessary to ionise the sample?
- To allow particles to be accelerated
- To generate a current (electrical impulse) in the detector
What is an electron orbital?
A region of space in which the electron spends most of its time
What does each orbital hold?
It can hold up to 2 electrons with opposite spins (one going clockwise and the other anticlockwise)
What is the Aufbau principle?
The idea that electrons enter the lowest energy orbital available first
What is the order of all of the sub levels?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶
What are the two exceptions of the expected pattern of electron sub level arrangements and why?
Cr - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ (because then the 3d level is half full which is a more stable arrangement)
Cu - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰ (because then the 3d level is full which is a more stable arrangement)
What is 1st ionisation energy?
The energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions
How does atomic radius affect ionisation energy?
The smaller the radius, the stronger the attraction between the nucleus and the electron
How does the number of protons affect ionisation energy?
The more protons, the stronger the attraction between the nucleus and the electron
What is shielding?
The repulsion by electrons in shells between the outer electron and the nucleus
How does shielding affect ionisation energy?
The more shielding, the weaker attraction between the nucleus and the electron
What is the trend of ionisation energy down a group?
Ionisation energy decreases as you go down the group because:
- there is going to be more shielding
- and the atomic radius increases
therefore weaker attraction between the nucleus and electron
What are the trends in ionisation energy down a group evidence for?
The electrons of atoms being organised in energy levels / shells
What is the trend in ionisation energy across a period?
Across a period, the ionisation energy increases because the nuclear charge (number of protons) increases
What are the two blips in the trend across period 3 and why?
Al - the P sub level is higher in energy so the electron is more easily lost
S - The electrons in the 3p sub level are in a pair and repel each other so the electron is more easily lost
What is the variation across period 3 regarded as evidence for?
The existence of atomic sub levels
