atomic structure Flashcards
Relative atomic mass
The average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12.
Relative isotopic mass
The mass of an atom of an isotope of an element compared to 1/12th the mass of an atom of carbon-12.
Relative molecular mass
The average mass of a molecule compared to 1/12th the mass of an atom of carbon-12
Mass number
The sum of neutrons and protons in an atom.
Atomic number
The number of protons in an atom.
history of atomic theory
dalton-stated that atoms are the smallest forms of matter
thompson- discovered electrons and proposed the plum pudding model which stated that atoms were a sphere of positive charge with electrons evenly distributed within it.
rutherford, geiger and marsden- conducted the alpha scattering experiment by firing alpha particles at a thin gold foil. mostly empty space as most particles went straight through, positive charge in the middle as some were deflected, small, dense nucleus as some bounced straight back.
bohr- proposed that electrons orbit the nucleus in shells.
chadwick-discovered neutrons
what is mass spectrometry
the mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes.
what can mass spectrometry be used for
to collect data on the mass and abundance of elements
electron impact ionisation
-sample is vapourised, high energy electrons are fired at it
-the electrons are emitted from a charged hot wire filament
-the electrons knock off one electron from each particle, forming 1+ ions.
X(g) + e^- —> X^+(g) + 2e^-
-used for substances with low RFM
-the ions often fragments which may also be detected
electrospray ionisation
-the sample is dissolved in a volatile solvent and injected through a fine, positively charged hypodermic needle so it become a fine mist.
-the particles are ionised by gaining a proton from the solvent as they leave the needle, producing XH+ ions
X(g) + H^+ —> XH^+(g)
-the solvent evaporates away
-it is used for substances with higher RFM such as proteins
-fragmentation is rare
acceleration
- the positive ions are accelerated so that they all have the same kinetic energyusing an electric field towards a negatively charged detection plate.
- as the kinetic energy is the same, the velocity depends on the mass of the ions
drift
- the positive ions travel through a hole in the negatively charged plate into a flight tube
- the TOF depends on its velocity
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
which causes a current to be produced
-This size of the current is proportional to the abundance of the ions hitting the plate and gaining an electron
-The detector plate is connected to a computer, which produces the mass spectra.
- a spectrum shows the mass to charge ratio (m/z) and the abundance of each ion that reaches the detector.
why might there be a trace at half the expected m/z value
during the ionisation process, a 2+ ion may be produced so the m/z ratio is halved
define first ionisation energy
the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms.
formulas for 1st 2nd and 3rd ionisation energy
1st: X(g) --> X^+(g) +e^- 2nd: X^+(g) --> X^2(g)+e^- 3rd: X^2+(g) --> X^3+(g) + e^-
factors affecting ionisation energy
proton number- more protons=stronger attraction between the nucleus and the outer e^-s.
atomic radius - smaller radius = outer e^-s are closer to nucleus so stronger attraction.
shielding - more inner electrons weakens the attraction between the nucleus and outer e^-s.
why is IE always positive
-energy is always needed to remove an e^- from an atom as it is attracted to the positive nucleus.
why is 2nd ionisation energy always bigger
the atoms becomes a positive ion so there is a stronger attraction between outer electrons and positive nucleus
why can there be a big jump in IE
the next electron being removed is in the next shell which is closer to the nucleus.
what are the periodic trends
atomic radii decreases as you go across a period. this is because the number of protons increases so the nuclear charge also increases. therefore, there is a stronger pull on the e- shells. ionisation energy increases across a period due to more protons but same shielding so stronger attraction between nucleus and outer e-s.
what are the different subshells, how many orbitals do they have and how many e-s can they hold
S sub-shell has 1 orbital which holds 2 e-s
P subshell has 3 orbitals which holds 6 e-s
D subshell has 5 orbitals which holds 10 e-s
F subshell has 7 orbitals which holds 14 e-s
why does the 4s subshell come before the 3d subshell
it has lower energy so fills first
exceptions to the electronic configuration order
copper: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^1, 3d^10
chromium: 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^1, 3d^5
explain the group 2 to 3 dip in ionisation energy
the outer electron is being lost from the 3p subshell rather than 3s in magnesium. it is a higher energy shell so the e- is removed easier
explain the 5 to 6 dip in ionisation energy
sulphur has paired electrons in the 3p subshell whereas phosphorous doesn’t. this causes electron-electron repulsion which decreases ionisation energy
how does melting point change across a period
increases until group 4 and then decreases. metals on the left then covalent
