3.1.1 atomic structure (just things not too good at) Flashcards
john dalton
atoms were solid spheres that make up elements, different spheres for each element
JJ Thompson
plum pudding model , showing atoms contain electrons
Ernest Rutherford
conducted the alpha scattering gold foil experiment to produce nuclear model .
- fired positively charged alpha particles at a very thin sheet of gold
- Ppm suggested : most particles slightly deflected by positive ‘pudding’
- actually : most straight through (most atom is empty space) some deflected (positive centre)
Niels Bohr
adapted nuclear model , putting electrons in shells/ orbitals of fixed energy .
- they raised electrons in a ‘cloud’ around the nucleus spiral down into the nucleus , causing it to collapse
- when electrons move to a new shell, electromagnetic radiation ( with fixed energy/ frequency) is emitted (down shells) or absorbed (up shells/ away from nucleus)
then
scientists discovered that not all electrons in the same shell have the same energy so refined the model to include sub shells. Not a perfect model but is useful as it is simple and explains many experimental observations e.g. bonding and ionisation energy
mass number [A]
total number of protons + neutrons in the nucleus. Nucleons are responsible for almost all the mass as electrons have a tiny mass
atomic number [Z]
the number of protons in the nucleus. This also equals the number of electrons in the atom meaning atoms are electrically neutral. The number of electrons in the outer shell of an atom determines the chemical properties of the element , how it reacts and what sort of element it is : its chemical identity. All atoms of the same element have the same atomic number
relative atomic mass [Ar]
the average mass of an atom of an element , taking into account its naturally occurring isotopes , relative to 1/12th of the mass of an atom of carbon -12
why C-12
too small numbers so relative to C-12 takes into account the percentage abundances of the isotopes
how to work it out
Ar = (isotope mass number x % abundance) / sum of % isotope abundance
relative isotopic mass
mass of an atom of an isotope relative to 1/12th the mass of an atom of carbon -12 . Always a whole number
relative molecular mass [Mr]
average mass of a molecule in relation to 1/12 the mass of a carbon -12 atom
how to calculate
Mr = sum of Ar in formula for the molecule
isotope
an atom of the same element with the same number of protons (atomic number) and a different number of neutrons (so different mass number)
about their properties
- same chemical properties as they have the same electron configuration
- slightly different physical properties as they depend on the mass of the atom e.g. density
radioactive?
isotopes can be radioactive if they are unstable (extra protons or neutrons in the nucleus creating extra energy)
- they emit radiation as they decay + the rate of decay is measured by half- life ( the time it takes for half of its radioactivity to decay)
- they have lots of uses e.g. : carbon dating. Carbon -14 isotope is radioactive with a long half life found in all living things . Scientists can work backwards to see how long a living organism has been dead for .
time of flight mass spectrometer
powerful instrumental technique that is useful for the accurate determination of relative isotopic masses and relative abundances. it determines the masses of separate atoms/molecules and is used by forensic scientists to help identify illegal drugs. can be used to work out relative atomic masses [Ar] to identify elements and relative molecular masses [Mr] .
It works by forming ions, then separating them according to the ratio of their charge: maths
stages of TOFMS
- ionisation
- acceleration
- ion drift
- detection
- data analysis
ionisation
this is where each atom becomes an ion and can be done one of two ways depending on its mass
low molecular mass
ELECTRON IMPACT IONISATION: for compounds with low molecular masses.
- sample is vaporised then high energy electrons are fired at the sample using an electron gun , one electron is knocked off each atom / particle forming a 1+ ion
equation
X(g) -> (X)+ (g) + e-
high relative molecular mass
ELECTROSPRAY IONISATION- for compounds with high relative molecular mass
- sample is dissolved in a volatile solvent + is injected into the ionisation chamber through a hypodermic needle which has a high voltage as is positively charged. The particles gain a proton and become 1+
ions as a fine mist. The solvent evaporates leaving the 1+ ions.
equation
X(g) + H+ -> (XH)+ (g)
acceleration
ions are accelerated using an electric field. the positively charged ions accelerate towards a negatively charged plate. this is so that all the ions have the same kinetic energy. they all have the same kinetic energy, but their velocity will differ as it depends on their mass lighter ions will have a higher velocity than heavier ions.
ion drift
ions pass through a hole in the plate into the flight tube where they enter a region with no electric field so they just drift through it towards the detector. ions with different masses have a different time of flight.
detection
The detector is a negatively charged plate and a current is produced when the ions hit the plate. the positive ions pick up an electron from the detector causing a current to flow
data analysis
The signal from the detector is passed to a computer which generates a mass spectrum
if electrospray what to do
the Mr is always 1 less than the highest peak
look at chlorine mass spectra
TOFMS calculations for a single ion
1) mass of one ion in Kg
2) mass using kinetic energy
3) kinetic energy of one ion
4) time of flight
5) distance of flight tube
TOFMS calculations for two ions
1-5
AND
6) TOF of one isotope ions with same kinetic energy
7) velocity of isotope ions with same kinetic energy
look at ppt for each step in more depth
blocks in periodic table
from left to right
s,d,p
electron configuration exceptions chromium
expect it to be 4S^2 3d^4 however, it donates one of its 4s electrons to the 3d sub shell so the 3d sub shell is more stable as its half - full. its configuration is actually 4s1 3d5
electron configuration exceptions copper
expect it to be 4s2 3d9 however it donates one of its 4s electrons to the 3d sub shell so the 3d sub shell is more stable as it’s now full. its configuration is actually 4s1 3d10
what’s lost first
4s before 3d
first ionisation energy
the energy needed to remove 1 electron from each atom of an element in one mole of gaseous atoms to form one mole of gaseous ions with a +1 charge
X(g) -> X+ (g) + e-
second ionisation energy
the energy needed to remove 1 electron from each ion of an element in 1 mole of gaseous +1 ions to form 1 mole of gaseous ions with a +2 charge
X+(g) -> X2+ (g) + e-
factors affecting ionisation energy
- atomic radius
- nuclear charge
- shielding
atomic radius
the smaller the atomic radius , the higher the first ionisation energy . Because the outer electron is more strongly attracted to the nucleus
nuclear charge
the higher the nuclear charge the higher the first ionisation energy because a higher nuclear charge means a smaller atomic radius because there are more protons to attract the outer electrons therefore more attracted to the nucleus
shielding
the more shielding the lower the first ionisation energy inner shells repel the outer electrons making them easier to lose
tends in ionisation energy across a period
increase
- higher nuclear charge , smaller atomic radius
- outer electrons are more attracted to the nucleus
- shielding is constant
trends in ionisation energy down a group
decrease
- larger atomic radius
- increase in shielding as more energy levels are added
- outer electron is less attracted to the nucleus and is therefore easier to lose
exceptions in ionisation energy trends between group 2 and 3
- dip in first IE
- mode to a new energy level with higher energy S -> P subshell
- further away from the nucleus and more shielding
exceptions in ionisation energy trends between group 5 and 6
- dip in first IE
- in group 5, P subshell has one electron in each orbital, no repulsion
- in group 6 , P subshell has one orbital with 2 electrons in
- the electrons in the same orbital repel each other
- so less energy is needed to remove the outer electron
successive ionisation energies look at slide
