Atomic Structure Flashcards
what is the plum pudding model?
atoms consisted of sphere of positive charge, with small negative charges distributed evenly within it
what is the nuclear model?
atom consists of a small, dense central nucleus surrounded by orbiting electrons in electron shells
relative mass of an electron?
1/1840
how to calculate maximum number of orbiting electrons that can be held by any single shell?
2n² where n is the number of the shell
what letter represents mass number?
A
what letter represents atomic number?
Z
what is relative atomic mass?
the mean mass of an atom of an element compared with one twelfth of the mean mass of an atom of carbon-12
what is an isotope?
Isotopes are atoms with the same number of protons, but different numbers of neutrons.
why do isotopes have similar chemical properties?
same electronic structure
why do isotopes have slightly varying physical properties?
different masses
why may a mass spectrometer be used?
to determine the isotopes present in a sample of an element + therefore identify elements
why must a tof mass spectrometer be under a vacuum?
- to prevent air molecules from colliding with ions
- air particles could ionise + register on the detector
list the 4 steps in tof mass spectrometry
1) ionisation
2) acceleration
3) ion drift/deflection
4) detection
List the 2 methods of ionisation
electron impact
electro spray
describe electron impact ionisation
- a vaporised sample is injected at low pressure
- electron gun fires high-energy electrons at sample
- this knocks out an outer electron forming positive ions with different charges
when would electron impact ionisation be used?
- elements + substances with low formula mass
- can cause larger organic molecules to fragment
describe electro-spray ionisation
- sample dissolved in a volatile, polar solvent
- injected through fine hypodermic needle giving a fine mist or aerosol
- tip of needle has high voltage
- proton gained at tip of needle from the solvent
- solvent evaporates away while ions move towards negative plate
when would electro-spray ionisation be used?
- for larger organic molecules
- softer conditions means fragmentation doesn’t occur
describe the acceleration stage of tof mass spectrometry
positive ions are accelerated towards a negatively charged detection plate to a constant kinetic energy
what determines the velocity of the ions in the acceleration stage of tof mass spectrometry?
- mass
- lighter particle = greater velocity
describe the ion drift/deflection stage of tof mass spectrometry
- ions deflected by a magnetic field into a curved path
- radius of path determined by charge + mass
- positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z + will move faster
- heavier particles take longer to move through drift area
- ions distinguished by different flight times
describe the detection stage in tof mass spectrometry
- when positive ions hit the negatively charged detection plate, they gain an electron producing a flow of charge
- greater abundance of the species = greater charged produced
what is m/z?
mass/charge ratio
what is the m/z of 24Mg2+ ?
12
mass=24
charge = 2
24/2 = 12
what is an atomic orbital?
- a region around the nucleus that can hold up to two electrons with opposite spins
- all orbitals in a subshell have the same energy
what is an electron?
a cloud of negative charge
what is the shape of an s orbital?
spherical
which electron shells contain s orbitals? how many?
every electron shell contains a single s orbital
what is the shape of a p orbital?
dumbell
which electron shells contain p orbitals? how many?
every electron shell apart from the first contains 3 p orbitals
which electron shells contain d orbitals? how many?
every shell apart from shells 1 and 2 have 5 d orbitals
which electron shells contain f orbitals? how many?
shells 4 and above have 7 f orbitals
what is a subshell/level
all of the orbitals of the same type in the same shell
as you move away from the nucleus, does the energy of the subshells increase or decrease?
increase
what are the rules for filling atomic orbitals?
1) orbitals with the lowest energy are filled first
2) can have up to 2 electrons in the same orbital but they must have opposite spins
3) electrons are put into individual orbitals with the same energy before they are paired
why are electrons put into individual orbitals first before they are paired?
electrons in the same orbital repel
how is an electron in an orbital represented?
half arrow in the direction of the electron’s spin
how is electron configuration written?
1s1,2s2,2p6,3s2,3p6,4s2,3d10
the first number represents the shell number
the letter represents the orbital
the other number represents the number of electrons in the orbital
why does the 4s subshell fill before the 3d subshell?
before it is filled, the energy of the 4s subshell is less than the energy of the 3d subshell
which elements are exceptions to the electron configuration rules?
why?
- chromium and copper
- 3d subshells more stable when half or completely full
- for chromium by having only 1 electron in 4s subshell it can have half full 3d subshell, so more stable
- for copper, by having only 1 electron in 4s subshell, it can have completely full 3d subshell, so more stable
when an ion forms, where do the electrons gained/lost go?
the subshell with the highest energy gains or loses electrons
where do d block elements lose electrons from? why?
- 4s subshell
- 4s subshell has a higher energy than 3d subshell once it contains electrons
what is the first ionisation energy?
the energy needed to remove one mole of electrons from one mole of atoms in their gaseous state to form one mole of 1+ ions (also in their gaseous state)
what is the second ionisation energy?
the energy needed to remove one mole of electrons from one mole of 1+ in their gaseous state to form one mole of 2+ ions (also in their gaseous state)
write an equation for the first ionisation energy of Mg
Mg -> Mg+ + e-
write an equation for the second ionisation energy of Mg
Mg+ -> Mg2+ + e-
list 3 factors that affect ionisation energy
1) atomic radius
2) nuclear charge
3) shielding
what is atomic radius? how does it affect ionisation energy?
- distance between nucleus and the outermost electrons
- as atomic radius increases, the force of attraction between the positive nucleus and the outer electrons decreases which lowers the ionisation energy
how does nuclear charge affect ionisation energy?
greater number of protons = greater force of attraction between the outer electrons and nucleus therefore increasing the ionisation energy
how does shielding affect ionisation energy?
- electrons in outer shell are repelled by electrons in inner shells
- this shielding effect reduces the attraction between the outer electrons and the nucleus and therefore lowers the ionisation energy
why does first ionisation energy decrease going down a group?
- atomic radius increases so outer electrons are further away from nucleus
- number of internal energy levels also increases so there is more shielding between the nucleus and the outer electrons
- therefore going down a group, attraction between the nucleus and outer electrons decreases, resulting in a lower ionisation energy
how does first ionisation energy change across a period?
tends to increase
why does first ionisation energy increase across a period?
- number of protons increases across a period (nuclear charge) which increases the attraction between nucleus and electrons
- atomic radius also decreases across a period
- therefore outer electrons are more attracted to the nucleus causing the first ionisation energy to increase across the period
why don’t boron and oxygen fit the trend in first ionisation energy across a period?
- boron has a lower 1st i.e than Be as the outer electron is in the 2p subshell which has higher energy than the 2s subshell so it takes less energy to remove the puter electron of boron compared to the outer electron of Be
- oxygen has one orbital in the 2p subshell that contains a pair of electrons - these repel each other so it takes less energy to remove one of these electrons than if they were in separate orbitals
what do the peaks on the mass spectra of an atom represent?
the different isotopes of the atom and their abundance
what is the formula to calculate Ar from the mean mass of isotopes?
[(m/z 1 x abundance 1) +( m/z 2 x abundance 2)/ total abundance
what do the peaks on a mass spectra of a molecule represent? why?
- molecular ion often breaks down + loses atoms forming fragments - represented by the peaks
- last peak on the mass spectrum represents mass of the whole molecule + therefore the Mr of the molecule
- may be a small peak (m+1) due to presence of isotopes
what is relative isotopic mass?
the mean mass of an isotope of an atom, compared with one twelfth of the mean mass of an atom of carbon-12
what is relative molecular mass?
weighted mean mass of a molecule of a compound or element compared with one twelfth of the mass of an atom of carbon 12
