Unit 1: Section 1 - Atomic structure CDS * Flashcards
the atom models of atomic structure relative mass and the mass spectrometer electronic structure ionisation energy trends in 1st ionisation energy
where is most of the mass in an atom concentrated?
the nucleus, but it’s diameter is tiny compared to the whole atom
what takes up most of the volume of the atom?
the electron orbitals
what is the relative mass and charge of a proton, neutron and electron?
proton: 1, +1
neutron: 1, 0
electron: negligible, -1
A X Z what does each of the symbols?
A=mass number Z=atomic (proton) number X=element symbol
what is the mass number?
the total number of protons and neutrons in the nucleus
what is the atomic number?
the number of protons in the nucleus all atoms of the same element have the same proton/atomic number
what are isotopes?
atoms with the same number of protons but different numbers of neutrons
how are different isotopes the same?
the number and arrangement of electrons decide an elements chemical properties. so isotopes have the same chemical properties
how are different isotopes different?
they have slightly different physical properties, like densities and rates of diffusion because it depends on the mass of the atom
what are the different isotopes of magnesium?
24, 25, 26
who thought of the different models of the atom?
John Dalton - 19th century
JJ Thompson - 1897
Ernest Rutherford - 1909
Niels Bohr
what was John Dalton’s model of the atom?
he thought they were solid spheres and different spheres make up different elements
what was JJ Thompson’s model of the atom?
plum pudding model
atoms aren’t solid and indivisible
electrons in a positive pudding
what did Ernest Rutherford do?
conducted the gold foil experiment with his students Hans Geiger and Ernest Marsden
what was the gold foil experiment?
positively charged alpha particles are fired at a very thin sheet of gold. most of the particles passed through and only a small number are deflected backwards
what do the results of the gold foil experiment mean?
the plum pudding can’t be right because then all the particles would be deflected
what model did Rutherford think of?
the nuclear model - tiny positively charged nucleus surrounded by a cloud of negative electrons - most of the atom is empty space
why was the nuclear model impossible?
the electron cloud would quickly spiral down into the nucleus, causing the atom to collapse
what was Niels Bohr’s model of the atom?
electrons exist in shells or orbits of fixed energy. when electrons move between shells, electromagnetic radiation (with fixed frequency) is emitted or absorbed. the Bohr model fitted experimental observations
why is the Bohr model flawed?
not all electrons in a shell have the same energy, so they were split into sub-shells
why is the nuclear model useful?
its simple and explains many experimental observations
what is the relative atomic mass (Ar)?
the average mass of an atom of of an element relative to 1/12 th of carbon 12
what is relative isotopic mass?
the mass of an isotope of an element relative to 1/12 th of carbon 12
what is the relative molecular mass (Mr)?
the average mass of a molecule relative to 1/12 th of carbon 12
what can a mass spectrometer be used for?
to find the relative atomic mass, relative molecular mass and relative isotopic abundance
what are the 4 things that happen in a time of flight mass spectrometer?
- ionisation
- acceleration
- ion drift
- detection
what are the 2 types of ionisation?
electrospray ionisation
electron impact ionisation
what happens in electrospray ionisation?
the sample is dissolved and pushed through a hypodermic needle at high pressure. a high voltage is applied to it, causing each particle to gain an H+ ion. the sample is turned into a gas made of positive ions
what happens in electron impact ionisation?
sample is vaporised, an electron gun is used to fire high energy electrons at it from a hot wire. this knocks 1 electron off each particle so they become +1 ions
when is electrospray used?
for large molecules that electrons can’t pass through to knock electrons off, and will get damaged instead
when is electron impact used?
on small molecules or elements
what happens in acceleration?
positively charged ions are accelerated by an electric field so they all have the same kinetic energy. (lighter ions move faster than heavier ones)
what happens in ion drift?
ions enter a vacuum with no electric field so they drift through and allow the different isotopes to separate
what happens in detection?
lighter ions travel faster in drift region so reach the detector first. the detector is a negative plate and every time a positive ion hits it a current is produced (current proportional to number of ions) and a mass spectrum is created
what are the axis labelled on a mass spectrum graph?
y-axis = abundance, height of peak = relative isotopic abundance
x-axis = mass/charge ratio
how is a mass spectrum from electrospray different to electron impact?
in electrospray each atom gains an H+ ion so the relative mass on the graph will be 1 unit greater than the relative mass of the isotope
how to work out relative atomic mass from a mass spectrum?
multiply abundance by relative isotopic mass for each peak add all of these values divide by combined abundance
how can a mass spectrum be used to identify elements?
each element has different abundances of each of its isotopes if the abundances match up with the ones on the mass spectrum its the same element
how is mass spectrometry used on molecules?
a molecular ion is formed by removing an electron from the molecule. this produces a peak with the same mass/charge ratio as the molecules relative molecular mass. this can be used to identify unknown molecules
how can a molecule produce different peaks on a mass spectrum?
if the elements that make it up have isotopes there’ll be a peak for every combination of isotopes that produces a different Mr
what does the currently accepted model of the atom say about electrons?
they have fixed energies
orbit the nucleus in shells/ energy levels
each shell has a number called the principal quantum number
what happens to a shell, the further it gets from the nucleus?
the higher its energy the larger the principal quantum number
what makes up shells in an atom?
shells are divided into sub-shells with slightly different energies sub-shells are made up of orbitals which can hold 2 electrons each, which spin in opposite directions
how may orbitals does an s sub-shell have?
1, so it can hold 2 electrons
how may orbitals does an p sub-shell have?
3, so it can hold 6 electrons
how may orbitals does an d sub-shell have?
5, so it can hold 10 electrons
how may orbitals does an f sub-shell have?
7, so it can hold 14 electrons
which sub-shells can fit into the 1st shell?
1s
which sub-shells can fit into the 2nd shell?
2s, 2p
which sub-shells can fit into the 3rd shell?
3s, 3p, 3d
which sub-shells can fit into the 4th shell?
4s, 4p, 4d, 4f
how are electrons arranged in atoms?
they fill the lowest sub-shell 1st they fill singly before they share (e.g. 1 electron in each orbital in p before doubling up)
which orbitals have the highest energy?
what is the electron configuration of chromium (24) and Copper (29)?
Cr = 1s2 2s2 2p6 3s2 3p6 3d5 41 Cu = 1s2 2s2 2p6 3s2 3p6 3d10 4s1
what happens to the electron structure when an atom becomes ionised?
they lose or gain atoms in the order they get them or would usually gain them transition metals lose their 4s electrons before the 3d electrons
what do the s block elements do when they react?
(group 1 and 2) they lose 1 or 2 electrons to form positive ions with an inert gas configuration
what do the p block elements do when they react?
(group 5, 6 and 7) they gain 1, 2 or 3 electrons to form negative electrons with an inert gas configuration (group 4, 5, 6 and 7) can share electrons to form covalent bonds
what do the group 0 elements do when they react?
they are inert gases and have completely filled their s and p sub-shells so don’t need to share electrons
what do the d block elements do when they react?
(transition metals) they lose s and d electrons to form positive ions
what is the 1st ionisation energy?
the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
what is ionisation?
when electrons have been removed from an atom or molecule, its an endothermic process
what is the equation for the 1st ionisation of oxygen?
O(g) —> O+ + e- 1st ionisation energy = +1314 kJ/mol
important points about ionisation energy:
- use gas state symbol
- refer to 1 mole of atoms
- the lower the ionisation energy, the easier it is to form an ion
what factors affect ionisation energy?
nuclear charge
distance from nucleus
shielding
how does nuclear charge affect ionisation energy?
the more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons
how does distance from the nucleus affect ionisation energy?
attraction falls off very rapidly with distance. an electron close to the nucleus will be much more strongly attracted than 1 further away
how does shielding affect ionisation energy?
as the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge. this lessening of the pull of the nucleus by inner shells of electrons is called shielding (or screening)
what does a high ionisation energy mean?
there’s a high attraction between the electron and the nucleus and so more energy is needed to remove the electron
what is secondary ionisation energy?
the energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
what is successive ionisation energy?
the energy needed to remove an electron from any ion of that element
what’s the equation for the 2nd ionisation of oxygen?
O+ —> O2+(g) + e- 2nd ionisation energy = 3388 kJ/mol
what is the equation for any successive ionisation energy? (nth ionisation energy)
x(n-1)+(g) —> x^n+ + e-
what does a graph of successive ionisation energies show?
it provides evidence for the shell structure of the atom
what pattern do successive ionisation energies have?
- within each shell, successive ionisation energies increase 2. big jumps in ionisation energy happen when a new shell is broken into - an electron is being removed from a shell closer to the nucleus
why do successive ionisation energies increase across a shell?
electrons are being removed from an increasingly positive ion - there’s less repulsion amongst the remaining electrons, so they’re held more strongly by the nucleus
how can you find which group an element is in from its successive ionisation energy graph?
by counting the number of electrons removed before the 1st big jump. this is the number of electrons in the outer shell and directly corresponds to its group
what are the trends in 1st ionisation energy?
down a group - decrease
across a period - increase
why does the 1st ionisation energy decrease down a group?
each group has an extra electron shell between the nucleus and the outer electron as you go down
so it has a greater atomic radius
more shielding
so weaker forces of attraction
why does 1st ionisation energy generally increase across a period?
more protons -
stronger nuclear attraction
same shielding
stronger forces of attraction decrease between group 2 and 3, and 5 and 6
why does the 1st ionisation energy across a period decrease between group 2 and 3?
new orbital (3s -> 3p) with a higher energy level
3p orbital has some shielding of the 3s electrons
outer electron further from the nucleus weaker forces of attraction
what does the decrease in 1st ionisation energies between group 2 and 3 show?
evidence of sub-shell structure
why does the 1st ionisation energy across a period decrease between group 5 and 6?
same shielding in group 6 there’s 2 electrons in the same orbital repulsion between the 2 electrons means its easier to remove weaker forces of attraction
what does the decrease in 1st ionisation energy between group 5 and 6 show?
evidence that electrons fill empty orbitals 1st before doubling up
what is Pauli’s exclusion principle?
no more than 2 electrons can occupy the same orbital 2 electrons in the same orbital must have opposite spin
what was the Aufbau principle?
electrons fill lower energy atomic orbitals before filling higher energy ones
what is Hund’s rule of maximum multiplicity?
every orbital in a subshell is singly occupied with an electron before it’s doubly occupied
all electrons electrons in singly occupied orbitals have the same spin
How to find the mass of 1 atom in kg?
the Ar is the mass in grams per mol of the atom
divide by 1000 to get the mass in kg/mol
then divide by avogadro’s constant to fin the mass in kg per atom
