Ph 1 Atomic Structure Flashcards
Democritus
Atoms are indivisible
John Dalton
1803
Atoms are indivisible.
Atoms of an element are identical
Different elements have atoms of different mass and size
Becquerel
1896
Radioactivity
Atoms can be split up
JJ Thompson
1897
Discovered the electron
Electrons are identical and negatively charged
Atoms are mainly positive with electrons scattered within the structure.
Plum pudding model
Rutherford
1911
Fired alpha particles at gold foil
Atoms are mainly empty space with a central positive nucleus surrounded by negatively charged electrons
Nuclear model
Bohr
1913
Electrons orbit the nucleus in fixed energy levels or shells.
Movement between shells emits energy
Chadwick
1932
Discovers the neutron
Arrangement of electrons
Electrons are arranged into main energy levels or shells.
These are split into sub-shells (s,p,d,f), each main shell has n number of sub-shells.
Each subshell consists of electron orbitals each holding a maximum of two electrons with opposite spins.
Orbitals should be thought of as a region of space where an electron in likely to be found.
Pauli’s Principle
No more than two electrons can occupy the same orbital
Electrons in the same orbital have opposite spins.
Aufbau Principle
Electrons fill lower energy atomic orbitals before filling higher energy ones
Hunds Rule of Maximum Multiplicity
Every orbital in a subshell is singly occupied with one electron before any one orbital in that subshell is doubly occupied.
All electrons in singly occupied orbitals have the same direction of spin.
order of electron fill up to 4p
1s 2s 2p 3s 3p 4s 3d 4p
The 4s and 3d orbital can overlap in energy level, the 4s orbital is actually a lower energy level so is filled with electrons first. 4s will also lose electrons before 3d.
s sub shell
found in all main shells
Contains one orbital = two electrons
p shell
Found from the second shell and upwards
contains three orbitals = six electrons
d shell
found from the third shell and upwards
contains five orbitals = ten electrons
Electronic configuration of copper
1s2 2s2 2p6 3s2 3p6 4s1 3d10
*Second numbers after subshell should be written in super script
Electronic configuration of Chromium
1s2 2s2 2p6 3s2 3p6 4s1 3d5
*Second numbers after subshell should be written in super script
Reason for chromium and copper anomalies in electronic configuration.
The 4s and 3d lie very close together in energy levels. The 4s sub-shell only contains one electron to allow the 3d sub-shell to be completely or half full as this is a more stable arrangement. Having a singly occupied 4s orbital decreases electron repulsion.
Proton overview
mass = 1.673x10^-27 (1)
charge= 1.602x10^-19 (+1)
Is a nucleon, found in the nucleus.
neutron overview
mass=1.675x10^-27 (1)
charge=0
Is a nucleon, found in the nucleus
electron overview
mass= 0.911x10^-30 (1/1840)
charge = -1.602x10^-19 (-1)
Found in shells orbiting the nucleus.
What is an isotope?
Atoms of the same element, meaning they have the same number of protons, but with a different number of neutrons changing the mass number.
Isotopes of an element are chemically identical as they have the same electronic configuration
Isotopes of Chlorine
35Cl 75% 37Cl 25%
Chlorine molecule abundance
35Cl+35Cl 35Cl+37Cl 37Cl+35Cl 37Cl+37CL multiply abundance of isotopes 72Cl = 6/16 74Cl = 1/16 70Cl=9/16
Isotopes of carbon
12C 98.89
13C 1.11
14C traces
Purpose of a mass spectrometer
To find the abundance and mass of each isotope in an element allowing us to calculate the Ar.
To find the relative molecular mass of a substance of molecules
What are the four stages in a TOF mass spectrometer?
Ionisation
Acceleration
Flight tube
Detection
Electron impact ionisation
Used for elements and substances with a low formula mass
Sample is vaporised so it is gaseous
An electron gun fires high energy electrons at the sample
This knock off one proton from each particle forming a +1 ion
What is an electron gun?
A hot wire filament with a current running through it.
Equation for electron impact ionisation
X(g) + e- -> X+1(g) + 2e-
Electrospray ionisation
Used for substances with a higher molecular mass
Sample dissolved in a high volatile solvent
Injected through a fine hypodermic needle to give a fine mist
The needle is attached to the positive terminal of a high voltage power supply.
The particles gain a proton from the solvent forming a +1 ion. (Mr increases by one)
The solvent evaporates.
Equation for electrospray ionisation
X(g) + H+ –> XH+(g)
Why is the mass spectrometer kept under a vaccum?
To prevent the ions produced from colliding with molecules in the air.
Why must ions be used in a mass spectrometer?
Only ions will interact with and be accelerated by the electric field to have the same kinetic energy
Only positive ions will gain an electron at the detector to generate a current and be detected.
What can be manipulated to guide ions produced to the detector?
The electric field
Acceleration in a mass spectrometer
Positive ions are accelerated using an electric field to have the same kinetic energy.
KE= 0.5mv^2
Lighter particles will have a faster velocity.
Flight tube in a mass spectrometer
The accelerated positive ions pass through a hole in a negative plate into a tube. The time of flight of each ion through this tube depends on its velocity which, as each ion has the same kinetic energy, depends on each mass. The ions separate with lighter, faster ions reaching the detector first.
Detection in a mass spectrometer
Positive ions hit a negatively charged plate.
Gain an electron as they hit the detector plate creating a movement of charge.
This generates a current that can be detected and is proportional to the amount of ions hitting the plate measuring their abundance.
What is a mass spectrum?
A graph produced by a computer using the data from the mass spectrometer.
x axis - m/z ratio
y axis - abundance
Element mass spectrum
can be used to calculate relative atomic mass
significantly lighter peaks are caused by 2+ ions of isotopes
Molecule mass spectrum
The highest peak to the right is the molecular ion peak and the m/z gives the Mr, however in electrospray ionisation the Mr is the m/z minus one (weight gained by the proton)
Significantly lighter peaks are caused by fragmentation.
Define relative atomic mass.
The average mass of one atom considering the existence of isotopes and their abundance divided by 1/12 the mass of one atom of carbon 12
How do you calculate relative atomic mass?
(Sum of isotope abundance multiplied by mass) / sum of isotope abundance.
Define first ionisation energy
The minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous one plus ions.
Define second ionisation energy
The minimum energy required to remove one mole of electrons from one mole of gaseous one plus ions to create one mole of gaseous two plus ions.
What must we remember about ionisation equations?
The atom/ion must be gaseous.
What is the trend of successive ionisation energies?
Electrons are removed one by one starting with the outer electron and moving inwards.
Successive ionisation energies increase slightly as the electron in being removed from a more positive species meaning their is a greater force of attraction between the nucleus and outer electron.
A large increase occurs when the electron is taken from a new shell as there is a large decrease in shielding.
Successive ionisation energy trends is evidence for what.
The existence of principle energy shells.
A change of shell is shown by a large increase between successive ionisation energies.
General trend of ionisation energy across a group
Increases more protons (greater nuclear charge) smaller atomic radius similar shielding greater force of electrostatic attraction between the nucleus and outer electron.
Group 2-3 trend in ionisation energy
Decreases
Group 2’s outer electron is taken from a s orbital
Group 3’s outer electron is taken from a p orbital.
Higher energy level, weaker force of attraction.
This is evidence for the existence of sub-shells.
Group 5-6 trend in ionisation energy
Decreases
Group 5 the electron is removed from a singly occupied orbital
Group 6 the electron is removed from a doubly occupied/ paired orbital.
Greater electron- electron repulsion
Less energy required
This is evidence for Hund’s rule
Trend in ionisation energy down a group
Decreases
Greater atomic radius
More shielding
Weaker force of attraction.
