Atomic number, relative atomic mass and isotopes.
atomic number: the number of protons in the nucleus of an atom of an element.
r.a.m.: the weighted average mass compared to the mass of a carbon-12 atom.
isotopes --> identical chemical properties (same electron configuration.)
How a mass spectrometer works
Bombardment Acceleration Deflection Detection
1. sample is vaporised
2. vaporised sample is bombarded w/ high energy electrons
3. accelerated by an electric field
4. deflected by a magnetic field
5. ions are detected and m/e ratio recorded on computer.
Uses of mass spectrometers
1. Determining r.a.m
e.g. two isotopes of Lithium 6Li and 7Li
(m/e 6Li x % 6Li) + (m/e 7Li x % 7Li) / 100
2. Detecting drugs in urine samples
steroids = certain mass/charge ratio
low energy mass spectrometer detects steroid molecules
3. Pharmaceutical industry
low-energy mass spectrometer prevents decomposition of molecule ion.
laser needle @ high electric potential
fog charged particles produced --> analyzed in mass spectrometer for m/e values
radioactive decay of carbon and other isotopes depending on m/e values can tell us how old an item is
Orbits and Orbitals
*when H+ atoms are heated, electrons are promoted from ground state to excited state.
1-s orbital is undivided.
2-s orbital and 2-p orbitals (b/c of penetrating towards the nucleus the 2s is lower than 2p).
4-s x 1, 4-p x 3, 4-d x 5, 4-f x 7
*if there is more than 18 protons 4s becomes lower than 3d (elements beyond Ar).
Electron configuration rules
rule one: one orbital can contain a maximum of two electrons with opposite spin
rule two: Afbau principle - electrons build succesively upwards from the lowest energy level.
rule three: Hund's rule- (seats on a bus) if there is more than one orbital in a subshell electrons are initially added so the electrons have parallel spins.
rule four: Pauli's exclusion principle - all electrons in an atom must be in different orbitals or have different spins (minimize repulsion).
Trends in periodic table
Li --> Ne
Na --> Ar
melting points increase steadily up to the 4th element; from the 5th onwards they fall dramatically.
Valency: number of electrons used by an element in bonding.
Li --> Fl, Na --> Cl (1,2,3,4,3,2,1)
* all elements in the same period have the same number of orbitals containing electrons (quantum shells).
Blocks of the periodic table
s-block group one and two elements
in an s-block element the last electron added goes into an s-orbital.
p-block elements in groups 3-7 and in group 0
in a p-block element the last electron added goes to a p-orbital
d-block Sc -> Zn in period 4 and elements below
*cannot be defined in terms of energy b/c the energy level of the d-orbitals is altered by the presence of electrons in the outer s orbital. (only defined x afbau terms).
last electron added goes to a d-orbital.
f-block 14+14 elements
14 rare earth metals (Ce --> Lu)
14 actinide elements (after actinium)
last electron added goes to an f-orbital
Metals and their properties
across a period elements become less metallic.
down a group elements become more metallic.
typical properties of a metal
more reactive metal + acid --> salt + H(g)
malleable and ductile
forms cations in compounds (looses e- itself)
reacts with oxygen to form oxides that are bases
Forces on the nucleus of an atom
inside nucleus - strong & weak nuclear forces ( hold the nucleus together)
*change in nuclear structure involves LARGE amounts of energy.
Radioactivity; the longer the half life - the more dangerous
Types of radiation
alpha rays - 2 protons & 2 neutrons, fast moving He nuclei, stopped by few cm air or a sheet of paper.
beta rays - fast moving electrons; stopped by a thin sheet of metal.
gamma rays- similar to X rays but higher frequency, therefore higher energy; stopped by a thick layer of concrete.
breathe in/ eat material w/ a/B emitters = v. dangerous
b/c radiation will destroy cells that absorb chemical w/ isotope.
Thyroid cancer is treated with iodine-131 isotope.
gamma rays are absorbed by human tissue (causes changes to cells) cancer cells are more likley to die than normal cells so radiotherapy (controlled cobalt-60)
Nuclei, fission and fusion
heavy nuclei are fissile (when the nucleus absorbs a neutron they become so unstable tehy break into much smaller pieces). - when this happens huge amounts of energy are released.
U-235 --> chain reaction
Nuclear fission is environmentally friendly!
+ no greenhouse gases
+ b/c so little uranium is used there is an unlimited supply
+ can answer threat to climate change
- nuclear fission makes reactors and materials used to build the reactor very radioactive
- ^ must be safely stored for 1000's of years
- nuclear energy = nuclear weapons?
nuclei of light isotopes fuse together realeasing LARGE amounts of energy
Forces on electrons in an atom are magnetic
- opposites attract; like repel
- the bigger the charge the stronger the force
- the larger the sum of the radii the weaker the force
electrons are attracted to the nucleus!
*the > the atomic number the higher the form of attraction & the further away from the nucleus, the less attraction.
nuclear charge in a group hardly varies b/c nuclear charge and the number of inner sheilding electrons increase by the same amount.
*the nuclear charge of d-block elements in the same period hardly varies.
Atomic radii decreases across a period (greater nuclear charge --> greater force of attraction on the electrons therefore they are drawn closer together).
Size of cations and anions
always smaller than its neutral atom. Smaller atomic radius because:
there is one less electron shell.
fewer electrions in ion than atom so there is less electron-electron repulsion --> further decrease in radius.
*for electrons with the same electron configuration the ion w/ the greatest charge will have the smallest radius.
a negative ion is always LARGER than a neutral atom
must gain e- to form negative ion so the forces of attraction are the same BUT there is extra repulsion due to the increased number of e- in same shell. the ion expands to balance forces of attraction and repulsion.
ionization energy: the energy needed to remove an electron from an atom, ion or molecule.
jumps in ionization energies = evidence for existence of quantum shells.
*find group of an element looking at the first big jump in ionization energies
1st ionization energies increase across a period
why? b/c... more energy is required to remove an electron across the group b/c atomic radii decreases so electrons are packed closer to the nucleus of an atom. also nuclear charge increases so increase in attraction for highest energy electron.
Group one elements after noble gases have lower first ionization energies (than prev. elements).
b/c they are well shielded
they hae a nuclear charge close to +1
atomic radius has a greater p.p.
In period 2 the energy level of 2-p is higher than 2-s so less energy is required to remove a 2-p electron.
1st electron affinity: energy change when one electron is added to each atom in a mole of neutral gas atoms.
A(g) + e- --> A- (g)
Second electron affinity = same as first electron affinity BUT applied when added to singly charged gaseous negative ions.