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elements of life 1(el1-5) (Atomic models (Rutherford (ALPHA SCATTERING/…
elements of life 1(el1-5)
Atomic models
origins
greek scientists though all matter was made of invisible particles
in the early 19th century Dalton said all atoms were SOLID SPHERES
experimental evidence against solid spheres (PLUM PUDDING)
performed by JJ Thompson in 1897
his measurements of charge and mass showed that elements must contain smaller negatively charged particles (electrons)
= a positive sphere of charge with negative electrons embedded in it
Rutherford (ALPHA SCATTERING/ GOLD FOIL)
Rutherford and his two students fired alpha particles at a sheet of gold
from plum pudding model they expected most of the alpha particles to be deflected slightly by positive sphere
most alpha particles went straight through the gold atoms, a small number were deflected backwards
nuclear model
tiny positively charged sphere at centre of atom - where most mass was concentrated
the nucleus was surrounded by cloud of freely orbiting electrons
most of atom was empty space
nuclear model modifications
existence of protons in nucleus
charges were explained by the different atoms of different elements having different numbers of protons in their nuclei
the existence of the neutron was made by Chadwick
the Bohr model
electrons existed in fixed orbitals
each shell/ orbital had fixed energy
when an electron moves between shells electromagnetic radiation is absorbed or emitted
energy shells are fixed so radiation will have a fixed frequency
the atom
made up of protons, neutrons and electrons
electrons: -1 charge, mass of 1/2000
protons: +1 charge, mass of 1
neutrons: 0 charge, mass of 1
nucleus
where most mass is concentrated
where protons and neutrons are
nuclear symbols show numbers of sub atomic particles
mass number= total number of protons and neutrons in atom
atomic number= the number of protons in the nucleus
ions have different numbers of electrons
negative charge
more electrons than protons
positive charge
more protons than electrons
isotopes
= same element with a different number of neutrons
can cause slightly different physical properties because they depend more on mass e.g. densities
the same chemical properties because they have the same number and arrangement of electrons
relative mass
relative masses are masses compared to carbon 12
the relative mass= the average mass of an atom of an element on a scale where an tom of carbon 12 is 12
relative isotopic mass= the mass of an atom of an isotope of an element on a scale where an atom of carbon 12 is 12
the relative molecular mass (mr)= the average mass of a molecule or formula unit on a scale where carbon 12 is 12
measured by MASS SPECTROMETER
1) vaporization= the sample is turned into gas using an electrical heater
2) ionisation= the gas particles are bombarded with high energy electrons to ionise them. electrons are knocked off the particles leaving POSITIVE IONS.
3) Acceleration= the positive ions are accelerated by electric field
4) detection= the TIME TAKEN for the positive ions to reach the detector are measured. depends on ions mass and charge - light highly charged ions will reach detector first. a mass spectrum is produced
mass spectrum
y axis gives abundance of ions (often as a percentage), the height of each peak gives the relative isotopic abundance
the x axis is the relative mass and units are given as mass/ charge ratio
each line represents a different isotope
CALCULATION EXAMPLE
step 1= for each peak multiply the % relative isotopic abundance and the relative isotopic mass together to get total mass for each isotope.
step 2= add up the totals
step 3= divide by 100
HOWEVER if the relative abundance is NOT a % then do steps 1 and 2 but divide by the total relative abundance (relative abundance/ total relative abundance x 100)
electronic structure
electron shells are made up of sub shells and orbitals
electrons move around nucleus in shells. these shells are given numbers known as principal quantum numbers
shells further from the nucleus have a greater energy level than shells closer to the nucleus
these shells contain different types of sub shells that have different numbers of orbitals
s sub shell
1 orbital
2 electrons
p sub shell
3 orbitals
6 electrons
d sub shell
5 orbitals
10 electrons
f sub shell
7 orbitals
14 electrons
periodic table
split into s, p and d block
s block have an outer shell electrons configuration of s1 or s2
p block have an outer shell electron configuration of s2p1 to s2p6
ionic bonding
ionic bonding is when ions are stuck together by electrostatic attractions
ions are formed when electrons are transferred from one atom to another
electrostatic attractions= forces that hold positive and negative ions together- theyre very strong
generally the charge on the metal ion is equal to the group number
giant ionic lattice
ionic crystals are giant lattices of ions
lattice= a regular structure
in the lattice different charges attract eachother and the ions with the same charge repel eachother. the ions arrange themselves to minimise repulsions and maximise attractions - overall attraction is ionic bonding
properties
ionic compounds conduct electricty when MOLTEN or DISSOLVED
ions in liquid or solution are free to move and carry charge whereas in solid theyre in a fixed position due to strong ionic bonds
high melting points
giant ionic lattices are held together by very strong electrostatic forces which take lots of energy to overcome
soluble in water
water molecules are polar- pull ions away from lattice and cause it dissolve
covalent bonding
in covalent bonding two atoms share electrons so they have full outer shells. both positive nuclei are attracted electrostatically to the shared electrons
as well as the electrostatic attraction between nuclei and electrons there is also repulsion between positive nuclei - to maintain covalent bond there has to be a balance
molecular substances
low melting points
weak intermolecular forces are easy to overcome
dont conduct electricty
no charge carriers free to move
insoluble
polar water molecules are more attracted to eachother than the molecular substance
dative bonding
dative bonding= where one atom donates both electrons to the bond
giant covalent and metallic strcutures
giant covalent
giant covalent lattices have a huge network of covalently bonded atoms
electrostatic attractions are stronger than in simple molecular molecules
e.g. carbon and silicon form diamond, graphite, silicon dioxide
properties
high melting points
strong bonds require lots of energy to break
hard
strong bonds in lattice arrangement
good thermal conductors
vibrations travel easily through stiff lattices
insoluble
atoms are more attracted to their neighbours in the lattice than to the solvent molecules and don't contain ions
cant conduct electricty
no charged or free electrons as all are localised in bonds EXCEPT GRAPHITE
giant metallic structures
in metallic lattices the electrons in the outermost shell are delocalised which leaves a positive ion
the metal ions are attracted to delocalised negative electrons and form a lattice of closely packed positive ions in a sea of electrons
properties
high melting points
strong metallic bonding and the more delocalised electrons per atom
ductile
no bonds holding specific ions together so metal ions can slide over eachother
good thermal conductors
delocalised electrons can pas kinetic energy to eachother
good electrical conductors
delocalised electrons are free to move and carry charge (impurities can reduce this- electrons transfer to them and form anions)
insoluble
strong metallic bonds
shapes of molecules
molecular shape depends on electron pairs around central atom
electron pairs repel eachother
electrons are negatively charged so electron pairs repel eachother
lone pairs repel MORE than bonding pairs
the greatest angles are between lone pairs of electrons and bond angles between bonding angles are often reduced because they are pushed together by lone pair repulsion
lone pair angles are biggest, then lone pair-bonding pair angles, bonding pair angles are smallest
the shape of the molecule depends on type of electron pairs surrounding the central atom as well as the number
electron pair repulsion theory is a way of predicting molecular shape
shapes
2 electron pairs on central atom
linear
180 degrees
3 electrons on central atom
trigonal planar
120 degrees
no lone pairs
bent
1 lone pair
120-2.5=117.5 degrees
4 electron pairs on central atom
tetrahedral
no lone pairs
109.5 degrees
trigonal pyramidal
1 lone pair
107 degrees
bent
2 lone pairs
104.5 degrees
5 electron pair on central atom
trigonal bipyramidal
no lone pairs
90 and 120 degrees
6 electron pairs on central atom
octahedral
no lone pairs
90 degrees
atomic spectra and nuclear radiation
electrons absorb and release energy in fixed amounts
atoms in ground state have all their electrons in the lowest possible energy level
if atoms ABSORB energy from their surroundings they can move to HIGHER energy levels
Electrons EMIT energy by dropping from a higher energy to a LOWER energy level
all energy levels have fixed values
so electrons have to absorb or emit a FIXED amount of energy to move
spectra
absorption
energy is related to frequency so when EM radiation is passed through gaseous element the electrons only absorb certain frequencies CORRESPONDING to differences between energy levels
this means radiation passing through has certain frequencies missing - show up as DARK LINES on a coloured background (atomic absorption spectra)
emission
when electrons drop to lower energy levels they give out certain amounts of energy- produces a line spectra similar to absorption
coloured lines on black background
the closer the lines are together the higher the frequency
E= h x v (difference in energy= planck's constant x frequency
C= v x y (speed= frequency x wavelength)
nuclear fusion
= when two smaller nuclei combine under high temp and pressure to make one larger nucleus
in stars hydrogen nuclei combine to make helium nuclei releasing lot of energy
when hydrogen runs out helium nuclei combine to make heavier elements until it explodes