Download Objectives: early history, laws for calculations, atoms, molecules

Objectives: early history, laws for calculations, atoms, molecules
Early history
3000 years ago:
iron was produced from mined minerals (weapons), in Egypt
embalming fluid were in use
2000 years ago:
4 fundamental substances (elements) were postulated by the
Greeks: fire, earth, water, air
1400 years ago:
Alchemy, introduced by Arabs, discovery of several true
elements, search for the conversion of cheap substances to gold
500 years ago:
in Europe systematic metallurgy came up, Paracelsus worked in
medicine
300 years ago:
first quantitative experiments: Boyle, Priestly (work in combustion with
O2
earlier the Chinese invented gunpowder, which Marco Polo brought to Europe
Lavoisier (1743 - 1794) in France
Law of conservation of mass (mass is neither created nor destroyed)
by carefully weighing products and reactants of chemical reactions
He discovered, that combustion and life both involve oxygen.
Proust (1754 - 1826)
Law of definite proportions:
a compound always contains exactly the same proportion of elements by mass, no matter from
where it comes.
CuCO3 copper carbonate: mCu : mC : mO = 5.3 : 1 : 4 (Proust)
and with masses from today's periodic tables:
mCu : mC : mO = 63.55 : 12.01 : 48.00 = 5.29 : 1 : 4.00
Dalton (1766 - 1844)
Law of multiple proportions:
When two elements form a series of different compounds together, the ratios of the masses
of the second element that combine with 1 g of the first element (arbitrary, which one you
call first and second), always can be reduced to small whole (integer) numbers.
Example: oxygen, O, and nitrogen, N, can form 3 different (by chemical properties)
compounds:
According to the old weights of the elements from 1800:
1 g O2 combines with
1.750 g of N2: compound A
0.815 g of N2: compound B
0.438 g of N2: compound C
Thus the ratios of N2 that combine with 1 g O2 between the compounds are
A : B = 2 : 1 (2.1 : 1 with old weights)
A : C = 4 : 1 (4.0 : 1 with old weights)
B : C = 2 : 1 (1.9 : 1 with old weights)
These data can be explained by using atoms as a model (atoms are tiny particles, the smallest
ones possible that still have the properties of the element):
A:
N N
O
N N
N:O=4:1
B:
N
O
N
N:O=2:1
C:
N
O
N:O=1:1
Thus: N/O (A) : N/O (B) = 4 : 2 = 2 : 1
N/O (A) : N/O (C) = 4 : 1
N/O (B) : N/O (C) = 2 : 1
But the same data can also be explained by the following model of molecules:
N
A:
O
B:
N
O
C:
ONO
N
N:O=2:1
N:O=1:1
N : O = 1 : 2 = 1/2 : 1
Thus: N/O (A) : N/O (B) = 2 : 1
N/O (A) : N/O (C) = 2 : 1/2 = 4 : 1
N/O (B) : N/O (C) = 1 : 1/2 = 2 : 1
Dalton's atomic theory (1808)
An element is composed of tiny particles called atoms. If these particles are cut to a smaller
size, they lose the properties of the element (atomos = undivisible).
Atoms of a given element are identical; atoms of another element are different in some
fundamental way (that time not known, but it is the number of protons in the nucleus).
Compounds are formed when the atoms of two or more elements combine.
In an ordinary chemical reaction no atom of any element ever disappears (is destroyed);
likewise no atom of any element can ever be created out of nothing.
From this atomic theory, he created the first table of atomic masses (atomic weights), where
the elements were sorted according to increasing weight.
His assumption was, when he assigns a weight of 1 to H, then the weight of O must be 8.
That time people assumed that water has the formula HO, which was later shown to be
wrong, but the idea of atoms and molecules existed.
Dalton believed, that nature is as simple as possible
Thus he proposed the simple formula HO for water
In his time it was known that 1 g hydrogen combines with 8 g oxygen to form 9 g water
But it was not known that hydrogen contains H2 and oxygen contains O2 molecules.
The assumption was that H reacts with O to form HO: H + O  HO
Really: H2 + 1/2 O2  H2O
The molecular weight of H2 is 2 g, that of 1/2 O2 is 16g, and that of H2O is 18 g (per mol)
so 1 g H2 combines with 8 g O2 and forms 9 g H2O
Avogadro (1811)
at the same temperature, T, and pressure, P, equal volumes of different gases contain the
same number of particles (molecules in normal gases, atoms in noble gases).
This was supported by the earlier experiments of Gay-Lussac:
If hydrogen and oxygen are monoatomic gases (H and O) then 2 volumes of H should
combine with 1 volume of O to form 1 volume of H2O (steam).
This was NEVER observed
Observed: 2 volumes of hydrogen combine with 1 volume of oxygen and form 2 volumes of
H2O (steam)
Thus it must be H2 and O2 instead of H and O:
2 volumes of H2 combine with 1 volume of O2 and form 2 volumes of H2O (steam)
and in the same way:
1 volume of H2 combines with 1 volume of Cl2 and forms 2 volumes of HCl gas
Objectives: atomic structure, molecules, and ions, periodic table
Dalton's atomic theory was long time not believed and it leads to further questions:
What is an atom made of?
First attempts to find out were Thomson's cathode ray experiments.
How do atoms of different elements differ from each other?
Thomson's cathode ray experiments (1898 - 1903): discovery of electrons
He used high voltage and different cathode (- pole) materials (mostly metals)
cathode and anode placed into a partially evacuated glas tube.
partial evacuation: charged particles hitting gas molecules cause them to glow:
like polar lights: at the poles charged particles coming along earth's magnetic field hit the
upper atmosphere (low pressure) and cause the molecules to glow.
Thomson observed glowing rays in his tube moving from the cathode to the anode:
they must be charged (glow)
cathode repels them, anode attracts them: negative charge
proof: when he applied another electric field perpendicular to the rays, they were deflected
towards the + pole:
So the rays consist of negatively charged particles, and they are produced by every material
he used as cathode.
Postulate: cathode rays are a stream of negatively charged particles which are called electrons
and are present in every element
They are also deflected by a magnetic field, in which the deflection is proportional to the
magnetic field strength and to the charge/mass, e/m, ratio of the particles
measuring the deflection at different field strength yields:
e/m = -1.76 x 108 C/g
(1 C = 1 As unit of charge "coulomb")
the negative sign comes from the negative charge of the particles.
With deflection in magnetic fields only e/m can be detected, not e or m alone.
Since cathode rays can be produced from different elements as cathode, Thomson concluded
that all elements contain electrons
Since all elements are electrically neutral and contain negative electrons, there must be also
a positive counter-charge in them.
Thomson believed, the positive charge is in a diffuse cloud (like smoke) around the negative
electrons.
Millikan (1909): determination of the electronic charge e, and in turn with e/m from
Thomson also the mass m
Millikan put a spray of oil droplets between two charged plates, the lower one negatively
charged, the upper one positively charged.
Then he bombarded the oil droplets with X-rays, which can cut electrons lose from oil
molecules. These electrons, usually just 1 or 2 per droplet, give rise to negatively charged
droplets.
These negative charges are repelled by the negative lower plate and attracted by the positive
upper one.
The electrostatic force acting on the charged droplets is proportional to the charge on the
droplets.
Changing the electric field strength, Millikan could find exactly that field that keeps the
droplets still (no more falling down)
With this field that exactly balances the droplets, he could calculate the charge on the
droplets.
Since this can be from 1, 2, 3, or even more electrons, he took the smallest charge found (1
electron), and that is the electronic charge e:
e = -1.602 x 10-19 C (1 C = 1 As)
Then with e/m from Thomson's experiment: m = 9.11 x 10-31 kg
Becquerel (1896) found, that when he puts an object between a mineral containing uranium
(U) and a fotografic plate, he obtains an image of the object on the film, because of the
radioactive radiation sent out of the mineral by the U nuclei.
In the early 1900, 1910 it was found that radioactive radiation contains 3 different kinds of
rays:
α particles: charge +2 (|e|), m = 7100 me (electron masses)
these are charged and thus dangerous for health, but they do not penetrate deeply.
A piece of paper between a man and α particle radiation is enough to save him.
But: if α ray emitting dust is inhaled by breathing or otherwise taken into the body, they
destroy it from the inside (deadly).
β particles: charge -1 (|e|), mass = me, they are electrons.
more penetrating than α rays; no paper would save from β rays, only another person between
one and the emitter could
γ rays: no particles, but high energy electromagnetic radiation
strongly penetrating, only a lead shield could help
Rutherford (1911): shooting of α-particles on a thin foil made of gold (other metal, same
effects).
the positive charge in an atom should repel and thus deflect the positively charged α
particles.
Around (in a circle) the foil he placed a detector for the α particles.
Believe: positive charge in an atom in a thin positive cloud surrounding the electrons.
Then all particles should be deflected, because the positive charge is smeared out throughout
the entire atom.
Unexpected observation: most particles are not or just a little bit deflected.
Only some particles are strongly deflected, some even in reverse direction.
Conclusion: the positive charge in the atom cannot be in a diffuse cloud, but must be
concentrated in a small solid, positive particle.
If the α particle does not hit this particle, it is not or only slightly deflected
If it hits it, then it is strongly deflected, even backwards if it is a direct hit.
Thus, there must be a nucleus, a very dense center of positive charge, no cloud
It must be very small, thus very dense, or more α particles would hit it than observed.
Modern view:
the nucleus has a diameter of 10-13 cm and it contains positively charged protons (p+) and
electrically neutral neutrons (n)
p, n have a diameter of about 1 fm (10-15 m) and are thus extremely small and dense
if a nucleon would have the size of a pea, it would weigh many tons
The electrons surround the nucleus at distances of about 10-8 cm
Thus an atom is mostly (>90 %) just empty space
particle
symbol mass(kg)
charge
electron
e or e- 9.11 x 10-31
-1
proton
p or p+ 1.67 x 10-27
+1
neutron
n
1.67 x 10-27
0
the charge is given in units of the electronic charge, 1.6 x 10-19 C
The electrons are orbiting the nucleus, very much like planets the sun (old ideas, not fully
correct: see Chapter 7)
different neutral atoms: different numbers of p and n, and different numbers of e
neutral: number of p = number of e
negative ion (anion): more electrons(-) than protons(+)
positive ion (cation): more protons(+) than electrons(-)
If 2 neutral atoms have the same number of p and e, but different numbers of n:
then they are atoms of the same element, having different weight. They are called isotopes.
If 2 neutral atoms have different numbers of p (and thus e if neutral), then they are atoms of
different elements
Way of writing: AZ E
E is the symbol of the element
A is the mass number: number of protons and neutrons together: A = p + n
Z is the atomic number: number of protons: Z = p, thus n = A - Z
For a neutral atom the number of electrons, e, is equal to p = Z
charge of a mono-atomic ion: p - e
for example there are 2 isotopes of sodium, 11Na, which differ just by 1 neutron:
23
11Na
and also 2411Na: isotopes of Na, different n
rules:
A = p + n = Z + n (mass number)
Z = p (atomic number) = e (if neutral atom)
How many p, n, and e are in 19779Au?
Z = p = e = 79
n = A - p = A - Z = 197 - 79 = 118
How many p, n, and e are in 19779Au3+?
Z = p = 79
n = A - p = A - Z = 197 - 79 = 118
e = 79 - 3 = 76 (3 positive charges, thus 3 electrons less than protons)
molecules and ions
When two atoms, A and B form a molecule, they share electrons with each other:
The sharing of negative electrons between the 2 positive nuclei, reduces and finally offsets
the repulsion between the 2 positive nuclei: formation of a chemical bond, here a covalent
bond
Chemical formulas: list the element symbols in the molecules and put the numbers of each
atom in 1 molecule as index: H2O, NH3, CH4
Better are structural formulas, giving more informations about bonds and geometry:
Each line stands for a covalent bond, where sharing of electrons between O and H occurs:
covalent compound
If there happens no sharing but a complete transfer of electrons from one atom to the other,
then ions are formed which attract each other.
If a sodium (metal) atom loses an electron, then a positive ion, a cation (attracted by the
negative cathode) is formed:
Na  Na+ + eIf a chlorine (non-metal) atom gains an electron, then a negative ion, an anion (attracted by
the positive anode) is formed:
Cl + e-  ClThe two ions attract each other and an ionic crystal is formed:
Na+ + Cl-  NaCl(s) ionic compound
short introduction of the periodic table of the elements
Arrangement not by the masses but by increasing atomic numbers Z, putting elements of
similar chemical properties in groups below each other
Representative or main group I: alkali metals (H is no metal)
Representative or main group II: earth alkaline metals
Representative or main group III: earth metals
Elements along the bold line: metaloids, between metals and non-metals
Representative or main group VI: chalcogenides
Representative or main group VII: halogens
Representative or main group VIII: noble gases (not diatomic, but mono-atomic gases)
Between groups II and III: transition metals
Starting from Lanthanum La: rare earths or lanthanides
Starting from Actinium Ac: actinides, all radioactive, beyond uranium all man made
Objective:
Names of Compounds
To discuss: binary compounds, polyatomic ions, oxoions, acids, 1 exception: Mn2O7, organic
acids
Binary ionic compounds (Type I)
To obtain the complete formula of an ionic compound:
Electroneutrality condition: In an ionic compound the total charge of all ions together must
be 0.
first step: find the charges of individual atoms (in type I compounds only cations of elements
that can have no more than 1 value of charge can appear)
second step: add cations and anions together, as many until the total charge is 0.
mono-atomic (only 1 atom) anions can be found in the periodic table right left of the noble
gases (non-metals): the number of negative charges is equal to the steps it takes to come
from the element to the next noble gas:
second period:
fluorine (F): 1 step to the next noble gas neon (Ne): F-, fluoride ion
oxygen (O): 2 steps to the next noble gas neon (Ne): O2-, oxide ion
nitrogen (N): 3 steps to the next noble gas neon (Ne): N3-, nitride ion
carbon (C): 4 steps to the next noble gas neon (Ne): C4-, carbide ion
the elements left of C are metals and thus they have cations, no anions
third period:
chlorine (Cl): 1 step to the next noble gas argon (Ar): Cl-, chloride ion
sulfur (S): 2 steps to the next noble gas argon (Ar): S2-, sulfide ion
phosphorus (P): 3 steps to the next noble gas argon (Ar): P3-, phosphide ion
the elements left of P are metaloids and metals and thus the metals have cations, no anions
fourth period:
bromine (Br): 1 step to the next noble gas krypton (Kr): Br-, bromide ion
the elements left of Br have no prominent anions, with the exception of (seldom found)
selenide (Se2-)
fifth period:
iodine (I): 1 step to the next noble gas xenon (Xe): I-, iodide ion
the elements left of I have no prominent anions
first period: Hydrogen can form cations, H+, and anions, hydride ion Hcations: main group I elements form + cations: Li+, Na+, K+, Rb+, Cs+
francium (Fr) is radioactive and not much is known
main group II elements form 2+ cations: Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ra2+
main group III elements form 3+ cations: B3+, Al3+, Ga3+, In3+
Those deeper in the table can form more than 1 cation
Formulas: first put the cation and then the anion with their numbers as indices
Names: first you put the name of the cationic element (don't name the charge, because in
type I we can have only elements with 1 cation type), then for the anionic element put the
root of the element name and add the ending "...ide".
calcium and oxygen:
CaO calcium oxide: Ca2+O2magnesium and chlorine:
MgCl2 magnesium chloride: Mg2+(Cl-)2
aluminum and sulfur:
Al2S3 aluminum sulfide: (Al3+)2(S2-)3: 6+ and 6calcium and nitrogen:
Ca3N2 calcium nitride: (Ca2+)3(N3-)2: 6+ and 6For electroneutrality, the cation gets as index the (absolute) chargenumber of the anion and
the anion gets as index the chargenumber of the cation
Type I binary ionic compounds are usually formed between main group metals (left in the
table) and main group non-metals (right in the table)
Type II binary ionic compounds
Some atoms (transition metals and main group elements in higher periods) can form several
cations having different charges.
In this type of compounds you start with the name of the cationic element and after it you
add in brackets the number of positive charges in roman numerals (1 = I, 2 = II, 3 = III,
4 = IV, 5 = V, 6 = VI), the anion is named as in type I
iron and bromine: that can be FeBr2 or FeBr3, because Fe has Fe2+ and Fe3+ cations.
charge neutrality: Fe2+(Br-)2 2+ and 2-, Fe3+(Br-)3 3+ and 3FeBr2: iron (II) bromide: II, because Fe2+
FeBr3: iron (III) bromide: III, because Fe3+
other examples:
Cu+, Cu2+
copper (transition metal)
Co2+, Co3+
cobalt (transition metal)
Mn2+, Mn3+
manganese (transition metal)
Au+, Au3+
gold (transition metal)
Hg22+, Hg2+
mercury (transition metal)
Sn2+, Sn4+
tin (main group element)
Hg2Cl2: mercury (I) chloride, also mercurous chloride
HgCl2: mercury (II) chloride
Hg22+ is like a diatomic molecule with 2 positive charges, 2 charges per 2 Hg, thus 1 charge
per 1 Hg and therefore mercury(I)
Some main group elements like Sn also have several cations and must be named in type II,
some transition metals like silver (Ag+) and zinc (Zn2+) have only 1 cation and must be
named in type I.
Type III molecular (covalent) compounds
These are not ionic and usually formed between two non-metal atoms, close together in the
table on the right side
Here in the name you put the two elements one after the other, and give each one its index
in the formula before the name (prefix), the second one you give the ending ide as in ionic
compounds
The first element must be that one which is more left (exceptions for oxygen) and more
down (larger period number) in the periodic table (don't forget the prefix, see below).
The second element must be that one which is more right and more up (smaller period
number) in the periodic table (don't forget the prefix, see below), and you add "ide" to it as
in ionic compounds.
Exceptions:
noble gases must be always the first element in the name
oxygen is usually the last one in the name, adding ide.
Only OF2 is oxygen fluoride
Prefixes: 1 = mono (usually not used, name alone means 1), 2 = di, 3 = tri, 4 = tetra,
5 = penta, 6 = hexa, 7 = hepta, 8 = octa, 9 = nona, 10 = deca
if tetra, penta, hexa, or hepta, with oxide, then use tetroxide, pentoxide, hexoxide, or
heptoxide (speaking more easy)
N2O5 dinitrogen pentoxide
but Cr2O3 chromium (III) oxide
SO2 sulfur dioxide; SO3 sulfur trioxide
PN phosphorus nitride; N2O dinitrogen oxide
Cl2O7 dichlorine heptoxide; CO carbon monoxide (mono is kept for historical reasons)
HCl hydrogen chloride (only if it is not dissolved in water)
P4O10 tetraphosphorus decaoxide, usually (because) shorter decoxide
1 exception
Mn2O7 should be a type II ionic compound (transition metal + non-metal), but ionic
compounds are solids with high melting points
Mn2O7 is a clear, colorless liquid which is highly explosive on shaking
Thus it is a molecular compound and the name is dimanganese heptoxide or heptaoxide
Polyatomic ions
these are ions that contain more than 1 atom, mostly anions
but there exist also some polyatomic cations:
NH4+ ammonium ion
Hg22+ mercury (I) or mercurous ion
mostly found as anions
examples
CO32- carbonate;
HCO3- hydrogencarbonate
CrO42- chromate;
Cr2O72- dichromate
K2Cr2O7 potassium dichromate
ClO3- chlorate;
CN- cyanide
OH- hydroxide
PO43- phosphate;
HPO42- hydrogen phosphate;
SO42- sulfate;
HSO4- hydrogen sulfate
H2PO4- dihydrogen phosphate
SO32- sulfite
NO3- nitrate;
NO2- nitrite
Common features of oxoions:
anions containing oxygen atoms and another kind of atom
in a series, that with the largest number of oxygen atoms is the "ate" ion
that with the smaller number of oxygen atoms is the "ite" ion
Only, if like for the halogens (Cl,Br,I, not F) 4 of them exist, then the highest oxygen count
is the "per"..."ate" ion, the next lower the "ate" ion, the next lower the "ite" ion and the
lowest the "hypo"..."ite" ion (the Cl- ion is the ide ion)
per...ate
...ate
...ite
hypo...ite
SO42-
SO32-
sulfate
sulfite
NO3-
NO2-
nitrate
nitrite
ClO4-
ClO3-
ClO2-
ClO-
perchlorate
chlorate
chlorite
hypochlorite
BrO4-
BrO3-
BrO2-
BrO-
perbromate
bromate
bromite
hypobromite
IO4-
IO3-
IO2-
IO-
periodate
iodate
iodite
hypoiodite
In compounds, the cation is named as in type I and typ II binary ionic compounds and then
comes the name of the oxoion:
BaS barium sulfide (S2-)
BaSO3 barium sulfite
NaClO sodium hypochlorite
NaClO3 sodium chlorate
Ca(NO3)2 calcium nitrate, type I, only Ca2+
Fe(NO3)2 iron (II) nitrate
Fe(NO3)3 iron (III) nitrate
BaSO4 barium sulfate
Acids
Some gases, dissolved in water produce free hydrated H+ ions: these are acids
HCl(g) is covalent molecular, and thus the gas is hydrogen chloride
HCl(aq), gas dissolved in water, is hydrochloric acid
An acid that produces "...ate" oxoions when dissolved, is an "...ic" acid
H2SO4(aq) sulfuric acid, because it produces sulfate ions in solution
HNO3(aq) nitric acid, because it produces nitrate ions in solution
HClO3(aq) chloric acid, because it produces chlorate ions in solution
H2CO3(aq) carbonic acid, because it produces carbonate ions in solution
H3PO4(aq) phosphoric acid, because it produces phosphate ions in solution
HClO4(aq) perchloric acid, because it produces perchlorate ions in solution
HClO3(aq) chloric acid, because it produces chlorate ions in solution
acids that produce "...ite" ions, are "...ous" acids
HClO2(aq) chlorous acid, because it produces chlorite ions in solution
acids that produce "hypo...ite" ions, are "hypo...ous" acids
HClO(aq) hypochlorous acid, because it produces hypochlorite ions in solution
Same sequence exists for bromine and iodine, but not for fluorine
H2SO3: sulfurous acid because it produces sulfite, SO32-, ions in solution
Organic acids
Same principle: the ic acid produces the ate anion
In organic acids, usually only H-s bound to O are acidic (dissociate to form H+ ions in
solution), but H-s bound to C are not acidic
acetic acid produces the acetate anion in solution:
HC2H3O2 + n H2O  H+(aq) + C2H3O2-(aq)
only the one hydrogen bound to O is acidic and gives a proton (H+) in solution:
oxalic acid produces the oxalate anion in solution and has 2 acidic H, bound to oxygens:
H2C2O4 + n H2O  2 H+(aq) + C2O42-(aq)
the two hydrogens bound to two oxygens are acidic and give protons (H+) in a solution:
Repetition how to find correct number of significant figures in a chain calculation
assume you have to calculate
a = [(2.623) x (4.912) - (0.6) x (0.03186)]/0.34175
first step: do the calculation in 1 step without any rounding, just type it in as it is:
a = 37.64465
to find out how many figures you must have in a, repeat the calculation step by step with
rounding in every step:
2.623 x 4.912 = 12.8842 (4 sign. figures in each number) = 12.88
0.6 x 0.03186 = 0.019116 (1 sign. figure in the first, 4 sign. figures in the second number,
lower one counts in multiplications) = 0.02 (leading zero-s do not count)
12.88 - 0.02 = 12.86 (2 sign. digits after the point in the first, 1 sign. digit after the point
in the second number, leading zero digits do not count, lower one counts in subtraction)
= 12.9 (1 sign digit after the point)
12.9/0.34175 = 37.74689 (3 sign. figure in the first, 4 sign. figure in the second number,
lower one counts in division) = 37.7
Thus a, from the first result, must have 3 significant figures:
a = 37.6
random errors
last digit in a number you read must be guessed:
the guess can be higher or lower than the true value
thus random errors average out when you measure many times and average
systematic errors
it can be, that something is wrong with your balance, when you measure a weight:
if something is wrong with the balance, then all measured values will be higher or all
measured values will be lower (depends on the kind of mistake in the balance) than the true
one
no matter how many times you measure, systematic errors will never average out.