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INTRODUCTION TO CHEMISTRY
Professor Paul Brint
These 12 lectures will consider the very basic aspects
of Chemistry: Atoms, Molecules, Matter.
Atoms
Democritus (350 BCE):
atoms are eternal and invisible; absolutely
small, so small that their size cannot be diminished (hence the name atomon,
or "indivisible"); absolutely full … differing only in shape, arrangement,
position, and magnitude.
Dalton (1800 CE): atoms are tiny, indivisible, indestructible particles
and that each one had a certain mass, size, and chemical behaviour that was
determined by what kind of element they were. (The following lectures are
the modern version of Dalton’s theory.)
Feynman (1960 CE): "Everything is made of atoms. That is the key
hypothesis. The most important hypothesis in all biology, for example, is that
everything that animals do, atoms do. In other words, there is nothing that
living things do that cannot be understood from the point of view that they
are made of atoms acting according to the laws of physics."
Richard Feynman, Six Easy Pieces: Essentials of
Physics Explained (1995)
If, in some cataclysm, all of scientific knowledge were
to be destroyed, and only one sentence passed on to
the next generations of creatures, what statement
would contain the most information in the fewest
words? I believe it is the atomic hypothesis (or the
atomic fact, or whatever you wish to call it) that
all things are made of atoms — little particles that move
around in perpetual motion, attracting each other when
they ore a little distance apart, but repelling upon being
squeezed into one another.
In that one sentence, you will see, there is an enormous
amount of information about the world, if just a little
imagination and thinking are applied.
Chemistry considers matter to be composed of minute
indestructible particles called atoms.
Volume
Mass
Atoms occupy space, have volume
or size.
Atoms have mass, contain matter.
There are over 100 different types of atoms known
These are incredibly small.
They ‘join’ together to form molecules and/or ions.
Matter is made of molecules or ions.
How small?
An atom of hydrogen (the smallest atom), sphere of
radius 3.5x10-11 m and has a mass of 1.67x10-27 kg.
An atom of lead (a large atom), sphere of radius
1.8x10-10 m and has a mass of 3.46x10-25 kg.
How many atoms of lead in 1 kg of lead? (2.89x1024)
Does this agree with the density of solid lead (1.13x104
kg m-3)?
(Yes, more or less; (mass of atom) / (volume of atom)
gives about 1.4x104 kg m-3)
If the diameter of carbon atom is 1.5x10-10 m, how many
carbon atoms are in a line across a diamond 4 mm in
size? (2.7x107)
How many atoms of hydrogen in 1 g of hydrogen?
(6.022x1023, called Avagadro’s Number!! )
Atoms are composed of ‘sub-atomic’ particles of
which
protons, neutrons and electrons
are important in Chemistry.
These particles have mass and charge:
Particle Charge /coulombs
Mass/ kg
electron
- 1.602 x 10-19 = e
9.109 x 10-31 = me
proton
+ 1.602 x 10-19= -e
1.672 x 10-27 = mp
neutron
0
1.675 x 10-27 ~ mp
The proton and neutron have almost the same mass
and both have much more mass than the electron.
(mp / me ) ~ 1860
The proton and electron have equal and opposite
charges.
An atom is electrically neutral and contains the same
number of electrons and protons.
An atom has protons and neutrons together in the
nucleus of the atom.
The electrons move around the nucleus.
protons
and
neutrons
In nucleus
electrons
This is only a ‘picture’.
The nucleus is tiny ~ 10-15m, compared to the radius of
atom ~ 10-10 m
The negatively charged electrons are attracted to the
positively charged nucleus.
The ‘electron cloud’ defines the volume of the atom.
The mass of the atom is concentrated in the nucleus.
Describing atoms:
Name.
eg. hydrogen, carbon, sodium, uranium
Symbol.
H
C
Na
U
Atomic number, Z = number of protons in the nucleus
1
6
11
92
Atomic mass number, A = number of protons plus the
number of neutrons in the nucleus
1
12
23
220
An atom is written as a combined symbol:
(Z written as its symbol)
1
H
12
C
23
Na
A
Z
220
U
Z determines the type of atom.
All C atoms have 6 protons (and 6 electrons); they
may have 6, 7 or 8 neutrons: 12C, 13C, 14C (isotopes)
Isotopes are atoms that have the same number of
protons (and electrons) and different numbers of
neutrons.
Z
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Name
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Sodium
Magnesium
Aluminium
Silicon
Phosphorous
Sulphur
Chlorine
Argon
Potassium
Symbol
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Atomic Numbers, Names and Symbols of the first 19
atoms in order of increasing atomic number.
Mass of atoms:
Define Atomic Mass Unit, amu, as 1/12 of the mass of
a 12C atom; 1 amu = 1.6605 x 10-27 kg
The mass of atoms is expressed in amu as a convenient
unit.
12
23
31
atom 1H
C
Na
P
mass 1.0078 12(exactly) 22.9898 30.9738 amu
Average Atomic Mass
Most types of atoms exist as a mixture of isotopes.
Carbon, for example, is 98.93% 12C, 1.07% 13C
Average atomic mass of Carbon is:
(0.9893)(12 amu) + (0.0107)(13.00335) = 12.01 amu