CHEM 100
Chemistry and Society
Winter Quarter 2008 SCCC
Lecture 3
Feel good, relax there is no quiz or homework and you
have almost made it through chapter 1!!
This week we have been exploring the definition:
“Chemistry is the scientific study of matter and its
transformations”
Key Concepts
1. Theories and physical laws are the output of the scientific
method.
2. The three principle states of matter are solid liquid and gas.
3. Matter may be either a pure substance or a mixture of pure
substances.
Can you recall what the definition of matter is ?
Pure substances may be either elements or compounds.
Elements are composed of atoms of a single type.
Compounds are composed of atoms of two or more types held
together by chemical bonds and may be either ionic or molecular.
Pure substances can not be separated into more than one
component without making or breaking chemical bonds.
Mixtures are composed of two or more pure chemical
substances.
They may be separated into pure substances using physical
means.
Mixtures may be classified as either homogeneous or
heterogeneous depending upon how evenly the pure substances
are distributed.
Chromatography is often used by chemists to separate mixtures
into their pure substances.
How does chromatography work ?
When we begin studying chemistry we see the
periodic table everywhere.
We may ask
• What are the origins of the periodic table ?
• Why is it so commonly used ?
In the 18th and 19th century many new elements were discovered.
The Russian chemist Dmitri Mendeleev recognized that a system
was needed to organize the elements.
Mendeleev organized the elements such that elements with
similar chemical and physical properties were placed next to one
another.
Mendeleev noticed that after a series of elements the properties
would repeat.
This lead Mendeleev to develop a periodic table of the elements
with columns containing elements of similar properties.
Mendeleev left several gaps in his table he proposed that these
gaps were needed for elements that were yet to be discovered.
He also predicted what the physical and chemical properties of
these elements were.
In the years following his predictions Mendeleev was proved
correct with amazing accuracy.
Mendeleev predicted that their should be an element with
properties similar to aluminum which he called eka-aluminum.
17 years after the development of Mendeleev’s table scientists
discovered this missing element and it gallium.
The experimental properties of gallium are very close to
Mendeleev’s prediction;
Prediction
Actual
Melting point
(oC)
Low
20oC
Density
(g/cm3)
5.9
5.9
Oxide
formula
X2O3
Ga2O3
Mendeleev’s table proved so successful it is one of the most
commonly used instruments of scientists.
Rather than memorizing the properties of all the elements we just
need to know how the properties vary across the table.
The modern periodic table looks like this
1A
8A
2A
3A 4A 5A 6A 7A
3B 4B 5B 6B 7B
8B
1B 2B
Elements are given either one or two letter symbols.
e.g. H for hydrogen and He for helium
In most cases these symbols are the first letter of the element
name possibly followed by either the second or third letter.
Some elements have symbols derived from their Latin names.
e.g. Na for sodium (natrium)
It is good to become familiar with some of these symbols Table
1.2 in our text book has a complete alphabetical list.
The periodic table can be divided into three main regions.
• The metals (the largest part of the table taking up the middle
and left side).
•The non-metals to the far right of the table.
•The metalloids lying between the non-metals and metals.
Metals have the following properties
• They are good conductors of electricity and heat.
• They are solids at room temperature (except for mercury).
• Generally have a high melting point.
Non-metals, however,
• Do not conduct heat or electricity very well.
• They maybe either solid, liquid or gas.
• Generally have lower melting points than metals.
Metalloids have properties that are intermediate between metals
and non-metals
We can combine the symbols of the elements to make formulas
that show the proportions of elements in a compound.
• The number of atoms of each element type in the molecule or
formula unit is written as a subscript after the symbol of the
element.
• Symbols for metals and metalloids are put before metalloids.
e.g. NaCl, H2O, Ga2O3
This saves time and space but you need to know the elemental
symbols !
There is no marked homework this week but
read Chapter 1 and complete
the problems on pages 22 and 23