CHEM1011 Lecture Notes - Lecture 1: Thulium, Californium, Molybdenum
2 Jul 2018
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TOPIC 1.
ELEMENTS, COMPOUNDS AND MIXTURES.
What is chemistry?
Chemistry is the study of matter and the interconversion of matter. Matter is anything
which has mass and occupies a volume. Sciences often begin by collecting and
classifying. Subsequent observations lead to generalisations and laws. In the science of
chemistry, there are numerous bases for classifying matter but the most fundamental is
to subdivide matter into pure substances and mixtures. Pure substances can be further
regarded as consisting of either elements or compounds.
MATTER
PURE SUBSTANCES MIXTURES
An infinitely
large number
of mixtures is
ELEMENTS COMPOUNDS possible
90 naturally occurring An infinitely large
elements + about 30 number of compounds
man-made elements is possible
What characterises each of these groups?
An ELEMENT is a substance which cannot be broken down into simpler component
substances. An ATOM is the smallest possible unit of an element. Atoms are
extremely small so any visible specimen of an element contains enormous numbers of
atoms. Each element’s atoms are unique to that element. There are only 90 naturally
occurring elements so it follows that there are only 90 different types of naturally
occurring atom. What distinguishes the atoms of each different element is the subject
of another Topic.
Elements are conveniently further subdivided into two groups, METALS and NON-
METALS, based on their physical and chemical properties. The physical properties of
metals are probably already familiar - shiny when freshly cut, conduct heat and
electricity well, malleable and ductile. The non-metals have the opposite properties to
metals - they are usually powders or gases, do not conduct well and, if solids, are
brittle. The chemical properties of each group will be discussed in future Topics.
I - 1
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I - 2
Elements as they occur in nature rarely consist of discrete, individual atoms. Usually
they consist of two or more atoms joined together by CHEMICAL BONDS of
various types. For example, oxygen, the essential life-supporting element in air, is not
present as individual oxygen atoms but instead, consists of two oxygen atoms bonded
together. This unit is called a MOLECULE of oxygen. Some elements occur as
discrete molecules containing even larger numbers of atoms such as 4, 6, 10 or 12
bonded atoms. However, most elements including all the metals do not normally exist
as discrete molecules containing a fixed number of atoms but instead, consist of very
large aggregates of atoms bonded together.
Regardless of whether an element occurs as single atoms or as atoms bonded together,
each element always consists of the same type of atom which imparts its own unique
properties to that element. For example, aluminium is recognised as a silver-coloured
metal while copper has a different colour. Apart from the obvious difference in
colour, there are many unique physical and chemical properties which each of these
two elements possess and which are attributable to the differences between their
atoms.
Table 1 (Page I-21) gives a complete alphabetical list of all the elements, including
some of those synthesised. In Table 2 (Page I-22), most of those elements are listed in
groups containing four to six elements. Some of the groups are given names - for
example, the elements of the first group collectively are known as the alkali metals,
the seventh group is the halogens and the eight group is the noble gases. Elements
within each group have many properties in common. In addition, another 11 elements
which are part of a much larger grouping known as the transition elements are listed in
Table 2. Note that hydrogen does not belong to any group and is regarded as being an
exceptional individual element.
Compounds.
A COMPOUND differs from an element in that an element contains only a single
type of atom while a compound consists of smallest units which contain at least two
different types of atom (i.e. atoms of at least two different elements) joined together by
chemical bonds. Compounds always contain atoms of their constituent elements in the
same numerical ratio regardless of how the compound was prepared. Consequently
analysis of any pure compound always returns the same result. Therefore a compound
when pure is HOMOGENEOUS. Many compounds exist as discrete molecules. The
smallest unit of the compound carbon dioxide is the carbon dioxide molecule, each of
which consists of one carbon atom bonded to two oxygen atoms - this molecule is the
smallest particle of carbon dioxide that can exist. The ratio of one carbon atom to two
oxygen atoms in the molecule of this compound applies equally well to the carbon
dioxide which may originate from burning of natural gas or petrol or from chemical
reactions such as when an acid is mixed with a substance such as limestone or washing
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I - 3
soda. Carbon dioxide molecules always consist of one carbon atom bonded to two
oxygen atoms regardless of the source of the compound.
However, not all compounds exist as discrete molecules - for example the compound
sodium chloride also known as table salt exists not as individual sodium chloride
molecules but as a highly structured crystal lattice, familiar as the rock salt commonly
used in salt grinders. There are several types of chemical bonds that join atoms in
elements and compounds and the type of bond present determines in part whether a
given compound exists as discrete molecules. Chemical bonds are examined in later
Topics.
Note that when elements have combined to form compounds, they lose their original
properties and the compound formed has its own characteristic properties. For
example, carbon - a black solid - combines with another element, the colourless gas
oxygen which supports combustion, to form the compound carbon dioxide which is a
colourless gas that is incapable of supporting combustion. Compounds can only be
converted back to their component elements by breaking the chemical bonds which
hold the atoms together in the compound. For example, the compound water, which
contains two hydrogen atoms and one oxygen atom combined in each water molecule
can be converted to the free elements oxygen and hydrogen by passing an electric
current through the water, a process called ELECTROLYSIS.
Whenever chemical bonds are broken or formed, the process is called a CHEMICAL
CHANGE. Processes such as the cooking of food, the burning of fuels and the
corrosion of metals are all examples of common chemical changes.
You need more calcium in your diet.
A very common mistake made by the general population is the failure to
differentiate between a pure element and compounds of that element, e.g. the
poisonous gaseous element chlorine and essential compounds of chlorine such as
sodium chloride. Likewise, the calcium referred to above is in the form of
compounds of calcium and definitely not the free element.
Mixtures
A MIXTURE, as the name implies, consist of different elements or compounds which
have been physically mixed together. Consequently, any given sample taken from a
mixture would contain at least slightly different amounts of its various components
compared with any other sample collected, because mixtures are
HETEROGENEOUS. Analysis of a mixture often requires specified procedures in
order to obtain a representative sample. Mixtures are always impure in that they
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Document Summary
Chemistry is the study of matter and the interconversion of matter. Matter is anything which has mass and occupies a volume. In the science of chemistry, there are numerous bases for classifying matter but the most fundamental is to subdivide matter into pure substances and mixtures. Pure substances can be further regarded as consisting of either elements or compounds. 90 naturally occurring elements + about 30 man-made elements. An infinitely large number of compounds is possible. An infinitely large number of mixtures is possible. An element is a substance which cannot be broken down into simpler component substances. An atom is the smallest possible unit of an element. Atoms are extremely small so any visible specimen of an element contains enormous numbers of atoms. Each element"s atoms are unique to that element. There are only 90 naturally occurring elements so it follows that there are only 90 different types of naturally occurring atom.
