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Nature of matter

General
Atoms are the smallest particles of matter whose properties we study in chemistry.
However, from experiments done in the late 19th and early 20th century it was deduced
that atoms were made up of three fundamental sub-atomic particles (see table below).

The early Greek philosophers proposed that all matter is made up of incredibly small but
discrete units (like the bricks in our wall example). Democritus (460 - 370 BC) was the
first to call these units ‘atomos’. From this phrase came the term atom that we use today.
Atomos is a Greek phrase which means ‘not cut’ or ‘that which is indivisible’.

Atoms can neither be created nor destroyed during chemical reactions.

All atoms are, crudely speaking, the same size and can be thought to consist of two main
parts. The outer part is composed of one or more orbits of electrons.

These orbits make up most of the volume of the atom yet contributes practically nothing
to its substance. The other part, located at the centre, is extremely small compared to the
atom as a whole, yet essentially all of the real substance of the atom can be attributed to
this small speck. We call this speck the nucleus.

Further investigation revealed that the nucleus is actually composed of two kinds of
particles of a roughly equal size and substance packed closely together. These nuclear
particles are the proton and neutron. When we refer to the amount of material or
substance in an object, we are really talking about the number of protons and neutrons in
that object.

Also, what we perceive as the mass of an object is related directly to the number of
protons and neutrons contained it.

The simplest atom is hydrogen which has a single proton for a nucleus. An atom of lead,
on the other hand, has 82 protons and 125 neutrons in its nucleus and so has 207 (125 +
82) times as much material or substance as an atom of hydrogen.

The size of an atom bears no simple relation to the number of particles in its nucleus. A
sodium atom, for example, with 11 protons and 12 neutrons is approximately the same
size as an atom of mercury with 80 protons and 121 neutrons.

In general, we can say that the size of an atom is determined by its electron orbits, its
substance is determined by the total number of protons and neutrons in its nucleus.

Dalton’s atomic theory
Around 1805, John Dalton proposed a theory to explain the nature of matter that is
remarkably similar to modern atomic theory. While this theory is stated in a variety of
ways, the key points of this theory are:

 All matter is composed of indivisible particles called atoms.
 All atoms of an element are identical, while atoms of different elements are different.
 Molecules are a combination of any two or more atoms. Compounds are a

combination of atoms from two or more different elements.
 Chemical reactions involve the rearrangement of atoms. The atoms themselves do

not change.

Total Training Support Ltd 1-9 Issue 2 – September 2016
© Copyright 2016 Module 2.1 Matter