2 · ELEMENTS, COMPOUNDS AND REACTIONS●They are not lustrous and not sonorous.
●In chemical reactions non-metals do not react with acids, they form acidic oxides
and form either negatively charged ionsor else they form covalent compounds.Examples of non-metals are iodine, oxygen, nitrogen and carbon.
As you will see later, some elements have intermediate properties between those
of metals and non-metals and are called metalloidsorsemimetals. Examples include
arsenic, silicon and germanium.Atoms and molecules
The ‘building bricks’ of elements are called atoms. They are the smallest part of an
element that chemically reacts like a bulk sample of the element. If it were possible to
take a piece of copper and divide it into smaller and smaller pieces, eventually we
would have one atom of copper. Atoms are incredibly small – 35 000 000 copper
atoms, laid end to end in a line, would cover a distance of about 1 cm. The existence
of atoms has been proposed since the time of the early Greeks and it explains the
behaviour of substances. The scanning tunnelling microscope, developed in 1981,
enables us to see atoms (see Box 2.2).
Two or more atoms can link together to form new particles called molecules. The
chemical behaviour of the new molecules is different from that of the atoms which
make them up. The molecules may be combinations of atoms of the same element.
For example, under normal conditions oxygen gas contains diatomicoxygen mol-
ecules (two atoms joined together). For this reason, the oxygen molecule is some-
times called ‘dioxygen’. If the atoms joined are of different elements, the molecules
form part of a compound. The compound known as water consists of molecules in
which two hydrogen atoms are joined to one oxygen atom (see Box 2.3).18
BOX 2.2
Scanning tunnelling microscope
We cannot look at atoms using a
microscope that uses visible light because
particles with diameters less than the
wavelength of visible light (about 400 nm)
cannot be distinguished.
Electron microscopes use electrons (small
particles found inside the atom) to produce
images of tiny objects. The scanning
tunelling microscope (STM) is the most
sophisticated of this type of instrument and
it can produce images of the surfaces of
elements which show the individual atoms.
The atoms show up as blurred spheres. The
STM has applications in physics, chemistry
and biology.
An ‘STM’ image of the surface of gallium arsenide.