Sandro Lanfranco
Department of Environmental Science, Junior College, University of
Malta, Msida
Overview: the small size of atoms
Much of the matter in the visible universe is composed of atoms. Atoms
are very small particles; so small, that their size is difficult to comprehend.
The average size of a hydrogen atom (the smallest atom) is 0.529 x 10-10m.
Put in simpler terms, this means that ten million hydrogen atoms, placed
end to end, would occupy a length of only ½
mm. A box measuring 1mm by 1mm by 1mm could contain 1.6 x 1021
atoms (16 followed by twenty zeroes). Such a box would only be filled if
each of the roughly six billion humans on Earth were to contribute 269
billion atoms each. If all the atoms in your body were placed end to end
they would circle the Earth 83 billion times. More than 100 different
kinds of atom are known.
Atoms, molecules and elements
Atoms are seldom solitary. Most of the known types of atoms have a
tendency to combine with atoms of the same type or of different types to
form molecules. Molecules are particles made up of two or more atoms bonded
together. Several substances may be made up of very large numbers of atoms
of the same type bonded together. These are called elements.
Example: a block of elemental iron is made up of large numbers of atoms of iron. The term element is also used as a general reference to all the atoms of a particular type (e.g. the element Helium, the element Hydrogen etc.)
Motion of atoms
All atoms are in constant motion. The velocity of motion depends on
the environmental temperature and on the mass of the atom. High temperatures
and low mass lead to higher velocities while low temperatures and large
masses result in lower average velocities. These velocities may vary. A
hydrogen atom at a temperature of 0°C is moving at a velocity of 2.4kms-1
while an atom of Radium at the same temperature moves at 161ms-1
as a consequence of its much greater mass. The mass and velocity of an
atom therefore influence the way it relates to other atoms. This in turn
influences the way in which matter changes from one form (or state) to
another.
States of matter
Matter in which the constituent atoms (or molecules) are bonded tightly
together is in a solid state. In this form, the atoms are not free to move,
although they would be in a state of continuous vibration. Increasing the
temperature of the atoms also increases their kinetic energy and hence
also their velocity. At a particular threshold called the melting point,
the atoms or molecules making up the solid would have acquired sufficient
energy to break these bonds with greater frequency and would be able to
move more freely, forming a liquid. Different solids have different melting
points. Increasing the temperature even further would increase the velocity
of the particles comprising the liquid, eventually severing the links between
them (a threshold called the boiling point). The atoms or molecules would
now not be bonded in any way and the resultant matter is in a gaseous state.
Solid liquid and gas are the three states of matter that are most likely
to be encountered in a relatively stable, low-temperature, low-gravity
context as the Earth. Other states of matter (plasma, degenerate, neutron,
black-hole, ylem) may exist in high-temperature, high gravity environments
such as stars.
Structure of atoms
All atoms are built along much the same lines, with a central nucleus
orbited by a number of electrons. Electrons are particles that carry a
negative electrostatic charge. The nucleus contains two other classes of
particles: protons, which are positively charged, and neutrons, which carry
no net charge. In any given type of atom, both the number of electrons
as well as the number of neutrons may vary. It is only the number of protons
that is invariant. Changing the proton number (or atomic number, as it
is also called) changes a particular atom into a different type of atom.
Arrangement of electrons (electronic configuration)
Electrons do not orbit the nucleus in a haphazard way, but are organised
into distinct shells, or orbitals, each of which holds a predefined maximum
number of electrons. These shells are numbered: the innermost shell (closest
to the nucleus) is termed Shell 1 (or orbital 1) and subsequent shells
are numbered according to increasing distance from the nucleus. The first
shell can only take two electrons whilst subsequent shells (in relatively
small atoms) can harbour up to eight. This basic principle varies in larger
atoms and is not within the scope of the present text. The arrangement
of electrons in shells also depends on the energy state of the atom in
question.
Example: an atom of Sodium contains 11 electrons. Two of these would be placed in the innermost shell, eight in the second shell and one in the third shell. The electronic configuration of Sodium (abbreviated as Na) would be 2-8-1. An atom of Nitrogen (N), with seven electrons would have an electronic configuration of 2-5. Calcium (Ca), with 20 electrons would configure as 2-8-8-2. The number of electrons in the outermost shell determines, to a large extent, the way in which an atom interacts with other atoms.
Mass of atoms
The mass of atoms and of subatomic particles is measured in terms of
atomic mass units (denoted as u). The mass of a proton and of a neutron
is approximately equal, although the neutron is very slightly heavier (proton
mass = 1.007 u; neutron mass = 1.009 u). The electron is much smaller and
lighter with a mass of 0.0005 u, only 0.05% (one-twentieth of 1%) of the
mass of either a proton or a neutron. The mass of any atom is therefore
approximated by simple addition of the number of neutrons and protons in
the nucleus. This quantity is termed the mass number, or atomic mass.
Example: an atom of sodium contains eleven protons and twelve neutrons. The mass number of an atom of Sodium would therefore be 23. In these approximate calculations, the mass of the electrons is disregarded since their mass is negligible compared to that of protons and neutrons. An atom of Calcium contains twenty protons and twenty neutrons and its mass number would therefore be 40.
Isotopes
Recall that atoms of the same element are characterised by the same
proton number (e.g. all atoms of the element Oxygen, anywhere in the universe,
would have eight protons). However, several elements occur in more than
a single variety. These varieties are called isotopes. Isotopes of the
same element are characterised by the same proton number but by different
mass numbers. In concrete terms, this is caused by differences in the number
of neutrons. Example: the element Uranium occurs in two forms (or isotopes).
One isotope (that is, one kind of Uranium) contains 92 protons and 143
neutrons, whilst the other kind contains 92 protons and 146 neutrons.
Some isotopes are unstable and tend to break apart. This process, called
radioactive decay, releases energy.