SCiENCE ZoNE
Atom,
tiny basic building blocks of matter. All the material on Earth is composed
of various combinations of atoms. Atoms are the smallest particles of a chemical
element that still exhibit all the chemical properties unique to that element. A
row of 100 million atoms would be only about a centimeter long.
Understanding
atoms is key to understanding the physical world. More than 100 different
elements exist in nature, each with its own unique atomic makeup. The atoms of
these elements react with one another and combine in different ways to form a
virtually unlimited number of chemical compounds. When two or more atoms
combine, they form a molecule. For example, two atoms of the element hydrogen
(abbreviated H) combine with one atom of the element oxygen (O) to form a
molecule of water (H20).
Atoms are made of smaller particles,
called electrons, protons, and neutrons. An atom consists of a cloud of
electrons surrounding a small, dense nucleus of protons and neutrons. Electrons
and protons have a property called electric charge, which affects the way they
interact with each other and with other electrically charged particles. The
nucleus contains nearly all of the mass of the atom, but it occupies only a tiny
fraction of the space inside the atom. The diameter of a typical nucleus is only
about 1 × 10-14 m (4 × 10-13 in), or about 1/100,000 of the diameter of the
entire atom. The electron cloud makes up the rest of the atom’s overall size.
If an atom were magnified until it was as large as a football stadium, the
nucleus would be about the size of a grape.
| Electrons
|
Electrons are tiny, negatively
charged particles that form a cloud around the nucleus of an atom. Each electron
carries a single fundamental unit of negative electric charge, or –1.
The electron is one of the lightest
particles with a known mass. A droplet of water weighs about a billion, billion,
billion times more than an electron. Physicists believe that electrons are one
of the fundamental particles of physics, which means they cannot be split into
anything smaller. Physicists also believe that electrons do not have any real
size, but are instead true points in space—that is, an electron has a radius
of zero.
Electrons act differently than
everyday objects because electrons can behave as both particles and waves.
Actually, all objects have this property, but the wavelike behavior of larger
objects, such as sand, marbles, or even people, is too small to measure. In very
small particles wave behavior is measurable and important. Electrons travel
around the nucleus of an atom, but because they behave like waves, they do not
follow a specific path like a planet orbiting the Sun does. Instead they form
regions of negative electric charge around the nucleus. These regions are called
orbitals, and they correspond to the space in which the electron is most likely
to be found. Orbitals have different sizes and shapes, depending on the energy
of the electrons occupying them.
| Protons
and Neutrons |
Protons carry a positive charge of
+1, exactly the opposite electric charge as electrons. The number of protons in
the nucleus determines the total quantity of positive charge in the atom. In an
electrically neutral atom, the number of the protons and the number of electrons
are equal, so that the positive and negative charges balance out to zero. The
proton is very small, but it is fairly massive compared to the other particles
that make up matter. A proton’s mass is about 1,840 times the mass of an
electron.
Neutrons are about the same size as
protons but their mass is slightly greater. Without neutrons present, the
repulsion among the positively charged protons would cause the nucleus to fly
apart. Consider the element helium, which has two protons in its nucleus. If the
nucleus did not contain neutrons as well, it would be unstable because of the
electrical repulsion between the protons. A helium nucleus needs either one or
two neutrons to be stable. Most atoms are stable and exist for a long period of
time, but some atoms are unstable and spontaneously break apart and change, or
decay, into other atoms.
Unlike electrons, which are
fundamental particles, protons and neutrons are made up of other, smaller
particles called quarks. Physicists know of six different quarks. Neutrons and
protons are made up of up quarks and down quarks—two of the six different
kinds of quarks. The fanciful names of quarks have nothing to do with their
properties; the names are simply labels to distinguish one quark from another.
Quarks are unique among all
elementary particles in that they have electric charges that are fractions of
the fundamental charge. All other particles have electric charges of zero or of
whole multiples of the fundamental charge. Up quarks have electric charges of
+?. Down quarks have charges of -?. A proton is made up of two up quarks and a
down quark, so its electric charge is ? + ? - ?, for a total charge of +1. A
neutron is made up of an up quark and two down quarks, so its electric charge is
? - ? - ?, for a net charge of zero. Physicists believe that quarks are true
fundamental particles, so they have no internal structure and cannot be split
into something smaller.
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