Atoms Imagine you have a piece of copper.
If you cut it in half, what do you get? Well, you get two (smaller) pieces
of copper, of course! But suppose you took one of those small pieces
chopped it in half. You'd have even smaller pieces of copper.
But could you keep doing that forever, cutting the smaller and smaller
pieces in half and always have copper? This was the question asked by
Leucippus and Democritus, Greek philosophers
of the 5th century
BC. Although they had no evidence that
we would today call scientific, they felt that the answer must
no. There had to be a point at which you can't cut the copper in half
anymore. They called the smallest-possible piece of anything an atom,
from the Greek word-roots a-, meaning 'no, not, without', and tomos,
'cutting, split'. They felt that the atom could not be cut because there
would be no space inside it. They knew that atoms, if they existed, must
be smaller than can be seen with the eye. This made it difficult to actually
prove they existed.
Atoms are really small. For example, a single
drop of sea water contains 50 billion atoms of gold. You would
need to extract all the gold atoms out of 200 tons of water to get
enough gold to make a tiny speck large enough to barely see with your naked
ETYMOLOGY: Other words that use the a- root include amoral
(without morals) and asymmetrical (not symmetrical). Some
words incorporating the root -tom include
tonsilectomy and appendectomy, operations in which the tonsils or appendix
are cut out of the body.
The existence of atoms wasn't definitively shown until the 1800s, and
there were no direct pictures until the 1980s. Here (to the right) is a picture of silicon atoms in a solid crystal.
In the 1800s it was known that the atom had to be made of positive parts
and negative parts, but the electron was not discovered
(by J.J.Thompson) until 1899, the nucleus (by Ernest
Rutherford, using the gold-foil experiment)
in 1911. Despite what Democritus thought, atoms are 99.999,999,999,999%
The nucleus contains protons (positive charge) and neutrons (no
charge). Protons and neutrons have almost identical masses, and each has over
1800 times the mass of an electron.
they reside in the nucleus, protons and neutrons are together called nucleons.
The radius of a proton or neutron is about 1x10-15m.
The radius of the H electron cloud is 0.5x10-10m; for the
largest atoms the radius is about three times as much. For Hydrogen,
is 50,000 of nuclear and electron cloud radii.
For a tennis ball proton model (radius 3 cm), the electron of the Hydrogen
atom is 50,000 times 3 cm away, or 1.5 km (roughly a mile). Size of the
is very small, so small that its size has never been measured.
There are 92 elements that occur naturally. They differ
in how many protons they have in their nucleus (and to neutralize that
the same number of electrons in orbit around the nucleus.) Hydrogen is
the simplest, with only one proton; Uranium has 92 protons. Elements with > 92
protons are artificial, created in nuclear reactors or particle accelerators. The largest artificial atoms have about 112 protons in the nucleus.
atomic number = # of protons
mass number = # of protons and neutron = # of nucleons
Atoms of the same element can have differing numbers of neutrons. These
differing versions of a given element are called isotopes.
For example, carbon atoms have 6 protons in the nucleus, and therefore
electrons orbiting the nucleus, making the entire atom neutral. The
number and energies of the electrons determine the chemical properties
of carbon: its melting and boiling points, what other atoms it bonds
with (or doesn't), etc. Most carbon atoms also have 6 neutrons in the
nucleus, keeping the protons company. About 1% of carbon atoms, however,
have 7 neutrons, and an even smaller percentage have 8 neutrons. The
extra neutrons have virtually no effect on the chemical
carbon. To distinguish between the different isotopes of an element, the
mass number is often said after the element name. For example, most carbon
atoms are Carbon-12 (6 protons, 6 neutrons), but the carbon atoms with
one extra neutron are called Carbon-13, and those with 2 extra neutrons
The periodic table lists
all the elements in order of atomic number, the number of protons in
the nucleus, which equals the number of electrons
in the electron cloud. The electron cloud
has differing energy levels or shells, but we aren't
going to go into detail about those here. The elements grouped in vertical
columns of the periodic table have the same number of electrons in the
outermost shell, and so those elements have very similar chemical properties.
Atoms can bond (stick) to each other, forming molecules.
Atoms and molecules are both called particles.
Activities & Practice
to do as you read
1. Listen to the Elements Song by
Tom Lehrer. Here's another version.
2. The radioactive isotope Polonium-210 was used to murder former KGB
agent Alexander Litvinyenko in 2006. Find Polonium on the periodic table.
have? How many neutrons does Polonium-210 have?
3. Name some elements that have similar chemical properties to Carbon.
4. Check out this website that graphically illustrates the different isotopes
of the first ten elements. Many isotopes are radioactive (unstable), meaning
that protons can spontaneously turn into neutrons (or vice versa), changing
the identity of the atom.
The arrangement of nuclei as in the above applet is call a nuclide chart. The one above only goes up to atomic number 10 (Neon). A complete version is much more impressive, such as the National Nuclear Data Center's Interactive Chart of the Nuclides.
STATES OF MATTER: There are five so-called states of
matter. The three you are most familiar with are solids, liquids, and
- Solids are made of particles (atoms and
molecules) that are attached to each other, usually
in a regular arrangement called a crystal. Solids
maintain their volume and shape because the particles are locked to
each other and can't move much or switch places.
- Liquids maintain their volume but not their shape.
The particles are still attached to each other, but more loosely so
they can flow and switch places.
- Gases are made of particles that are not
attached to each other at all. Instead, they move freely and bounce
off each other when they collide. Gases maintain neither their shape
nor volume: a gas can be squeezed under pressure and change its volume.
By the way, the word fluid refers to anything that can flow, that is, to both liquids and gases.
Matter can, of course, change from one state to another, by the addition or subtraction of heat at certain temperatures. We use different words to label each kind of change of state, some of which are likely familiar to you, and some maybe not. The diagram to right summarizes these vocabulary words. Melting and sublimation take an input of energy, as does evaporation. The reverse transitions (freezing, deposition and condensation) all release energy instead.
Two states of matter that are less familiar are
- Plasma. A plasma is a gas that is hot enough that
the atoms collide with each other so violently that electrons are knocked
free from the
atoms. In other words, the gas is ionized. The plasma
is a mixture of positively-charged ions and loose electrons. Plasmas
with which you are familiar include:
- The large electrical discharges (sparks) in the atmosphere that
we call lightning raise the temperature of the
air through which it passes to over 30,000°C, stripping electrons
from the atoms in the air and creating (for a brief time) a plasma.
- The gases inside a fluorescent light
- Arc welding
- Plasma TVs
Although plasmas are relatively rare in your everyday experience,
most of the matter in the Universe is in the form of
plasma. Stars are almost entirely ionized gas (albeit very,
very dense in the
center due to the crushing force of gravity), and most
of the (very, very thin) gas that permeates interplanetary
and interstellar space
is also ionized.
- Bose-Einstein Condensate. A Bose-Einstein condenstate
(BEC) is a state of matter that occurs when some atoms are cooled
to a couple billionths of a degree above absolute zero. Essentially,
the atoms lose their individual identities, forming a single blob.
We'll discuss this more when we talk about Quantum Mechanics. BECs
are named after Satyendra Nath Bose, an Indian physicist, and Albert
collaborating in the new field of quantum mechanics when they predicted the existence of this state of matter, but at the time there wasn't the technology to cool a sample of atoms so close to absolute zero. The first BEC was created in a lab in 1995 by Eric
Cornell and Carl Wieman at NIST and the University of Colorado.
The processes of solidification, deposition or condensation all require the release or extraction of thermal energy (heat) from the material. The addition of heat to a substance can cause the opposite changes, namely melting, sublimation and evaporation, respectively. Here's a video showing heat released when a liquid solidifies.
5. Read more about BECs at http://www.colorado.edu/physics/2000/bec/what_is_it.html