Download Energy – Section 2-1

Structure of Atoms – Section 3-2
Objectives:
 Describe the evidence for the existence of electrons, protons, and
neutrons, and describe the properties of these subatomic particles.
 Discuss atoms of different elements in terms of their numbers of
electrons, protons and neutrons, and define the terms atomic number
and atomic mass.
 Define isotope and determine the number of particles in the nucleus of
an isotope.
It took awhile for Dalton’s ideas to gain support. However, chemists started
testing his ideas, and gradually new information made modifications
necessary. In contrast to Dalton’s theory, it appeared that in fact atoms were
not the smallest piece, that they had smaller substructure. You could say
that atoms were the smallest complete unit of matter, but that there were
smaller pieces.
http://www-outreach.phy.cam.ac.uk/camphy/electron/side6_electron.jpg
In 1897, J.J. Thomson, an English physicist was working with electricity,
using a device called a Cathode Ray Tube (CRT).
This tube had a passed a beam of electricity between magnetic plates.
Thomson discovered that the beam was deflected they way a negative
particle would be. It was this information that led him to hypothesize that
the atom had a negative particle inside, which he called the electron.
http://content.answers.com/main/content/wp/en/thumb/7/77/300px-JJ_Thomson_exp2.jpg
As a result of this discovery, Thomson proposed a new model of the atom, to
update the Dalton view. This new model was called the Plum Pudding
Model. Thomson knew that the atom was overall electrically neutral, so if
there was a negative component, there must also be a positive component (to
balance it out and keep it neutral). The Plum Pudding Model described the
atom as a mass of positive charge in a matrix (like the dough of the
pudding), with negatively charged electrons scattered inside the matrix (like
the plums in the pudding).
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BohrModel/
plupudding.gif
In 1909, Ernest Rutherford was testing the Plum Pudding Model (which he
assumed to be correct). He had a thin foil of gold mounted so he could pass
a beam of alpha particles (a radioactive particle made of 2 protons and two
neutrons) through the foil, and detect the radiation on photographic film
around the whole set up. He expected the beam of positive particles to go
straight through the foil and made a spot on the other side.
http://mpimichelet.free.fr/rutherford.jpg
http://bhs.smuhsd.org/science-dept/marcan/apchemistry/rutherford_exp.gif
That isn’t what happened. What happened was that most of the particles
went through, but some were deflected off, at all angles, including
deflections that went straight backwards. By 1911, Rutherford explained
this result by saying that the atom appeared to be mainly empty space.
However, sometimes the positive beam came close to a dense positive
area, which caused the beam to deflect. When the alpha particles hit this
dense space head on, they bounced back entirely. In this way, Rutherford
was the first to describe the atomic nucleus, where most of the mass and
positive charge in the atom was concentrated. This positive charge was
later isolated, and the particle was named the proton.
http://www.visionlearning.com/library/modules/mid50/Image/VLObject-497-021202031205.gif
Rutherford proposed the Nuclear Atom Model (or Solar System Model)
of the atom, where a dense positive center was orbited by moving
electrons. It resembled this:
http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Rutherford_atom.svg/500pxRutherford_atom.svg.png
In our modern understanding of the atom, we know that the nucleus is
composed of protons and neutrons. Neutrons are particles found in the
nucleus with no charge at all. They are large in size, like the proton.
Each electron is assigned a charge of 1- and protons are 1+. These
opposite charges (positive in the nucleus and negative outside of the
nucleus) create an electrostatic attraction that holds the atom together.
Opposite charges are attracted to each other, while similar charges will
repel. For this reason, it may seem strange that a nucleus can hold
together with all that positive charge in one place.
The nucleus is held together by a special force of nature called the strong
nuclear force. Basically, the neutrons act as a buffer zone between the
protons in the nucleus, reducing the repulsion between them. This is why
for smaller atoms, there is usually an equal number of protons and neutrons
in the nucleus. As atoms get larger, the number of neutrons starts to increase
disproportionately in order to hold the protons together.
It turns out that the number of protons is extremely important. The number
of protons in the nucleus determines the identity of the atom. If you have 6
protons, the atom is carbon. If you add one more, the atom changes to
nitrogen. The number of protons is absolutely specific to an element. The
number of protons is referred to as the atomic number. Atomic number is
the method of arranging the atoms on the periodic table. Atoms on their
own are electrically neutral. This means that they have no charge. If there is
a certain amount of positive in the nucleus, then there must be an equal
amount of negative on the outside. So, not only does atomic number tell us
the number of protons, it also tells us the number of electrons.
Another important value is the atomic mass. Atomic mass is the mass of
the atom in a.m.u. (atomic mass units). Protons and neutrons are large, and
both are assigned a mass of 1 amu each. Electrons are so much smaller, they
have basically no affect on the mass of an atom. For this reason, they have a
mass value of 0 amu. If an atom had 17 protons, 17 electrons, and 18
neutrons, it would have an atomic mass of 35 amu (17 + 18).
There is one twist with this. Atoms of the same element will have identical
numbers of protons, but the number of neutrons can vary. This means that
there are different versions of the same atom! Some will have heavier
atomic masses than others due to the extra neutrons. These different
versions of the same atom are called isotopes. Isotopes are identical in
every way except the number of neutrons (ex. Carbon has two versions, 12C
and 14C).
http://www.atomicarchive.com/Physics/Images/isotopes.jpg
The impact of this is that when you look at atomic mass on the periodic
table, they are decimals, rather than the whole numbers we’d expect. The
reason for this is that the atomic masses on the periodic table are averages of
all the different isotopes in nature. You can often do the appropriate
rounding up or down to find the most common isotope, and therefore figure
out how many protons and neutrons are possible in each.