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Atomic Structure
Atoms are the smallest part of matter that acts as a unit. Atoms have two main
regions, the nucleus and the electron cloud. The nucleus contains protons and neutrons,
which make up most of the mass of the atom. The nucleus is surrounded by the electron
cloud, which contains electrons that are in energy levels within the cloud.
The number of protons in an atom is the atomic number of the atom (element). This
gives the atom (element) its identity. For example, hydrogen can’t be helium, because
hydrogen can only have ONE proton and helium must have TWO protons.
The number of neutrons and protons in an atom are added together to determine the
mass number of the element. The mass number is taken from the average atomic mass
reported on the periodic table. Mass number is always a whole number, while average
atomic mass is a decimal number. If the decimal portion is .5 or greater, the mass number
is rounded up. If the decimal portion is .4 or smaller, the mass number is rounded down.
An example of this principle is shown below.
Phosphorus has an average atomic mass of 30.974, so it would have a mass number of 31.
The atomic number of Phosphorus is 16. So, using the definition of atomic number and
mass number, we can say that Phosphorus has 16 PROTONS and 17 NEUTRONS. (Mass
number – atomic number (# of protons) = number of neutrons.)
If you look at the periodic table, each box generally looks like this:
6
C
Carbon
12.011.
The 6 is the atomic number, the C is the symbol of the element,
Carbon is the name, and 12.011 is the average atomic mass.
To find the average atomic mass, you have to do a weighted average of the mass of
all the isotopes (atoms of the element with different numbers of neutrons, and therefore
different masses) of the element. An example calculation is shown here.
You have a sample of carbon atoms. The sample contains 86.7 %
Carbon-12, 10.3% Carbon-13, and 3.0% Carbon-14. To find the
average atomic mass:
1. Change each percentage to a decimal by dividing by 100.
2. Multiply the decimal by the mass of each isotope.
3. Add these numbers together.
.867 X 12 = 10.404
.103 X 13 = 1.339
.030 X 14 = .42 _
12.163
Niels Bohr had been studying how some atoms can give off light when they are
heated. He concluded that the light must be given off when electrons move from place to
place within an atom. Bohr called the places that the electrons move to and from energy
levels. These energy levels surround the nucleus in rings, similar to the planets orbiting the
Sun. Bohr concluded that the first energy level could hold two electrons, the second can
hold eight, and the third can hold 18.
Recently, scientists have built onto Bohr’s model and changed one important part.
We now know that the electrons do not move in circles around the nucleus of the atom.
Scientists believe that the electrons are confined to energy levels, but within those energy
levels, there are orbitals where the electrons are believed to be. You see, electrons are so
tiny that they do not contribute to the mass of the atom. They are also moving so incredibly
fast that it is impossible to locate their exact position. Scientists have labeled the orbitals on
atoms with the following letters: s, p, d, f. The s orbital can hold two electrons, the p orbital
can hold 6, the d orbital can hold 10, and the f orbital can hold 14 electrons.
In a neutral atom, the number of electrons should equal the number of protons. The
energy level an electron is in is determined by the amount of energy the electrons have.
Electrons can jump between levels by gaining or losing energy. The electrons can gain
energy when the substance is heated, for example, and then lose energy as it cools. The
electrons may give off light as they lose energy and fall back to their original energy level.