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Earth Lab: Atomic Structure
By Trista L. Pollard
When scientists design models of atoms, they
usually show a simplified version of the atom's nucleus
and its subatomic particles. The nucleus is made up of
protons and neutrons (picture red and blue gumballs
stuck together) with electrons moving at high speeds
around the outside of the nucleus (imagine gumballs on
a circular wire). Over the years, scientists have found
out that the highly fast-moving electrons form a cloud
around the nucleus. In fact, within this electron cloud, the electrons are
spaced at different distances from the nucleus. These areas of electrons are
called energy levels or shells. Each shell can only have a certain number of
electrons. Since electrons are negatively charged and opposite charges
attract, the electrons are attracted to the nucleus, which has a positive charge.
It is this attraction that keeps the electrons inside the atom. Speaking of the
nucleus, you should know that most of the atom's mass is the nucleus.
However, the nucleus's size is extremely small compared to the size of the
atom. Most of the atom's volume is empty space, due to the extremely small
size of the electrons, protons, and neutrons.
Scientists use information in the atom to describe elements. The nucleus
of an atom has a specific number of protons. This number of protons
determines the atom's atomic number and the name of the element. An
element's atomic number distinguishes it from other elements. Atoms that
are "free" or uncharged have the same number of electrons as protons.
Therefore an atom's atomic number is also equal to the number of electrons
within that atom. If you think about it, an element is a huge collection of
atoms (with neutral charges) that all have the same atomic number.
Remember the name of the element? Well, all atoms with six protons are
classified as carbon atoms, whereas all oxygen atoms have eight protons.
All elements are organized on a periodic table of elements. Elements with
similar arrangements of electrons in their atoms are put together in the same
column of the periodic table or groups. All of the elements that are part of
the group have similar chemical properties. Below is a diagram explaining
how elements appear on the periodic table.
An atom's atomic mass refers to the sum of its protons and neutrons.
Subatomic particles are too small to define the mass in units of grams.
Therefore, atomic mass unit (amu) is used to describe the mass of protons,
neutrons, and electrons. The mass of a proton and a neutron is about 1 amu
each. Electrons, which are much smaller than protons and neutrons, are not
included in the calculated atomic mass of an atom. In fact, if you were to
compare the mass of electrons with the mass of protons, you would need
about 1,840 electrons together to equal the mass of one proton.
Even though all atoms of an element have the same number of protons,
they may have different amounts of neutrons. These atoms are called
isotopes. Isotopes will have the same atomic number as other atoms of the
same element. However, they will have a different atomic mass due to the
different number of neutrons. Isotopes of a specific element will have
different properties. To account for the different masses of an element's
isotopes, the periodic table uses average atomic mass. The average atomic
mass of each element is the weighted average of the three naturally
occurring isotopes of that element. For example, there are two isotopes for
carbon: carbon-12 and carbon-14. Since carbon-12 is more common, the
average atomic mass on the periodic table is about 12 amu. Remember, all
carbon atoms have six protons. This means that carbon-12 has six neutrons
and carbon-14 has eight neutrons. The diagram below shows one of the
naturally occurring isotopes for Hydrogen.
Although isotopes of the same element have different amounts of
neutrons, they are chemically almost identical. Isotopes of an element react
the same during chemical reactions. In fact, different isotopes from the same
element may end up in the same mineral. (We'll talk about minerals later.)
Keep in mind that isotopes may be similar, but they also have properties of
their own.
1. The areas of electrons in electron
clouds are called ______.
Stages or views
Shells or energy levels
Power levels or valences
All of the above
3. If the element Helium (He) has
two protons and two neutrons,
what are the atomic number and
the atomic mass?
2. Scientists sometimes compare the
orbiting of electrons around the
nucleus of an atom to the planets
orbiting around the sun. Explain
the comparison.
4. Isotopes of elements have the
same number of protons but a
different number of ______.
7. Based on the diagram, how many 8. What is the difference between
protons, neutrons, and electrons
does this element's atom contain?
the atoms of these two isotopes
for sodium?