2. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
An atom is the basic structure from which all matter is composed, in the
same manner as a brick is basic to the structure of a wall. Although atoms
are too small to be seen with our eyes, scientists have long had indirect
evidence for the existence of atoms.
3. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
Model of Proton, Neutron, and Electron
Atoms are made of small particles called protons, neutrons, and electrons.
Each of these particles is described in terms of measurable properties,
including mass and charge. Mass is the amount of matter that an object
contains. The proton and neutron have roughly the same mass and have
approximately one thousand times the mass of the electron. The proton and
electron have equal, but opposite, electrical charges. A neutron does not have
an electrical charge.
4. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
If the proton and
neutron were enlarged,
and each had the
approximate mass of a
hippopotamus, the
electron, enlarged to
the same scale, would
have less mass than
an owl.
5. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
Most of an atom is empty
space. The nucleus of the
atom contains almost all of
the mass of the atom. A
greatly enlarged atom
might look like a marble
(the nucleus) inside an
empty football stadium.
(The electron probability
cloud is where the electron
is likely to be).
Electron Probability Cloud Around a Nucleus
6. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
Diagram Comparing a Beryllium Atom and a Positively-Charged Beryllium Ion
The attractive electric force between the positively-charged protons in the
nucleus and the negatively-charged electrons around the nucleus holds the
atom together. Atoms containing the same number of protons and electrons
have no net charge. Atoms that have extra electrons or are missing
electrons have a net electrical charge and are called ions. Ions can interact
with other ions due to the electrical attraction between opposite charges.
7. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
Model of Covalent and Ionic Bonds
Atoms interact with other atoms by sharing or transferring electrons that are
farthest from the nucleus. These outer electrons determine the chemical
properties of the element, such as how readily it interacts with other elements
and the allowable ratios for its combinations with other substances.
8. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
An element is a substance
Element Symbol Relative % of
made up of a single type of
Earth’s Mass atom. It can’t be broken into
Oxygen O 46.6 simpler components by
Silicon Si 27.7 chemical processes. There
Aluminum Al 8.1
are 92 naturally occurring
Iron Fe 5.0
Calcium Ca 3.6 elements. They may be
solids, liquids, or gases. The
Sodium Na 2.8 elements are distributed
unevenly, with some much
Potassium K 2.6
more common than others.
Magnesium Mg 2.1
The ten most abundant
Titanium Ti 0.4 elements on earth make up
Hydrogen H 0.1 more than 96% of our planet.
9. Education/Public Outreach
Cosmic Chemistry: An Elemental Question
Gold is one example of an element.
A bar of gold can be shaved
into gold dust, and still be How fine can the dust
recognizable as gold. become and still be
considered gold?
The smallest particle that would still have the
properties associated with gold is an atom.
How small is an atom? Consider that a
small gold coin may contain over
20,000,000,000,000,000,000,000 atoms.
10. Atomic mass and atomic number
• The atomic mass (ma) is the mass of a specific isotope, most often
expressed in unified atomic mass units (amus). The atomic mass is
the total mass of protons, neutrons and electrons in a single atom.
• The atomic number (also known as the proton number) is the
number of protons found in the nucleus of an atom.
Atomic number
Atomic mass
12. Using the periodic table to find information
• The periodic table gives you lots of information. One of the typical
things that you must know is how to find the number of protons,
neutrons and electrons in a given element. In order to do this:
Step 1:
- Find the element in the periodic table.
- Find the atomic mass and the atomic number.
Atomic number
Atomic mass
13. Number of protons
• Step 2 - The Number of Protons is...
The atomic number is the number of protons in an atom of an element. In
our example, krypton's atomic number is 36. This tells us that an atom of
krypton has 36 protons in its nucleus.
The interesting thing here is that every atom of krypton contains 36
protons. If an atom doesn't have 36 protons, it can't be an atom of
krypton. Adding or removing protons from the nucleus of an atom
creates a different element. For example, removing one proton from an
atom of krypton creates an atom of bromine.
14. Number of electrons
• Step 3 - The Number of Electrons is...
By definition, atoms have no overall electrical charge. That means that
there must be a balance between the positively charged protons and the
negatively charged electrons. Atoms must have equal numbers of
protons and electrons. In our example, an atom of krypton must
contain 36 electrons since it contains 36 protons.
An atom can gain or lose electrons, becoming what is known as an ion.
An ion is nothing more than an electrically charged atom. Adding or
removing electrons from an atom does not change which element it is,
just its net charge.
15. Number of neutrons
• Step 4 - The Number of Neutrons is...
1. Round the atomic mass to the nearest whole number. To get the mass number. The mass
number is a count of the number of particles in an atom's nucleus. Remember that
the nucleus is made up of protons and neutrons. So, if we want, we can write:
Mass Number = (Number of Protons) + (Number of Neutrons)
For krypton, this equation becomes:
84 = (Number of Protons) + (Number of Neutrons)
If we only knew how many protons krypton has, we could figure out how many neutrons it
has. Wait a minute... We do know how many protons krypton has! We did that back
in Step 2! The atomic number (36) is the number of protons in krypton. Putting this into
the equation, we get:
84 = 36 + (Number of Neutrons)
What number added to 36 makes 84? Hopefully, you said 48. That is the number of neutrons
in an atom of krypton.
16. Isotopes
• The interesting thing here is that adding or removing neutrons from an
atom does not create a different element. Rather, it creates a heavier or
lighter version of that element. These different versions are
called isotopes and most elements are actually a mixture of different
isotopes.
