ATOMIC STRUCTURE: a model of the atom for beginners
Atoms and molecules are small. If a bb was an atom and a marble was a small molecule
then you would be the size of North America! Atoms (and molecules, atoms bonded
together by their electrons) are about 10-10 meters. But what do they look like?
Although we’ll never actually see them, the structure of an atom is actually very simple.
Atoms contain protons and neutrons in the central nucleus (The nucleus is not something
separate from the protons and neutrons. It is the protons and neutrons.) Electrons are
found 10,000 times distant from the nucleus. Electrons are so small compared to protons
and neutrons (2000 times smaller) that their mass is ignored. Electrons orbit in “clouds”
called shells (electrons are responsible for electricity & magnetism) at nearly the speed of
light around the nucleus. There are a theoretical infinite number of shells for any atom,
but for practical purposes there are as many shells as there are rows (periods) of the
periodic table. (This is not the whole story on electrons and energy levels, but simplifies
it for beginners.) The elements of the first row of the periodic table can hold up to 2
electrons in an inner shell. The elements of the second row of the periodic table have 2
electrons in the first inner shell and can hold up to 8 electrons in the second shell. The
elements of the third row of the periodic table hold 2 electrons in their inner shell 8
electrons in their second shell and up to 8 electrons in their outer shell. This pattern in
the periodic table is continued although there is an important exception in the way
electrons fill shells from element 19 on. Electrons within the outer shell (the highest
energy electrons) of any atom are known as the valence electrons and are important in
bonding with other atoms to make molecules. All rows (periods) except row 1 have 8
valence electrons. In any neutral atom the number of electrons is equal to the number
of protons. As you move across the periodic table by atomic number the number of
protons and electrons increase by 1 for every square (element). And when you get to the
end of a row (period) you have filled all the valence (outer) electrons for that shell and
any additional atoms are filled in the next shell. In this way electron shells are filled.
1. Look at page 134 in your textbook. How many protons are in the nuclei of the
helium, carbon, beryllium and neon atom? How many shells of electrons are in a
nitrogen atom? How many shells are in an Iron atom?
2. What about the neutrons? The number of neutrons in an atom can vary in a sample of
any element! Atoms with different numbers of neutrons than average for a given element
are called isotopes. Isotopes are important in many chemical and nuclear tests. For
example, isotopes of carbon, uranium and potassium are used to date rocks (billions of
years old!) To find the number of neutrons in an average atom for any element subtract
the atomic number from the rounded atomic mass. How many neutrons are in lead,
potassium, hydrogen, helium, oxygen, uranium, fluorine, and tin?
3. Draw some atoms. Use your periodic table and what you just read (remember the
rule about atomic number = protons, and electrons = number of protons, as a general rule)
Make sure to draw electron shells as you learned above and fill them appropriately as you
learned above! The example of the Bohr Model atom on page 127 is a good model of
how you should draw your atoms. Draw pictures of the following atoms (use the back of
this paper): hydrogen, helium, lithium, carbon, oxygen, neon, phosphorous, argon,
aluminum, potassium, nitrogen, sulfur, chlorine, and sodium.