Download ATOMIC STRUCTURE AND MOLECULAR BONDING

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
and light too!) 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.)
Electron Shell Rules! The elements of the first row of the periodic table have a shell that can
hold up to 2 electrons. The elements of the second row of the periodic table have 2 electrons in
an inner shell and can hold up to 8 electrons in a 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 a third 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 (Can you guess how many valence electrons are in row 1?). 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.
Do the problems below and use the back of the page for your answers if you need more room.
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.