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, which are made
from atoms) are about 10-10 meters. But what do they look like? 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 the 92 different atoms (elements) there are
as many shells as there are rows of the periodic table. The elements of the first row of the
periodic table can hold up to 2 electrons in a 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 with the exception of the transition metals
(elements 57-70 and 89-102). The electrons of the outer shell of any atom are known as
the valence electrons and are important in bonding with other atoms to make molecules.
In any 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. 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 shells are filled in.
Look at page 503 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 a Iron atom? 2; 6; 4; 10; 3; 4
What about the neutrons? The number of neutrons in an atom can vary in a collection 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 (hundreds of
millions and billions of years old!) To find the number of neutrons in an average atom
for any element subtract the rounded atomic mass from the atomic number. How many
neutrons are in lead, potassium, hydrogen, helium, oxygen, uranium, fluorine, and tin?
125; 20; 0; 2; 8; 146; 10; 69
Draw some atoms. Use your periodic table and what you just read (remember the rule
about atomic number = protons, and electrons = number of protons) Make sure to draw
electron shells as you learned above and fill them appropriately as you learned above!
The example of nitrogen on page 503 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, vanadium, chromium, aluminum, potassium,
nitrogen, sulfur, chlorine, and sodium. See examples below