Prescriptive Activities: Facet Cluster 1.6: PF#64
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Prescriptive #64:
64 The student doesn't know how many electrons are in a full shell (octet) for most atoms
versus how many electrons are in a full shell for H, He, and Li1+, or Be2+ limiting their ability
to predict bonding.
Materials:
Student handout
Background:
You have learned that electrons exist in different energy levels around the nucleus and that
atoms that have a full outermost energy level are particularly stable. While it is true that
most atoms require a full “octet” of 8 electrons in their outermost energy level to be stable, a
few atoms are “full” with less than this. In particular, the four lightest elements, hydrogen
(H), helium (He), lithium (Li) and beryllium (Be) tend to exist with only two electrons. Two
electrons fills the first energy level .Thus, whereas we often refer to the “octet rule” for
determining bonding, in the case of these four elements we can refer to the “duet rule”
which simply means they are stable with two outermost electrons. For hydrogen, this means
sharing one electron with another atom. This explains why hydrogen only forms covalent
bonds with a single atom at a time, such as in HF or HCl.
A helium atom consists of 2 protons, 2 neutrons and 2
electrons (illustrated at right). Since it already contains 2
electrons in its outermost shell it is stable and un-reactive
and exists as individual atoms.
Protons and neutrons are shown in red and blue and
electrons are represented by the small grey circles which
exist within the “electron cloud”.
Note: at this scale the electrons would be much smaller and
about ½ mile away from the nucleus.
Helium atom
Prescriptive Activities: Facet Cluster 1.6: PF#64
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When lithium reacts it tends to lose one electron,
to form a Li+ ion, with only two electrons in its
outermost energy level.
A beryllium atom contains 4 protons and 4
electrons. Two of these electrons fill the first
energy level and the other two are in the 2nd
energy level. When beryllium reacts with another
element do you think that it would be “easier” for
beryllium to gain six electrons to have eight in its
outermost shell or to lose its two outer electrons,
thereby having a full shell containing only two
electrons?
Lithium atom
Beryllium does in fact lose two electrons when
reacting with nonmetals and becomes stable with
the remaining two electrons in its outermost shell.
Questions:
1. How many electrons can exist within the first energy level of an atom? ______
2. How many electrons can exist within the second energy level of an atom? ______
3. How many valence electrons does lithium (Li) have? ______
4. When lithium metal reacts with a nonmetal, does lithium obtain a full outer shell, by
gaining or losing electrons?
5. How many electrons are gained or lost by lithium?
6. How many electrons are now in the outermost energy level of lithium?
7. Draw a representation showing hydrogen sharing electrons with carbon (C) in
methane (CH4). Hydrogen has 1 electron and carbon has 6 (2 in its inner energy level
and 4 in its valence energy level).
8. Why can hydrogen form only one bond?
Teacher Notes:
Students must already understand about energy levels of electrons, and that we cannot
define exactly where electrons exist, but only a region in which they exist most of the time
(usually defined as 90% of the time).
Prescriptive Activities: Facet Cluster 1.6: PF#64
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Beryllium does in fact lose two electrons when reacting with nonmetals and becomes stable
with the remaining two electrons in its outermost shell.
When reacting with nonmetals, the nonmetal does in fact remove two electrons from
beryllium, which becomes stable with the remaining two electrons in its outermost shell.