Unit 2 Class Notes and Worksheet:
Name: ___________________________
Why do atoms form Cations and Anions? What does becoming “isoelectronic” with a noble gas mean?
The “alkali metal” family of elements (column 1 elements) are very chemically reactive metals. These atoms
are so reactive because they have an outermost energy level with only 1 electron. Their “valence shell”
electron configuration is: “s1” . The alkali metal atoms lose this 1 electron very easily (more easily than any
other kind of atom loses any of its electrons). After losing the electron, the alkali metal atom is then a
positively-charged ion called a “cation”; and, its charge is 1+.
Each alkali metal cation has 1 less electron than its number of protons; thus it will have the same
number of electrons as one of the “noble gas” atoms (column 18, or group “8A” elements), and it is said to be
“iso-electronic” to the noble gas atom. [“iso”: prefix meaning “same”; “electronic” refers to electrons]. Not
only is the number of electrons now identical, but also, the configuration of the electrons is now identical.
The noble gas family of element are known to have stable, un-reactive atoms, each having a “full” (or
complete) “outer shell” (full “outermost energy level”). Each noble gas atom (other than helium) has an
outermost energy level that is “_s2_p6 ”, where “ _” is the row number of the noble gas. Example: Neon’s
outermost energy level is “2s2 2p6”. The row number is not important; but, knowing that “s2p6” is a full outer
shell is very important! This is because having a full outer shell represents “chemical stability” to an atom.
Atoms will lose or gain electrons to arrive at a full outer shell of electrons. Alkali metal atoms lose 1
electron, forming cations, because having the“s2p6” full outer shell is more stable than having their original
“s1” outer shell configurations.
Are the alkali metal cations identical to noble gas atoms? NO!! The number of protons are different
(which really is a big deal), and therefore the charges are different: Noble gas atoms are neutral (have no
charge); but, alkali metal cations are 1+ charged. Noble gas atoms aren’t attracted to anything (they’re
gases); alkali metal cations are attracted to anything negatively charged. In fact, …. Alkali metals only lose an
electron if some other atom is around to take the electron. And, when an atom takes an electron, it becomes
a negatively charged “anion”. So, a cation and an anion form at the same time; and then are spontaneously
attracted to each other – this is how an ionic compound is formed.
Last idea: When ionic compounds form, even though they are each made from millions of charged
particles, all of the positive charges will exactly cancel out all of the negative charges: Every ionic
compound is neutral! This is the reason why the “drop/swap/simplify” method works – mathematically, you
are creating a neutral compound. Each subscript (the dropped/swapped numbers) no longer represent
charges when they are dropped/swapped. Instead, subscripts represent a count of the number of cations
and the number of anions.
Each subscript belongs to the ion to its left. In the ionic compound, Al2O3, the 2 tells you the number
of aluminum ions. The 3 tells you the number of oxide ions. The 2 tells you that there are two Al3+ ions; the 3
tells you that there are 3 O2- ions. If you add together the number of each ion times its charge, you get:
2 ( +3) + 3 ( -2) = 0. “0” represents neutrality! Ionic compounds’ ions’ charges must always sum together to
give you zero. If you drop/swap the charges correctly, you will always get a sum of zero.
Similar to the alkali metal atoms, the alkaline earth family of metals (column 2 elements) have atoms
whose outer shell electron configuration is “s2”. These atoms will lose these 2 outermost electrons to
become isoelectronic with atoms of the noble gas elements. The alkaline earth family forms cations that are
all 2+.
The halogen family of non-metal atoms all have an outer shell electron configuration of “s2p5”. They
will all gain 1 electron, forming 1- anions, each anion now being isoelectronic with an atom of a noble gas.
The four elements, oxygen, sulfur, selenium, tellurium all have an outer shell with an “s 2p4” electron
configuration. They will all gain 2 electrons, forming 2- anions, as each anion is now isoelectronic with a
noble gas atom. The three elements, nitrogen, phosphorus, arsenic all have an outer “s 2p3” electron
configuration and will thus gain 3 electrons to fill the outer shell, forming 3- anions, which are isoelectronic
with a noble gas.
Follow the example given below (sodium) in order to show your understanding of ion formation.
An atom of Na
An ion of Na
# of
protons
# of electrons Charge
Symbol
11
11
11
10
Na0
Na1+
0
1+
Cation,
anion, or
neutral?
neutral
cation
Iso-electronic
with which noble
gas?
---------------------Ne (because it
has 10 electrons)
An atom of F
An ion of F
An atom of Al
An ion of Al
An atom of Ca
An ion of Ca
An atom of S
An ion of S
An atom of P
An ion of P
Answer the questions below to show your understanding how an ion is isoelectronic with a noble gas atom.
1. What is the electron configuration for an atom of Ne?
2. What is the electron configuration for Na1+?
3. What is the electron configuration for F1-?
4. What is the electron configuration for Al3+?
5. What is the electron configuration for an atom of Ar?
6. What is the electron configuration for Ca2+?
7. What is the electron configuration for S2-?
8. What is the electron configuration for P3-?