Lesson 2.4 Ions
Suggested Reading:
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Zuhdahl Chapter 2 Sections 2.6 & 2.7
Essential Question:
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What are ionic and molecular compounds and how are they formed?
Learning Objectives:
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Describe how ions are formed as a result of electron transfer.
Deduce which ions will be formed when elements in groups I, II & III
lose electrons.
State that transition elements can form more than one ion and
memorize the most common transition metal ions.
Memorize the formulas and names of common polyatomic ions.
Describe the ionic bond as the electrostatic attraction between
oppositely charged electrons.
State the names of compounds using oxidation numbers.
Ion Basics:
Recall that an ion is an electrically charged particle obtained from an atom or
chemically bonded group of atoms (polyatomic) by adding or removing
electrons. A cation is a positively charged ion and an anion is a negatively
charged ion.
An ionic compound is a compound composed of cations and anions. Ionic
compounds are not molecules. A molecule is a group of atoms that are
chemically bonded together by the sharing of electrons (covalent bonds).
Ionic compounds are held together by the electrostatic attraction between
oppositely charged ions.
Watch the video on ion formation.
The video mentions effective nuclear charge, which is an important concept
that plays a role in some of the period trends we will be reviewing later in this
course. Lets make sure you understand this concept. We know that electrons
are attracted to the positive charge of the nucleus. However, not all electrons
in an atom feel this attraction in the same way. In an atom with multiple shells
of electrons the outmost electrons feel less charge then the inner electrons.
Thus, the effective nuclear charge is the charge felt by outer electrons after
you take into account the shielding effect of inner electrons. This video on
effective nuclear charge will illustrate this for you.
Effective Nuclear Charge
Why Ions Form
Atoms will either lose or gain electrons in order to achieve a noble gas
configuration. Recall that noble gasses (Group VIII: He, Ne, Ar, Kr, Xe &
Rn) have full valence electron shells which makes them very stable. Thus,
atoms will either gain or lose electrons in order to achieve a stable state.
This make it easy to predict what type of ion an atom will form in order to
achieve a noble gas electron configuration.
Types of Ions Formed:
Groups that Lose Electrons:
When Group 1 electron lose an electron they adopt the noble gas
configuration of the noble gas in the previous period. For example, when Na+
has the same electron configuration as Ne with a full n=2 valence. Hydrogen
is an exception. It only has one electron so when it loses its single electron a
proton is formed.
Similar to Group I, Group II elements must lose 2 electrons in order to
achieve the noble gas configuration of the noble gas in the previous period.
Group 13 is also called Group 3A on some periodic tables. Group 13
elements must lose three valence electrons in order to adopt the noble gas
configuration of the noble gas in the previous period.
Group 14 (4A) Can Lose or Gain
Electrons:
Groups that Gain Electrons:
For Group 15 elements, it is easier to gain rather than lose electrons in order
to achieve a noble gas configuration. Notice that these atoms adopt the
configuration of the noble gas at the end of their period.
Similar to Group 15 (5A), Group 16 (6A) elements will gain to electrons to
achieve a noble gas configuration.
Get the picture?
For the transition metals, you must recognize that these elements sometimes
form more than one ion. Additionally, you need to memorize the most
common transition metal ions. When naming compounds involving
transition metals that form more than one ion, you must use a roman
numeral to indicate the oxidation state. Students often forget to do this.
This is what the learning objective "State the names of compounds
using oxidation numbers"
Play the monatomic ion concentration game!
Polyatomic Ions:
A list of the common polyatomic ions is given in your chapter 2 study guide.
You must memorize this list. This list was provided in the summer work
packet.
Play the polyatomic ion concentration game!