In this chapter, we will learn to :
- Understand the existence of ions and how they are formed
- Distinguish between a cation and an anion
- Name some ions
- Distinguish between monoatomic and polyatomic ions
Mineral water is not pure water. It contains certain chemical species that are labeled on the bottle.Have you ever read what is writted on the label? What are the chemical species that appear on the label of a mineral water bottle?
1. Existence of Ions:
These chemical species that you saw are called ions. Ions are constituents of many substances that we use in our everyday life, such as salt, potassium permanganate, and sodium bicarbonate - they are charge carriers. The word "ion" is, in fact, Greek in origin, and it means "wanderer". Ions, as we will see next, may be positive or negative. Essentially, ions are formed from their corresponding neutral atom either by a gain of electron(s), to form the negative ion, or by a loss of electron(s), to form the positive ions. At this stage, it is important to note that ions are extremely mobile in solutions - this can be easily understood as they are essentially free atoms with either an excess or deficit of electrons not bonded in any way to anything else.
1.2 Characteristics of Ions
A) Ionic charge:
Ions can be of two different opposite charges:
- Positively charged called cations
- Negatively charged called anions
B) Colors of Ions:
Most ions are colorless in aqueous solution ( Na+, K+, Cl-, NO3-, ...), while some ions possess characteristic colors, such as ( Cu2+ blue,
Fe2+ greenish, Cr2O72- yellow-orange, MnO4- violet).
2. Monoatomic Ions:
Generally, monotaomic ions ar atoms which have fulfilled their octets either by gaining or losing electrons. To illustrate, consider the sodium atom which has an atomic number of 11 and the following electron configuration: 1s2 2s2 2p6 3s1 (NE has 3s1). To acquire the electron configuration of the noble gas neon, the sodium atom loses one electron and becomes the ion Na+.
The sodium atom, like atoms of many metallic elements, loses one electron to obey the octet rule. The loss of valence electrons from an atom produces a positively charged ion or cation.
Consider, on the other hand, the chlorine atom that has 17 as an atomic number and the following electron configuration: 1s2 2s2 2p6 3s2 3p5. By the same reasoning as above, the chlorine atom tends to gain one electron to aquire the electron configuration of the noble gas argon. Thus, it becomes the
ion Cl- and carries a negative charge.
The florine atom, like atoms of many non-metallic elements, gains one electron to obey the octet rule. The gain of electrons by an atom produces a negatively charged ion or anion.
To illustrate the above concepts regarding the loss of gain of electrons, look at the following figure (hint: place the mouse over the images):
It is very important to note that there are some elements called the transition metals whose atoms may produce different ions, like iron or copper for example. Copper atoms may lead to two different ions Cu+ and Cu2+. Such atoms do not obey the octet rule.
2.2 Symbol and Nomenclature
An atom is represented by a symbol and a molecule is represented by a formula. A monoatomic ion is represented by the symbol of the atom, followed by a superscript indicating the number of positive or negative charges which it carries: Ag+, Ba2-, Al3+, O2-, F-.
As for naming these ions, the rule is as follows:
made up of
made up of more
than one ion
In this case, the elements tend to lose electrons and form positively charged cations. The name of such ions is simply the name of the element that generated it followed by the word "ion". For example,
Ag+ silver ion Ba2+ barium ion
Al3+ aluminum ion
Transition Metal Elements:
These kind of metals usually form more than one cation (cation because they are essentially metals and so have tendanct to lose valence electrons). to distinguish between them, the number of excess charges written as roman numerals follows the name of the metal that generated the cation followed by the word "ion". For example, Fe2+ iron (II) ion
Fe3+ iron (III) ion
Monoatomic ions generated form non-metallic elements are usually negatively charged anions. These have the name of the non-metallic element from which it is generated, and end with the suffix "ide" instead of the ending letters in the name of the element. It is also followed by the word "ion". For example,
F- fluoride ion
S2- sulfide ion
N3- nitride ion
In this category, there is no specific naming rule except that: When two or more polyatomic ions consist of the same ions, those containing more oxygen atoms end with the suffix "ate" while those with the fewer oxygen atoms end with "ite".
3. Polyatomic Ions:
symbol of hydrogen atom
|symbol of Nitrogen atom||
number of positive charges for the whole ion
|Number of Nitrogen atoms is one(no subscript)||
Number of Hydrogen atoms
Thus, from the figure above, it is obvious that :
- Polyatomic ions are those that are composed of many atoms connected to each other ( by covalent bonds which we will see in the next chapter).
- A polyatomic ion is represented by a formula.
- The charge written as a superscript to the formula is for the entire ion and NOT individual constituent elements.
As for the geometry of the polyatomic ion, it is determined by writing the Lewis structure and applying the VSEPR method. We will learn more about that in the next chapter.