2.6 Molecular and Ionic Compounds

You might have noticed something about the atomic mass of some elements while studying the periodic table.

Most of the elements with atomic numbers 84 and higher have their atomic mass given in square brackets.

You will learn more about radioactivity in the nuclear chemistry chapter, but this is done for elements that are completely unstable.
An average atomic weight can't be determined for these elements because their radioisotopes can vary greatly depending on the source and nature of the element.
The number in brackets is the atomic mass number of the most stable element.

The nucleus of each atom remains the same in ordinary chemical reactions.

Transferred from other atoms, lost by transfer to other atoms, or shared with other atoms are some of the ways in which electrons can be added to atoms.

The chemistry of the elements depends on the transfer and sharing of electrons among atoms.
During the formation of some compounds, atoms gain or lose electrons.

The periodic table can be used to predict whether an atom will form an anion or a cation.

Many main-group metals have the same number of electrons as the preceding noble gas.
An atom of an alkali metal loses one electron and forms a cation with a 1+ charge; an alkaline earth metal loses two electrons and forms a cation with a 2+ charge.
A neutral calcium atom has 20 protons and 20 electrons.
The preceding noble gas, argon, has the same number of electrons as it does.
The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca2+ is called a calcium ion.

When atoms of nonmetal elements form ion, they usually have the same number of electrons as the next noble gas in the periodic table.

The neutral bromine atom has 35 protons and 35 electrons and can gain one electron to give it 36 electrons.
It has the same number of electrons as the next noble gas.

The periodic table can be used to predict likely ion formation and charge.

The main-group elements form cations with a charge equal to the group number when moving from the far left to the right on the periodic table.
Group 1 elements form 1+ ion; group 2 elements form 2+ ion.
A negative charge is created when elements are moved from the far right to the left on the periodic table.
Group 17 elements form 1- ion; group 16 elements form 2- ion; and so on.
When moving toward the center of the periodic table, the trend's predictive value decreases.
Transition metals and some other metals have variable charges that are not predictable.
Iron and copper can form ion with a 1+ or 2+ charge.

Some elements have a pattern of ionic charge.

13 protons and 10 electrons are contained in an ion found in some compounds.

When an atom forms an ion, the atomic number of the element must be 13.

The periodic table can be used to identify the element as Al.

The Al atom has more positive charges than it has electrons.

The aluminum cation is Al3+.

The ion has 34 protons and 36 electrons.

Nitrogen and magnesium form an ionic compound.

Predict which forms an anion and which forms a cation.
Write the symbol for each ion.

The periodic table tells us that magnesium is a metal.

Positive cations are formed by metals.
A magnesium atom has the same number of electrons as a neon atom.
A magnesium atom will form a cation with two less electrons than the protons.
The symbol for the ion is magnesium.

Nitrogen is a nonmetal in the periodic table.

Nonmetals form anion.
To have the same number of electrons as an atom of neon, a nitrogen atom must gain three electrons.
A nitrogen atom with three more electrons than the protons will form an anion.
The symbol for the ion is N3.

An ionic compound is formed when aluminum and carbon react.

Predict which forms an anion and which forms a cation.
Write the symbol for each ion.

An aluminum ion will be formed with a charge of 3+.

A charge of 4-: C4 will form an anion with carbon.

A group of bonds of atoms with an overall charge act as ion units.

Some of the more important polyatomic ion are listed in Table 2.5.

You should memorize the names, formulas, and charges of the most common polyatomic ion at this point in your study of chemistry.

chlorite and hypochlorite have the same chemical composition.

This will be covered in the next module.

The attractive forces that hold atoms together within a compound are the basis of chemical bonding.

The attractive forces experienced between objects of opposite electrical charge are called ion bonds.
The attractive forces between the positively charged atoms and one or more pairs of electrons are called covalent bonds.
On the basis of the bonds present in them, compounds are classified as ionic or molecular.

A transfer of electrons occurs when an element composed of atoms that lose electrons reacts with an element composed of atoms that gain electrons.

The compound formed by this transfer is stable because of the ion bonds between the two charges present in the compound.

Each calcium atom can give up two electrons and transfer one to each of two chlorine atoms to form CaCl2, which is composed of Ca2+ and Cl- ion to two Cl- ion.

The periodic table can help us identify compounds that are ionic, when a metal is combined with one or more nonmetals.

The guideline works well for predicting ionic compound formation for most of the compounds encountered in an introductory chemistry course.
It is not always true, for example, aluminum chloride is not ionic.

Ionic compounds can be seen because of their properties.

At high temperatures, ion compounds tend to boil at even higher temperatures.
In solid form, an ionic compound can't be charged because it's not able to flow.
It can conduct electricity when molten because its ion are free to move through the liquid.

You can see a mixture of salts melt and conduct electricity in this video.

The total number of positive charges of the cations is the same as the total number of negative charges of the anions.

Even though there are positive and negative ion compounds, they are all neutral.
The formula of an ionic compound can be written using this observation.
The numbers of positive and negative charges are equal in the formula of an ionic compound.

The OpenStax book is available for free at http://cnx.org/content/col11760/1.9 aluminum cations, Al3+, and oxygen anions, O2.

Blue sapphires characteristic color is due to trace amounts of iron and titanium.

The ionic compound must have the same number of positive and negative charges.

Six positive and six negative charges would be given by two aluminum ion and three oxide ion each with a charge of 2-.
The formula would be Al2O3.

Predict the formula of the ionic compound formed between Na+ and S2.

There are many ionic compounds that contain both the anion and the cation.

As with simple ionic compounds, these compounds must also be neutral, so their formulas can be predicted.
A group of atoms are indicated by parentheses in a formula.
One of the minerals in our bones is Ca3(PO4)2.

The PO4 groups have an overall charge of 3- and are composed of one phosphorus atom and four oxygen atoms.

The compound has a total count of three Ca, two P, and eight O atoms.

The positive and negative charges have to balance.

We need two negative charges to balance the calcium ion.

We put the formula for the dihydrogen phosphate ion in parentheses and added a subscript 2.

The formula is Ca.

These formulas are only used for compounds with only monatomic ion and for compounds with polyatomic ion.

The formulas for some ionic compounds are not empirical.

The formula is written as Na2C2O4.

The empirical formula, NaCO2, is not the smallest possible whole number, as each subscripts can be divided by 2 to yield it.

There are many compounds that do not contain ion.

Covalent bonding is an important concept in chemistry and will be covered in a later chapter of this text.
We can identify compounds based on their physical properties.
Many important exceptions exist under normal conditions.

ionic compounds are formed when a metal and a nonmetal combine, while covalent compounds are formed by a combination of nonmetals.

The periodic table can help us recognize many of the compounds that are covalent.
While we can use the positions of a compound's elements in the periodic table to predict whether it is ionic or covalent at this point in our study of chemistry, this is a very simplistic approach that does not account for a number of interesting exceptions.
There are shades of gray between ionic and molecular compounds.