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How do compounds stick together?

When you think of bonding, you may not think of ions. Like most of us, you probably think of bonding between people. Like people, molecules bond — and some bonds are stronger than others. It's hard to break up a mother and baby, or a molecule made up of one oxygen and two hydrogens!

Chemical Bonding

Periodic Table of the Elements.

Ions come together to create a molecule so that electrical charges are balanced; the positive charges balance the negative charges and the molecule has no electrical charge. To balance electrical charge, an atom may share its electron with another atom, give it away, or receive an electron from another atom.

The joining of ions to make molecules is called chemical bonding. There are three main types of chemical bonds that are important in our discussion of minerals and rocks:

  • Ionic bond: Electrons are transferred between atoms. An ion will give one or more electrons to another ion. Table salt, sodium chloride (NaCl), is a common example of an ionic compound. Note that sodium is on the left side of the periodic table and that chlorine is on the right side of the periodic table. In the figure below, an atom of lithium donates an electron to an atom of fluorine to form an ionic compound. The transfer of the electron gives the lithium ion a net charge of +1, and the fluorine ion a net charge of -1. These ions bond because they experience an attractive force due to the difference in sign of their charges.

Lithium (left) and fluorine (right) form an ionic compound called lithium fluoride.

  • Covalent bond : In a covalent bond, an atom shares one or more electrons with another atom.

In the picture of methane (CH4) below, the carbon ion (with a net charge of +4) shares a single electron from each of the the four hydrogens. Covalent bonding is prevalent in organic compounds. In fact, your body is held together by electrons shared by carbons and hydrogens! Covalent bonds are also very strong, meaning it takes a lot of energy to break them apart.

Methane is formed when four hydrogens and one carbon covalently bond.

  • Hydrogen bond: These weak, intermolecular bonds are formed when the positive side of one polar molecule is attracted to the negative side of another polar molecule.

Water is a classic example of a polar molecule because it has a slightly positive side, and a slightly negative side. In fact, this property is why water is so good at dissolving things. The positive side of the molecule is attracted to negative ions and the negative side is attracted to positive ions.

Demonstrating dipole in a water molecule

Water is a polar molecule. Because the oxygen atom has the electrons most of the time, the hydrogen side (blue) of the molecule has a slightly positive charge while the oxygen side (red) has a slightly negative charge.

A video about chemical bonding, from Khan Academy: http://www.khanacademy.org/video/ionic--covalent--and-metallic-bonds.

Water is a covalently bonded, polar molecule. Watch this animation to see how it forms: http://www.youtube.com/watch?v=qmgE0w6E6ZI.

Summary

  • In an ionic bond, an atom gives away one or more electrons to another atom.
  • In a covalent bond, two atoms share one or more electrons.
  • A hydrogen bond is a relatively weak bond between two oppositely charged sides of two or more molecules. Water is a polar molecule.

Practice

Use this resource to answer the questions that follow.

1. What is ionic bonding?

2. How many valence electrons does sodium have?

3. How many valence electrons does chlorine have?

4. What is the charge on a sodium ion?

5. What is covalent bonding?

6. How many valence electrons does oxygen have?

Review

1. How is a covalent bond different from an ionic bond?

2. Why is a hydrogen bond a relatively weak bond?

3. Diagram the polarity of a water molecule.

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