Learn about covalent bonds and their two types: nonpolar covalent bonds and polar covalent bonds. Discover how to predict the type of bond that will form based on the periodic table. Learn what ionic character means and how to determine it.
Do you like to play basketball? Do you like to drive in a car that has tires? You can thank Mr. Goodyear. Naturally, rubber is sticky when warm and brittle when cold. It doesn't make good balls or good tires. In 1839, Charles Goodyear accidentally dropped a mixture of sulfur and natural rubber on a hot stove. The mixture heated up and became tough and elastic, forming vulcanized rubber. Why did it change properties? It changed because of the covalent bonds that formed between the rubber and sulfur.
Covalent bonds transform sulfur and natural rubber into vulcanized rubber when heated
A covalent bond is formed when atoms share one or more pairs of electrons. This is opposed to an ionic bond, where electrons are actually transferred from one atom to another. You can remember the difference by thinking that the 'C' in 'covalent' is kind of like an 'S' in 'share.' Covalent bonds share electrons. Ionic bonds transfer electrons. The 'I' in 'ionic' is like the 'T' in 'transfer.'
Just to review for a quick moment, electrons are negatively charged, so if an atom has more electrons than protons, it's going to have a negative charge. Electronegativity is the ability of an atom to attract shared pairs of electrons to it. So, the more electronegative the atom is, the more it will attract electrons.
In general, metals, which mostly reside on the left side of the periodic table, are less electronegative than the non-metals, so non-metals would be more likely to attract electrons. The size of an atom also contributes to its electronegativity. In smaller atoms, the electrons are closer to their nucleus, which has its own strong attraction to its electrons and to the electrons of the other atom, so they have greater electronegativity.
Smaller atoms have greater electronegativity because their electrons are closer to the nucleus
So, to summarize this part, the smaller, non-metal atoms have greater electronegativity than the bigger metal atoms. The upper right on the periodic table is more electronegative than the lower left.
Polar and Nonpolar Covalent Bonds
When two atoms that are sharing electrons attract the electrons with the same force, a nonpolar covalent bond is formed. A nonpolar covalent bond is a covalent bond in which the shared electrons are shared equally. A polar covalent bond is a covalent bond in which the shared electrons are more attracted to one of the atoms than the other, thereby forming a polar molecule. The shared electrons are more likely to be near the atom whose electronegativity is higher.
If the difference in electronegativity is too great, the atom in a molecule won't share the electrons. Instead, one atom will take them from the other. When this happens, the bond won't be a covalent bond, but rather an ionic bond because the electrons were transferred, or taken from one.
Just remember that electronegativity is only a general guide for what kind of bond is formed, and like all rules, there are exceptions.
Remember that a covalent bond is when electrons are shared between two atoms. When two different elements are covalently bonded, the electrons aren't shared equally. One of the elements will attract the electron more than the other. This makes it behave a bit like an ionic bond (which is when electrons are transferred between elements).
Covalent bonds behave similarly to ionic bonds; one element attracts electrons more than the other
Ionic character refers to the percentage of difference between the electronegativity of two covalently bonded atoms. Calculating the ionic character isn't something we're going to get into here, but just know that a bond between two atoms of different electronegativity typically results in a high ionic character, while bonds between atoms of similar electronegativity typically results in a low ionic character.
The greater the percentage of the ionic character, the more one element has the electrons than the other. This will give one of the elements a partial negative charge from having the electrically charged electron more often and the other element a partial positive charge from not having the electron. So, the bigger the difference between the electronegativity of two atoms, the more likely it is to be ionic. If the electronegativity is the same between elements, then the bond is likely covalent. To find out the actual electronegativity numbers of atoms, there are several charts available in textbooks and on the web.
When atoms come together to form molecules and need to fill their outer electron shells, they can either transfer electrons from one atom in the molecule to the other, forming an ionic bond, or they can share electrons between the atoms, forming a covalent bond. The electronegativity of an atom determines if the covalent bond is polar or nonpolar. The atom with greater electronegativity will have more attraction to the electrons and pull them toward it, making that part of the molecule slightly negative. In general, the smaller, non-metal atoms will have a higher electronegativity than the bigger, metal atoms.
After watching this lesson, you should be able to:
- Differentiate between ionic and covalent bonds
- Define the two types of covalent bonds
- Explain electronegativity and ionic character