Single Covalent Bond: Definition & Examples

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  • 0:00 When Atoms Share Electrons
  • 0:50 What's the Motivation?
  • 1:35 Ionic vs. Covalent
  • 3:26 Single Covalent Bonds
  • 4:37 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
Single covalent bonds hold much of the world together. When atoms meet in just the right way with just the right needs, they will form a single covalent bond, an arrangement that holds the two atoms together and gives them lower energy than they would have if apart.

When Atoms Share Electrons

A chemical bond is an attraction between two atoms and a force that holds them together. There is an attractive force between positively and negatively charged particles that is called the electrostatic force. The positive protons in the nucleus of one atom are attracted to the negative electrons of another atom. A bond forms when the two atoms attached to each other is a lower energy state than the two atoms being apart.

When two atoms share their electrons in order to create a bond, we call it a covalent bond. The other possibility is a little greedy. An ionic bond forms when one atom completely takes an electron away from another. This turns the atoms into ions, one positive and one negative. The two oppositely charged ions are attracted to each other and stick close by to form an ionic bond.

What's the Motivation?

A good general rule for why atoms come together to form covalent bonds is called the octet rule. Atoms are most stable (and in a lower energy state) when they have eight valence electrons, meaning eight electrons in their outer energy level. The noble gases all have full outer energy levels and are very stable. This explains why they rarely react with other atoms. The rest of the elements on the periodic table react and form bonds to achieve this ideal set by the noble gases. This rule dictates which atoms react with which and how they do it. For example, sodium has just one lonely electron in its outer energy level, while chlorine has seven. Together they make the octet and the perfect pair.

Ionic vs. Covalent

The example of sodium and chlorine coming together is a clear illustration of the octet rule, but these two elements form an ionic bond not a covalent one. Sodium gives up its lonely outer electron to chlorine. Sodium becomes an ion with a +1 charge and chlorine becomes an ion with a -1 charge. They stick together in what we call an ionic bond.

Hydrogen is similar to sodium in that it has just one outer electron. It also pairs up well with chlorine, but when it does it forms a single covalent bond. The outer energy level of the hydrogen with its one electron overlaps with the outer energy level of chlorine with its seven electrons. Together, by sharing their outer electrons, hydrogen and chlorine each achieve the octet of eight outer electrons. They each share one unpaired electron to create a matched pair and the full octet.

Whether two elements will form an ionic or a covalent bond depends on the electronegativity of each. This is a measure of how strongly an atom pulls on electrons from another atom. Fluorine is the element on the periodic table with the highest electronegativity. Francium has the lowest. When atoms with very different electronegativity values come together, they will form an ionic bond because one atom is pulling strongly on its neighbor's electrons, as chlorine does to sodium. When two atoms with similar electronegativity come together, they are more evenly matched and will share.

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