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Lesson · Grade 9 · Science

Covalent bonds: when atoms share electrons

  • Grade 9
  • Science
  • 1 min 55 s
  • British Columbia
  • BC9-SC-15

One dot and cross diagram builds up step by step. Oxygen's outer shell needs two more electrons, so it shares one pair with each hydrogen to make water, and carbon shares two pairs (a double bond) with each oxygen to make carbon dioxide, so every outer shell ends up full. The video then shows that a molecule can be made of one element (O2), lists the seven diatomic elements, and explains that substances made of small molecules usually melt and boil at low temperatures and do not conduct electricity, before a one-line recap.

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In one sentence

The question this lesson answers

Water is made of two non-metals, oxygen and hydrogen. How do their atoms join?

An oxygen atom has two electrons in its inner shell and six in its outer shell.

Transcript

What the lesson says

Water is made of two non-metals, oxygen and hydrogen. How do their atoms join?

An oxygen atom has two electrons in its inner shell and six in its outer shell.

Only the outer shell takes part in bonding. It is full with eight, so oxygen needs two more.

A hydrogen atom has one electron. Its shell is full with two, so it needs one more.

Each hydrogen shares its electron with oxygen, and oxygen shares one back.

A shared pair of electrons is called a covalent bond.

Now oxygen has eight outer electrons and each hydrogen has two, so every shell is full.

Atoms joined by covalent bonds form a molecule. This is a water molecule, H2O.

In carbon dioxide, carbon has four outer electrons, so it needs four more.

Carbon shares two pairs with each oxygen. Two shared pairs make a double bond.

Now all three atoms have eight outer electrons. This is carbon dioxide, CO2.

A molecule does not need two different elements: in O2, two oxygen atoms share two pairs.

Seven elements are diatomic, made of two-atom molecules: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine.

Covalent bonds are strong, but forces between molecules are weak, so these substances usually melt and boil at low temperatures.

Their molecules have no overall charge, so they usually do not conduct electricity.

Non-metal atoms share pairs of electrons to fill their outer shells, forming molecules.

Curriculum

Where this fits

British Columbia, Grade 9

Science, Chemistry: The arrangement of electrons determines the compounds formed by elementsBC9-SC-15

Previous: Ionic bonds: when atoms give and take electronsNext: Naming Compounds and Writing Their Formulas

The outcomes and strands follow the British Columbia curriculum.

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