ChemistryHow Atoms Bond Into Compounds
Atlas the guide kneels at a glowing lab bench, holding two oversized model atoms whose outer electrons spark and click together like puzzle pieces under safety goggles.
- Explain that atoms bond by sharing or transferring outer electrons.
- Distinguish a covalent bond (sharing) from an ionic bond (transferring).
- Predict that a compound's properties differ from the elements that formed it.
Key terms
- Covalent bond
- A bond formed when two atoms share one or more pairs of electrons.
- Ionic bond
- A bond formed when one atom transfers an electron to another, creating attracting charges.
- Compound
- A substance made of two or more different elements bonded together.
- Outer shell
- The outermost region of electrons that atoms try to fill by bonding.
- Ion
- An atom that has gained or lost electrons and now carries an electric charge.
Two Ways Atoms Bond
Atoms bond because they are most stable when their outer electron shell is comfortably filled. They reach that comfort in two ways. In a covalent bond, two atoms share one or more pairs of electrons, holding together as they pool their electrons; water forms this way when hydrogen and oxygen share. In an ionic bond, one atom transfers an electron to another, so one becomes positive and one becomes negative, and the opposite charges attract like magnets; sodium and chlorine bond this way to form table salt.
Compounds Have New Properties
The most striking idea in bonding is that a compound behaves nothing like the elements that built it. Sodium is a soft, reactive metal that reacts violently with water, and chlorine is a poisonous green gas. Yet when sodium transfers an electron to chlorine and they bond into sodium chloride, the result is harmless table salt that you eat every day. Bonding rebuilds properties from scratch, so you cannot judge a compound by the elements inside it.
Worked examples
Decide whether the bond in water is covalent or ionic.
- Check whether electrons are shared or transferred.
- In water, hydrogen and oxygen share pairs of electrons rather than transferring them.
- Shared electron pairs define a covalent bond.
Answer: Water has covalent bonds, because hydrogen and oxygen share electrons.
Explain how sodium and chlorine bond to form salt.
- Sodium has one outer electron it tends to give away, and chlorine needs one to fill its shell.
- Sodium transfers its electron to chlorine, so sodium becomes positive and chlorine becomes negative.
- The opposite charges attract, holding the two together as an ionic bond.
Answer: Sodium transfers an electron to chlorine, forming an ionic bond that makes table salt.
Activity
Sort each example into 'electrons shared' or 'electrons transferred' to show the bond type.
Practice
Decide whether magnesium oxide forms by sharing or transferring electrons and name the bond.
Explain why table salt is safe to eat even though chlorine gas is poisonous.
Common mistakes to avoid
- A compound keeps the properties of its elements.Bonding rearranges electrons, so the compound has entirely new properties unlike its elements.
- Covalent and ionic bonds are the same thing.Covalent bonds share electrons while ionic bonds transfer them, creating attracting charges instead.
Check your understanding
When two atoms form a covalent bond, what happens to the electrons involved?
Sodium is a reactive metal and chlorine is a poisonous gas. What is true of the compound table salt they form?
Which statement describes an ionic bond?
A classmate says a compound always keeps the properties of the elements inside it. Why is this wrong?
Recap
Atoms bond to fill their outer electron shells, either by sharing electrons in a covalent bond or transferring them in an ionic bond. The resulting compound gains entirely new properties, which is why dangerous sodium and chlorine combine into safe, edible table salt.
Reflect
Why can't you predict a compound's safety from the elements inside it?