Why do atoms join together?
Most atoms aren’t stable on their own. They join with other atoms by forming chemical bonds. A bond is an attraction that holds atoms together. Bonds explain why water is a liquid, salt forms crystals, and metals bend.
The key: outer electrons
Atoms bond using their valence electrons, the electrons in the outer shell. Atoms are most stable when their outer shell is full, usually with eight electrons (the "octet rule"). Hydrogen is happy with two.
Noble gases like neon and argon already have full outer shells. That’s why they almost never bond.
Other atoms reach a full shell by:
- giving away electrons
- taking electrons
- sharing electrons
Ionic bonds
An ionic bond forms when a metal atom gives electrons to a non-metal atom.
Example: sodium chloride (table salt)
- Sodium (Na) has 1 valence electron. It gives it away and becomes Na⁺.
- Chlorine (Cl) has 7 valence electrons. It takes one and becomes Cl⁻.
- The positive and negative ions attract each other strongly.
Millions of ions stack into a regular pattern called a crystal lattice. That’s why salt grains are tiny cubes.
| Properties of ionic compounds |
|---|
| High melting points (salt melts at 801 °C) |
| Hard but brittle; they shatter when hit |
| Often dissolve in water |
| Conduct electricity when melted or dissolved, but not as solids |
Covalent bonds
A covalent bond forms when two non-metal atoms share electrons. Each shared pair counts toward both atoms’ outer shells.
| Molecule | Bonds |
|---|---|
| Hydrogen, H₂ | One shared pair (single bond) |
| Water, H₂O | Oxygen shares one pair with each hydrogen |
| Oxygen, O₂ | Two shared pairs (double bond) |
| Nitrogen, N₂ | Three shared pairs (triple bond, very strong) |
| Carbon dioxide, CO₂ | Two double bonds |
Atoms joined by covalent bonds form molecules.
| Properties of covalent compounds |
|---|
| Usually low melting and boiling points |
| Often gases or liquids at room temperature |
| Usually do not conduct electricity |
Some covalent substances form giant networks instead of small molecules. Diamond is a network of carbon atoms, each bonded to four others. That’s why it’s the hardest natural material.
Sharing unequally
In some covalent bonds, one atom pulls the shared electrons more strongly. In water, oxygen pulls harder than hydrogen, so oxygen gets a slightly negative charge and the hydrogens slightly positive. This makes water a polar molecule. Polarity explains why water dissolves so many substances and why water droplets stick together.
Metallic bonds
In a metallic bond, metal atoms release their valence electrons into a shared "sea" that flows around all the atoms.
This explains why metals:
- conduct electricity and heat (electrons move freely)
- can be bent and hammered into shapes (atoms slide without breaking the bonds)
- are shiny (free electrons reflect light)
Comparing bond types
| Ionic | Covalent | Metallic | |
|---|---|---|---|
| Atoms involved | Metal + non-metal | Non-metal + non-metal | Metal + metal |
| Electrons | Transferred | Shared | Pooled in a sea |
| Example | Salt (NaCl) | Water (H₂O) | Copper (Cu) |
| Melting point | High | Usually low | Usually high |
| Conducts electricity? | When melted or dissolved | Usually not | Yes |
Bonds and energy
Breaking bonds takes energy. Forming bonds releases energy. In a chemical reaction, old bonds break and new ones form. When burning wood releases heat, it’s because the new bonds formed (in carbon dioxide and water) release more energy than was needed to break the old ones.
Common mistakes
- Thinking ionic compounds are made of molecules. They form lattices of ions.
- Thinking salt conducts electricity as a solid. Ions must be free to move.
- Thinking noble gases bond easily.
Try it yourself
- Predict the bond type: magnesium oxide (MgO), methane (CH₄), aluminum foil.
- Use paper circles for atoms and pom-poms for electrons to model how sodium gives an electron to chlorine.
- Test whether salt water and sugar water conduct electricity using a simple circuit with a bulb (with an adult). Explain the result.
(Answers: ionic, covalent, metallic; salt water conducts because it contains ions, sugar water doesn’t)
Quick recap
- Atoms bond to fill their outer shells.
- Ionic bonds transfer electrons between metals and non-metals.
- Covalent bonds share electrons between non-metals, forming molecules.
- Metallic bonds pool electrons in a sea.
- The type of bond decides a substance’s properties.