Ionic and Covalent Bonding Explained

Why does salt water carry an electric current, while sugar water barely does? From above, these salt ponds beside San Francisco Bay look like an enormous paint box. Water evaporates, dissolved salts become more concentrated, and crystals can form. Now put a salt crystal beside a sugar crystal. Both are solids. Both dissolve in water. But dissolve them, and their invisible structures tell two very different stories. To solve the puzzle, we need to follow the particles.

First: making ions. A neutral atom has equal numbers of positive protons and negative electrons. Change the number of electrons, and you change its charge. A sodium atom has one electron in its outer shell. A chlorine atom has seven. In our dot and cross model, sodium loses one electron and chlorine gains it. Sodium is now a positive ion; chloride is a negative ion. The protons haven't moved. Losing something negative makes the sodium positive. That's the sign trap: losing an electron does not make a negative ion.

Here is the bond: electrostatic attraction between oppositely charged ions. Electron transfer makes the ions; attraction holds them together. But don't draw salt as lots of tiny sodium chloride couples. A salt crystal is a giant lattice: one repeating, three dimensional network of ions. Positive and negative ions attract in every direction. The formula for table salt tells you the ratio: one sodium ion for every chloride ion, not a separate molecule. This large network helps explain why sodium chloride needs eight hundred and one degrees Celsius to melt.

Now return to our electrical puzzle. In solid salt, the ions are locked into the lattice. They cannot travel through it to carry current. Melt the salt, and the ions can move. Dissolve it in water, and water molecules separate the ions and surround them. The ions can move through the solution, so salt water conducts. Notice what carries charge here: moving ions. Not electrons hopping from one dissolved ion to the next. The electrons moving in the wires are a different part of the circuit.

Second: sharing electrons. Sugar uses a different kind of bond, called a covalent bond. Start with something smaller: two hydrogen atoms. Each contributes one electron. Put a dot from one atom and a cross from the other between the two symbols. That's one shared pair. Both positive nuclei attract that pair, holding the atoms together. Each hydrogen now has access to two electrons, filling its first shell. Dots and crosses track where electrons came from. They are not two different kinds of electron. One shared pair makes one single covalent bond.

Let's draw water, step by step. Oxygen has six outer electrons. Each hydrogen has one. Use dots for oxygen's electrons and crosses for hydrogen's. Put one dot and one cross in each overlap between oxygen and hydrogen. That makes two shared pairs, so two single covalent bonds. Leave the other four oxygen electrons as two lone pairs. Count around oxygen: eight electrons, including both shared pairs. Count around each hydrogen: two. A line can replace a shared pair in a simpler drawing. But the lone pairs are still there, even when a drawing leaves them out.

Third: solve the sugar mystery. A sucrose molecule contains twelve carbon atoms, twenty two hydrogen atoms and eleven oxygen atoms, joined by covalent bonds. In a sugar crystal, many separate molecules pack together. When sugar dissolves, water separates whole molecules from one another. It does not tear apart the covalent bonds inside them. Those dissolved molecules are neutral. So sugar does not supply the mobile ions that salt does. Disappearing into water does not mean becoming ions. Same visible result; different particles.

Here's another trap: a covalent bond is not automatically a weak bond. Many substances made of small molecules melt relatively easily because melting overcomes attractions between molecules, not the strong bonds inside each molecule. And covalent substances don't all make small molecules. Diamond is a giant network of covalent bonds. To explain a property, ask two questions: what particles are present, and what holds them together? For electrical conduction, add a third: can charged particles move? The word ionic or covalent alone is only the start of the explanation.

Quick check. Solid sodium chloride does not conduct, but its solution does. Which model fits: fixed ions that become mobile, or neutral molecules that stay neutral? The first model fits. Now our salt ponds make sense: evaporation removes water, and ions can come together again as crystals. No new salt has to be created. Keep these phrases: ionic bonding, electrostatic attraction between oppositely charged ions. Covalent bonding, a shared pair of electrons attracted to both nuclei. Conductivity, charged particles free to move. Salt and sugar may look alike. Follow their particles, and the difference becomes clear.