How atoms bond: sharing and transferring electrons
Chemistry · Key Stage 3 · The School
Ionic bonding: giving an electron away
A metal atom often gives away its outer electrons completely to a non-metal atom. The metal atom becomes a positive ION, having lost a negative electron. The non-metal atom becomes a negative ion, having gained one. Opposite charges attract strongly, and that attraction is the IONIC BOND. Sodium chloride forms exactly this way: sodium gives an electron to chlorine.
Covalent bonding: sharing instead
Two non-metal atoms often share a pair of electrons instead of one giving them away. Both atoms count the shared pair as part of their own outer shell. This sharing is a COVALENT BOND. Water forms this way: each hydrogen atom shares a pair of electrons with the oxygen atom.
Why the properties differ
Ionic compounds form a rigid lattice of alternating charges, giving high melting points and the ability to conduct electricity once melted or dissolved. Covalent compounds form separate molecules held together only loosely, giving low melting points and no electrical conductivity at all. The bonding type decides the properties, long before you ever test the substance.
Sodium chloride, common salt, is IONIC. It melts at 801°C, and molten or dissolved salt conducts electricity, because the charged ions are free to move and carry current. Water is COVALENT. It melts at just 0°C, and pure water barely conducts electricity at all, because its molecules carry no overall charge to move. Same rough idea — atoms joined together — but two different bonding types, and two dramatically different sets of properties as a direct result.