The vocabulary of this chapter tells one story: atoms bond to reach a stable octet or duplet, either by transferring electrons to make ions held in a giant ionic lattice or by sharing electrons to make molecules, and the strength of these bonds explains every physical property that follows.
The terms in Chemical Bond are best learned as one connected story rather than a list, because every one of them flows from a single question: why do atoms bond at all? Once you see that atoms bond to reach the same stable electron arrangement that a noble gas already has, the whole chapter turns from a set of separate definitions into a chain of cause and effect. That chain is also where the marks are, because in this chapter the precise wording of a definition is frequently the mark itself.
Why atoms bond. Everything begins with stability. The octet rule states that atoms lose, gain or share electrons to reach a stable outer shell of eight, the arrangement of a noble gas. Small atoms are the exception: hydrogen and helium reach a duplet of two electrons instead. Reaching this stable electron arrangement is the whole reason a bond forms, so a good answer to almost any “why” question in this chapter comes back to the octet or the duplet. The first pair students confuse is exactly this one, octet versus duplet, and remembering that hydrogen needs only two electrons prevents a very common diagram error.
Ionic bonding. The first way to reach stability is to give electrons away or take them. When a metal meets a non-metal, electron transfer takes place: the metal loses its outer electrons to become a positive cation, and the non-metal gains them to become a negative anion. Opposite charges attract, and this electrostatic force of attraction between the ions is the ionic bond itself. Because the attraction acts in every direction, the ions build up into a giant ionic lattice, a huge, orderly, three-dimensional structure. Here students confuse cation with anion; a useful anchor is that a cation is positive and moves to the cathode, while an anion is negative and moves to the anode.
Covalent bonding. The second way to reach stability is to share. Two non-metal atoms form a covalent bond by sharing one or more pairs of electrons. Each shared pair is a bonding pair, and the leftover outer electrons that are not shared stay on one atom as a lone pair, the second pair students confuse, since only the bonding pair is actually a bond. Sharing can involve one, two or three pairs, giving single, double and triple bonds. Because the shared electrons hold only the bonded atoms together, covalent substances usually form a simple molecular structure: small separate molecules with strong bonds inside them but only weak forces between them. How evenly the electrons are shared depends on electronegativity, the pull each atom exerts on the shared pair.
Dative and hydrogen bonds. Two important extensions build on the covalent idea. A dative (coordinate) bond is a covalent bond in which both shared electrons come from the same atom, which is how ions such as NH4+ and H3O+ form. A hydrogen bond is a weak attraction between molecules, formed when a hydrogen bonded to fluorine, oxygen or nitrogen is pulled towards a lone pair on a neighbour. It is the strongest of the intermolecular forces, the weak attractions between whole molecules that must not be confused with the strong covalent bonds inside them.
Diagrams and properties. All of this is drawn with a dot-and-cross diagram, which shows the outer electrons as dots and crosses so you can see what was transferred or shared. The type of bonding then explains the properties. A giant ionic lattice gives high melting points, brittleness and conductivity only when molten or dissolved; a simple molecular structure gives low melting points and no conductivity. Solubility follows the same logic: because water is a polar molecule, it dissolves ionic compounds well but not non-polar covalent ones.
These terms build directly on the atom and the periodic table you met earlier, and they lead into every later chapter that involves compounds and reactions. Our teachers work through this vocabulary in its connected order in online one-to-one lessons, so that each definition reinforces the last and the exact wording that SPM Chemistry rewards becomes second nature under exam pressure.
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