Elements & Bonds

• An element is one of the basic real substances everything else is made from — oxygen and iron are both elements, and neither can be broken down into anything simpler and still be itself.
• Every element has its own short chemical symbol, like O for oxygen or Na for sodium.
• A bond is a real force that holds two atoms together. Two or more atoms joined by a bond make a molecule.
• A covalent bond is when atoms share a pair of electrons between them — the water you drink is held together this way.
• An ionic bond is when one atom gives an electron to another atom completely, instead of sharing one — the salt on your table is held together this way.
• A compound is a molecule made of two or more different elements bonded together, like water (hydrogen and oxygen) or table salt (sodium and chlorine).

The one real thing that decides it

• Every atom pulls on the electrons it shares in a bond with a real, measurable strength called electronegativity — how hard that atom pulls shared electrons toward itself.
• The real rule: look at how different the two atoms' pull is. That difference alone is what decides covalent or ionic, not anything about the atoms looking "similar" or "different" in general.

The three real ranges

• Nearly the same pull (the two atoms are the very same element, or close to it): the electrons are shared evenly. A plain covalent bond — the hydrogen gas (H2) in this page's own list is a real example, with no difference in pull at all.
• A real but modest difference in pull: still a covalent bond, but shared unevenly — one atom holds the electrons a little closer. Hydrogen Chloride (HCl) is a real example of this.
• A big difference in pull: the pull is so lopsided the electron does not really get shared at all — it moves from the weaker-pulling atom to the stronger-pulling one. That is an ionic bond. Table Salt (NaCl) is a real example of this.

Why "metal plus nonmetal" is really a shortcut for this

• Metals pull weakly on electrons and let go of them easily. Nonmetals pull strongly and hold onto them.
• So a metal bonded to a nonmetal almost always means a big difference in pull — an ionic bond. A nonmetal bonded to another nonmetal almost always means a small or modest difference — a covalent bond.
• The "metal versus nonmetal" rule above is not a separate rule at all — it is this same real difference in pull, just described by where the two elements sit instead of by a real number.
• This is a real, reliable guide, not an absolute law — a few real elements sit close to the covalent/ionic dividing line and do not fit either description perfectly.