Oxygen in the air is not a single atom, and salt is not made of molecules at all. In this course we see how atoms join to make molecules and ions, and we learn to write the formula of a compound from the valency of its parts.
3 lessons, about 1 h 20 min3 simulations10 problems6 practice questionsPython in every lesson
Created by StemPy LabA molecule is a group of two or more atoms joined together that acts as one particle. A molecule of an element has atoms of one kind, such as H2, O2, O3, P4 and S8. A molecule of a compound has atoms of different kinds in a fixed ratio, such as H2O, CO2, NH3 and CH4. Noble gases are monatomic. The number of atoms in a molecule is its atomicity. In a formula, a small number after a symbol gives the atoms in one molecule, and a number before the formula gives the number of molecules.
An ion is an atom or a group of atoms that carries an electric charge, because it has lost or gained electrons. A cation is a positive ion, made when an atom loses electrons, and an anion is a negative ion, made when an atom gains electrons. Compounds of metals with non-metals, such as sodium chloride, are made of ions held together by the attraction between opposite charges, in a regular pattern with no separate molecules. They are neutral overall. Polyatomic ions are groups of atoms with one charge, such as hydroxide, nitrate, ammonium, carbonate, sulphate and phosphate.
The valency of an element is its combining capacity. We write the formula of a compound by writing the symbols with their valencies, crossing the valencies over, and simplifying.
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