Electrolysis Electrolysis is the decomposition of an ionic compound, molten or aqueous, by an electric current. The electrolyte contains mobile ions. Cathode: negative electrode in an electrolytic cell; reduction occurs. Anode: positive electrode in an electrolytic cell; oxidation occurs. Cations move to the cathode and gain electrons. Anions move to the anode and lose electrons. In general, a metal or hydrogen forms at the cathode; a non-metal other than hydrogen forms at the anode. Electrons move through the external circuit; ions move through the electrolyte. ...

Formulae and equations A molecular formula gives the actual number of atoms in one molecule. A formula for an ionic compound gives the simplest whole-number ratio of ions. Formulae can be deduced from models, diagrams, or ion charges. To deduce an ionic formula, balance total positive and negative charge. For example, $\mathrm{Mg^{2+}}$ needs two $\mathrm{Cl^-}$ ions, giving $\mathrm{MgCl_2}$. Two $\mathrm{Al^{3+}}$ ions and three $\mathrm{O^{2-}}$ ions give $\mathrm{Al_2O_3}$. Common elemental formulae include $\mathrm{H_2}$, $\mathrm{N_2}$, $\mathrm{O_2}$, $\mathrm{F_2}$, $\mathrm{Cl_2}$, $\mathrm{Br_2}$, and $\mathrm{I_2}$. The noble gases are monatomic. ...

Elements, compounds and mixtures An element contains only one type of atom. A compound contains atoms of two or more elements chemically bonded in fixed proportions. A mixture contains two or more substances that are not chemically bonded. Its components can be separated by physical methods. Atomic structure Particle Relative charge Relative mass Location Proton $+1$ $1$ Nucleus Neutron $0$ $1$ Nucleus Electron $-1$ $\frac{1}{1840}$ Shells around nucleus The proton number, or atomic number, is the number of protons. The mass number, or nucleon number, is the total number of protons and neutrons. In a neutral atom, number of electrons equals number of protons. ...

Solids, liquids and gases State Arrangement and separation Motion Macroscopic properties Solid Particles closely packed in a regular arrangement Vibrate about fixed positions Fixed shape and volume; almost incompressible Liquid Particles close together in an irregular arrangement Move and slide past one another Fixed volume; takes container shape; almost incompressible Gas Particles far apart in a random arrangement Move rapidly and randomly in all directions No fixed shape or volume; easily compressed; low density These descriptions apply to particles, not necessarily individual atoms. A particle may be an atom, molecule, or ion. ...