An atom is the smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. As such, the atom is the basic building block of chemistry.

Atomic number is the number of protons (positively charged particles) in an atom's nucleus. All the atoms of an element have the same number of protons. Some forms of an element have a different number of neutrons than do other forms of that element.

Isotopes are atoms of the same element having different mass number. Isotopes have different number of neutrons in their nucleus.

Atomic weight of an element is its weight relative to 1/12 th of that of the isotope of carbon whose atomic mass number is 12. This carbon isotope has been assigned an atomic weight of exactly 12 atomic mass units (a.m.u.) .

Ion is an atom or a group of atoms that has an electric charge. Atoms and molecules become charged if they gain or lose electrons. Each atom has a cloud of negatively charged electrons around a small, heavy nucleus. The nucleus contains positively charged protons. If the number of electrons around the nucleus equals the number of protons inside the nucleus, the atom is neutral. The process of removing electrons from atoms or molecules to produce positive ions is called ionization. The electrons removed may then join other atoms or groups of atoms, causing them to become negative ions. The amount of electric charge an ion has is determined by the number of electrons gained or lost by the atom or molecule.

Many common substances contain ions. For example, table salt consists of equal numbers of positively charged sodium ions and negatively charged chloride ions. In forming table salt from the elements sodium and chlorine, each sodium atom loses an electron and becomes a positive sodium ion. Likewise, each chlorine atom gains an electron and becomes a negative chloride ion. Seawater contains many kinds of ions. The most common ones, in order of their amount, are chloride, sodium, sulphate, and magnesium. The earth's atmosphere also contains ions. They are concentrated in a layer called the ionosphere

Ions in solids usually fit together in a regular, repeating, three-dimensional arrangement. Such a substance is called an ionic crystal. For example, sodium ions alternate with chloride ions in a crystal of table salt. Ions in an ionic crystal are held together by electrostatic attraction, the attraction between opposite charges.

Ions in liquids can migrate throughout the liquid. In a solution, each ion attracts one or more molecules of the solvent (dissolving liquid). Ionic crystals, such as sodium chloride, usually dissolve only in solvents that contain polar molecules. Polar molecules have a positive end and a negative end. Each ion on the surface of the crystal attracts the oppositely charged end of polar molecules. This attraction weakens the attraction between ions in the crystal. The ions then break away from the crystal and enter the solution, combined with polar molecules. Ions combined with polar molecules of the solvent are said to be solvated.

Ions in gases are too far apart at normal pressures to attract each other strongly. As a result, single ions in a gas may drift for a long time before they combine with other ions. A mixture consisting of ionized gas and electrons is called a plasma.

Differences between Ions and Atoms

Sodium Atom

Sodium Ion

Many particles in space are ions. Some of these ions are trapped by the earth's magnetic field and make up part of the Van Allen belt.

Behaviour of ions: All ionic solids and liquids, and most ionized gases, are electrically neutral. The total charge of all their positive ions equals the total charge of all their negative ions. This general rule also applies to all other kinds of matter, and is called the principle of electroneutrality.

Like neutral atoms and molecules, ions in a liquid or a gas are constantly moving. Each one changes its direction of motion billions of times each second because of collisions and the forces exerted on it by other particles. After each change in direction, an ion usually is no more likely to be moving in one direction than in any other. Such random motion is called Brownian motion. When two oppositely charged electrodes are placed in a liquid or gas, each ion loses part of its random motion and starts to drift toward one of the electrodes. Negative ions drift toward the anode (negative electrode) and are called anions. Positive ions drift toward the cathode (positive electrode) and are called cations. The movement of the charges carried by the drifting ions makes up an electric current.

The ability of a solution to conduct electric current depends on the concentration of ions in the solution. For example, drinking water drawn from a typical tap contains few ions and therefore is a poor conductor of current. But seawater, with significant amounts of dissolved sodium chloride, magnesium sulphate, and other ionic compounds, is a good conductor.

Producing ions: Any process that can add or remove electrons from an atom or a molecule can produce ions. Radiation and chemical reactions are such processes.

Radiation can increase the energy of the electrons in an atom or a group of atoms. If this energy is increased enough, one or more electrons can overcome the attraction of the nucleus and escape from the atom. The loss of negative charge results in the atom becoming a positive ion. Radiation that can produce ions includes light, X rays, gamma rays, atomic nuclei, subnuclear particles, and electrons.

High-energy radiation absorbed by plant or animal tissues produces unnatural ions in the tissues. These ions become involved in potentially harmful chemical reactions. In human beings and other animals, the symptoms of these reactions are called radiation sickness. The amount of radiation absorbed by tissues is measured in units called roentgens or radiation units (rads). One roentgen or rad corresponds to the formation of more than a billion pairs of ions in the tissues. Death is likely to occur if a person's body absorbs 500 roentgens of radiation over a short period.

Ions are formed in a chemical reaction if molecules split into electrically charged parts. For example, molecules of hydrogen chloride (HCl) split when added to water. They form positive hydrogen ions and negative chloride ions.