Atom

For alternative meanings see atom (disambiguation).

Atom
Classification
Smallest division of a chemical element
Properties
Mass: atomic mass Electric Charge: 0 C Diameter: 10pm to 100pm

An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of subatomic particles: electron, proton, and neutron. Atoms usually have the tendency to combine into molecules. For example, the molecule of water is made of 2 atoms of Hydrogen, and 1 atom of Oxygen.

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions. Only 91 types of chemical building blocks, or chemical elements, can be found naturally on earth, and are classified in the Periodic table. This classification depends only on the number of protons in the atom. Other types of atoms have been artificially created, but they are unstable and spontaneously decay into natural chemical elements by nuclear fission(s).

Atoms of the same chemical element can display different radioactive properties, based on the number of neutrons. Atoms with different numbers of neutrons are called isotopes of the chemical element. Because of their ubiquitous nature, atoms have been an important field of study for many centuries. Current research focus on quantum effects, such as in Bose-Einstein condensate.

Atomic theory

Main article: Atomic theory

The atomic theory is a theory of the nature of matter. It states that all matter is composed of atoms.

Structure

Sub-atomic particles

Atoms are composed mostly of empty space, but also of smaller subatomic particles. At the center of the atom is a tiny positive nucleus composed of nucleons (protons and neutrons). The rest of the atom contains only the fairly flexible electron shells. In a neutral atom, the numbers of protons and electrons are identical, and the charges balance each other out. Charged atoms are called ions. Atoms with less electrons than protons have a positive charge and are known as cations. Atoms with more electrons than protons have a negative charge and are known as anions. Electrons orbit around the nucleus at high speeds. Electrons orbit at different distances from the nucleus. We say those closest to the nucleus are in one "shell," and those a little further away are in another "shell." Each shell is given a number. The one closest to the nucleus is shell 1, the next one out is shell 2, and so on. The nucleus is more than 100,000 times smaller than the atom - the size of the atom is determined by the size of the outer electron shell. If an atom was expanded to the size of Heathrow airport, the nucleus in the centre would be the size of a golfball.

Elements and isotopes

Atoms are generally classified by their atomic number, which corresponds to the number of protons in the atom (in neutral atoms, this is the same as the number of electrons). The atomic number decides which family or element the atom belongs to. For example, carbon atoms are those atoms containing 6 protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical behavior. The various kinds of atoms are listed in the Periodic table. The mass number or nucleon number is the total of protons and neutrons. The number of neutrons does not have any effect on the element of the atom - within an element family are several members, each with the same atomic number but different mass numbers. These are called isotopes. To write the name of an isotope, we write the name of an element followed by its mass number, eg. Carbon 14 (which contains 6 protons and 8 neutrons in each atom).

The simplest atom is the hydrogen atom, having atomic number 1 and consisting of one proton and one electron. The hydrogen isotope containing 1 additional neutron is called deuterium; the hydrogen isotope with 2 additional neutrons is called tritium. It has been the subject of much interest in science, particularly in the early development of quantum theory.

Outer shells and bonding

The chemical behavior of atoms is largely due to interactions between the electrons. In particular the electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.

Each shell around the nucleus can hold a limited number of electrons

Shell 1 - 2 electrons
Shell 2 - 8 electrons
Shell 3 - 8 or 18 electrons (depending on the element)

Every atom is much more stable (less energetic) with a full (or empty) outer compliment of electrons. This is achieved either by sharing electrons with neighboring atoms or by completely removing electrons from other atoms. When electrons are shared a covalent bond is formed between the two atoms. Covalent bonds are the strongest type of atomic bond. Thus, some atoms stay together in groups called molecules. The electrons in the outer shells of a molecule orbit around all the atoms, leaving each one satisfied with a full shell of electrons.

When one or more electrons are completely removed from one atom by another, ions are formed. Ions are atoms that possess a net charge due to an imbalance in the number of protons and electrons. Some atoms, such as sodium, have one electron in the outer shell. Others, like chlorine, need just one more electron to have a full outer shell. If sodium atoms are allowed to come into contact with chlorine atoms, the sodium atoms will give their outer electrons up to chlorine. Sodium will gain a positive charge, whereas chlorine will gain a negative charge. The ion that stole the electron(s) is called an anion and is negatively charged. The atom that lost the electron(s) is called a cation and is positively charged. Cations and anions are attracted to each other due to coulombic forces between the positive and negative charges. This attraction is called electrovalent or ionic bonding and is weaker than covalent bonding.

As mentioned above covalent bonding implies a state in which electrons are shared equally between atoms, while ionic bonding implies that the electrons are completely confined to the anion. Except for a limited number of extreme cases, neither of these pictures is completely accurate. In most cases of covalent bonding, the electron is unequally shared, spending more time around the more electronegative atom, resulting in the covalent bond having some ionic character. Similarly, in ionic bonding the electrons often spend a small fraction of time around the more electropositive atom, resulting in some covalent character for the ionic bond.

Models of the atom

• Democritus' shaped-atom model (for want of a better name)
• The plum pudding model
• Cubical atom
• The Bohr model
• The quantum mechanical model

Etymology

The word atom is derived from the Greek atomos, indivisible, from a-, not, and tomos, a cut. Till the 19th century many people believed atoms to be tiny hard spheres, like minute billiard balls. They thought they had no internal parts and could not be created, divided or destroyed.

See also

• Atomism
• Exotic atom
• Radioactive isotope
• List of particles
• Individual (same literal meaning)
• Infinite divisibility

External link

• The American Heritage Dictionary (contains popups)