For other uses, see Atom (disambiguation).
Helium atom

An atom, is made up of the nucleus and the electron cloud distribution. The nucleus is spherically symmetric, although for more complicated nuclei this is not always the case.

...

For other uses, see Atom (disambiguation).
Helium atom

An atom, is made up of the nucleus and the electron cloud distribution. The nucleus is spherically symmetric, although for more complicated nuclei this is not always the case.

Classification
Smallest recognized division of a chemical element

Components: Electrons and a compact nucleus of protons and neutrons

The atom is the smallest unit of an element that retains the chemical properties of that element. An atom has an electron cloud consisting of negatively charged electrons surrounding a dense nucleus. The nucleus contains positively charged protons and electrically neutral neutrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a net positive or negative charge. An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element.

The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was not indivisible. The principles of quantum mechanics were used to successfully model the atom.

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons occupy a set of stable energy levels, or orbitals, and can transition between these states by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

No one has ever seen an atom and no one will because they are smaller than the wavelengths of the light with which we see, but the evidence gives us two useful models of atoms, particle and wave.

In the particle model the simplest atom, hydrogen, is 10^-8 cm in diameter. This is 10 billionths meter, i.e. very, very small. But the two particles of which it is made are 10,000 times smaller or 10^-12 cm. This means if y...

No one has ever seen an atom and no one will because they are smaller than the wavelengths of the light with which we see, but the evidence gives us two useful models of atoms, particle and wave.

In the particle model the simplest atom, hydrogen, is 10^-8 cm in diameter. This is 10 billionths meter, i.e. very, very small. But the two particles of which it is made are 10,000 times smaller or 10^-12 cm. This means if you were to increase the size of each particle to that of a softball they would be separated by one kilometer, 0.62 miles or 3274 ft. with both doing pirouettes at about 10% the speed of light and not permitting anything in their space. While we see atoms as spheres that is only approximate and their shapes are probably more like potatoes. But, they join together in very regular crystals when solid.

The wave model is more mysterious and hard to visualize, but convenient when working with matters electrical or transferring energy. The concepts include such things as waves passing through each other without interference, absorbing energy by changing their positions in the atom instantly and all the other esoteric concepts of quantum mechanics.

All that we say of atoms as particles or waves is theoretical, based on evidence and subject to future interpretation.