Atom

The atom is defined as the smallest subdivision of an element.  Atoms of the same element have the same size, mass and chemical properties^[1]. 

An atom is composed of three different particles: protons (p⁺), neutrons (n⁰) and electrons (e^-) .

Protons and neutrons are located in the nucleus of the atom, whereas the electrons spin around this nucleus in specific orbits^[2][3].

The addition of the number of protons and neutrons produces the atomic mass of an element, whereas the number of protons equals with the number of the electrons of the same element and are equal with the atomic number of this element^[4]. The number of neutrons can vary significantly, especially in radioactive elements. The number of neutrons in the atom defines the isotope of the element^[5].

How the atom is really structured was a serious matter for many centuries and different theories raised from Democritus (c.300 BC), Dalton (c.1800), J.J. Thomson (c.1850), Rutherford (c. 1905), Neils Bohr, Bohr-Rutherford (c. 1920) to “Modern theory” which describes the orbits of the spinning electrons around the nucleus as a "cloud of energy levels"^[6][7].

References

1. ↑ http://js082.k12.sd.us/My_Classes/Physical_Science/atoms/atoms_1.htm
2. ↑ http://js082.k12.sd.us/My_Classes/Physical_Science/atoms/atoms_1.htm
3. ↑ http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookCHEM1.html
4. ↑ http://www.clickandlearn.org/Gr9_Sci/atoms/modelsoftheatom.html
5. ↑ ^ Leigh, G. J., ed. (1990). International Union of Pure and Applied Chemistry, Commission on the Nomenclature of Inorganic Chemistry, Nomenclature of Organic Chemistry – Recommendations 1990. Oxford: Blackwell Scientific Publications. p. 35. ISBN 0-08-022369-9. "An atom is the smallest unit quantity of an element that is capable of existence whether alone or in chemical combination with other atoms of the same or other elements."
6. ↑ http://www.clickandlearn.org/Gr9_Sci/atoms/modelsoftheatom.html
7. ↑ http://www.clickandlearn.org/chemistry/atomic_theory.htm