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[[Image:Atoms1.jpg|thumb|right|Subatomic resolution of atoms by AFMan atomic force microscope]]

a [[particle]] of [[matter]] indivisible by [[chemical]] means <ref>[http://www.lbl.gov/abc/Glossary.html Glossary of Nuclear Terms] </ref> which form the building blocks of molecules[[molecule]]s.

In the atomic [[nucleus]] there are positively charged [[proton]]s and electrically neutral [[neutron]]s. Spinning around Surrounding the nucleus are negatively charged [[electron]]s. The simplest atom is that of [[Hydrogen]], in its most common [[isotope]], . It has only one a single proton and electron and no neutrons.

Protons Each proton and neutrons neutron are comprised of three [[quark]]s and are contained closely together in the center of an atom, forming the [[nucleus]]. The nucleus is extremely dense, typically having a density of 10¹⁷ kg m^-3, or 10¹³ that of [[lead]]. Electrons move in the space around the nucleus, and are arranged around it in a series of layers, known as [[electron shell|shells]] or energy levels. Since protons and neutrons are approximately 2000 times as heavy as electrons, the vast majority of an atom's [[mass (science)|mass]] is found in the nucleus. Currently quarks and electrons are considered truly [[fundamental particle|elementary particles]]. Atoms are mostly empty space, as the relative size of the nucleus compared to the area of the lowest electron shell is about that of a pea in a stadium. Another common analogy for the atom along the same lines is the "fly in the cathedral", where the cathedral is the whole atom and the fly is the nucleus.

Those who understand [[electrostatics|electrical theory ]] might notice that positively charged particles packed closely together would repel one another. The As the protons and neutrons in the nucleus stays together because are hadrons, they are affected by another of what is the four fundamental forces, known as the ''[[Strong force|strong nuclear force'']]. Unsurprisingly given its name, the strong nuclear force is stronger than the electrical repulsion of the protons at these small scales and so the nucleus stays together. The quantization of this force is a tiny particle called a "[[gluon]]". ===Atomic Number===

===Ions===

Atoms are normally electrically neutral; they have no charge. However, electrons in the [[valence shell]] can be gained or lost (depending on the element and the conditions) to form an [[ion]]. An atom that loses electrons becomes positively charged and is known as a cation. An atom that gains electrons electrons becomes negatively charged and is known as an anion. Anions of the common [[Halogen]]s (Group 7 elements ) fluorine, chlorine, bromine and iodine are known as fluoride, chloride, bromide and iodide (replacing -ine with -ide), respectively.

Many common substances are made up of ions. For example, [[sodium chloride]] (NaCl), otherwise known as table salt, is made up of [[sodium cations]] cations (Na⁺) and [[chlorine]] anions (chloride, Cl^-) in equal proportions. The negatively charged chloride ions are attracted to the positively charged sodium ions, forming an [[ionic bond]]. This results in a lattice structure, which is responsible for sodium chloride being crystalline in its solid state.

===Isotopes===

Atoms of the same element (having the same number of protons) that have different numbers of neutrons are known as [[isotope|isotopes]]s. Some isotopes are more stable than others, and occur more often in nature, but there is no "standard" number of neutrons in a given element. The atomic weight of an element is a weighted average of the atomic weights of all naturally occurring isotopes (isotopes that are not radioactive). The atomic weight of an isotope is approximately equal to the number of neutrons and protons (number of protons remains constant in a given element) in all , For example, [[chlorine]] has two naturally occurring isotopes: chlorine-35 and chlorine-37, with relative abundances of 75.78% and 24.22%.<ref>{{cite web|url=https://www.webelements.com/chlorine/isotopes.html|title=Chlorine: isotope data|accessdate=2019-01-24}}</ref> The atomic weights of those isotopes are 34.96885268 and 36.96590259, respectively. The atomic weight of chlorine is the weighted average, so it is (0.7578×34.96885268)+(0.2422×36.96590259)=35.45293758. So the atomic weight of an element is not necessarily an integer for two reasons: the element as it occurs in nature may be (as in this case) a mixture of two or more isotopes, and the individual isotopes may have non-integral atomic weights due to the mass defect. Mass defects are calculated relative to the [[carbon]]-12 isotope, which is defined to have an atomic weight of exactly 12. Other isotopes generally have non-integral atomic weights because the nuclear binding energy ties up some of the mass according to the formula E=mc². See [[Quantitative Analysis of Alpha Decay]]. Many isotopes are [[radioactive]] and [[radioactive decay|decay]] over time. ==History of the Atomic Model== The early Greek philosophers [[Category:ChemistryDemocritus]]and [[Leucippus]] first proposed that all matter in the universe consisted of particles, which are the smallest units of [[matter]], though [[Aristotle]] used his fame to convince the scientific community that matter consisted of various ratios of [[earth]], [[fire]], [[water]], and [[air]], which can exist in any quantity. This mislead them for centuries. [[CategoryJohn Dalton]] developed a realistic atomic theory based on four principles:Physics that all matter is composed of atoms, that each element's atoms are of one size, atoms combine chemically to form compounds, and chemical reactions occur when atoms are joined or separated. His theory relied on [[Antoine Lavoisier|Lavoisier's]] [[Law of the conservation of mass|Law of Conservation of Mass]] and Joseph Proust's [[Law of definite proportions|Law of Definite Proportions]]. J. J. Thompson later developed the [[Plum Pudding]] Model, a result of his discovering the [[electron]] using [[cathode ray tube]]. [[Ernest Rutherford]] later discovered in his [[Geiger–Marsden experiment|gold foil experiment]] that the atom was mostly empty space, with a nucleus of protons in the center. James Chadwick discovered the neutron and complexities of the nucleus, paving the way for [[nuclear fission]]. ==See also==*[[Element]]