Changes

[[Image:Atoms1.jpg|thumb|right|Subatomic resolution of atoms by AFM]]

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

An '''atom''' is

An '''atom''' is

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 [[molecule]]s.

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 [[molecule]]s.

Although the word "atom" comes from the Greek term for indivisible, ''átomos'', atoms are actually made up of three different kinds of subatomic particles; some of these are composed of yet smaller particles.

Although the word "atom" comes from the Greek term for indivisible, ''átomos'', atoms are actually made up of three different kinds of subatomic particles; some of these are composed of yet smaller particles.

In the atomic [[nucleus]] there are positively charged [[proton]]s and electrically neutral [[neutron]]s. Surrounding the nucleus are negatively charged [[electron]]s. [[Hydrogen]], in its most common [[isotope]], has only one proton and no neutrons.

In the atomic [[nucleus]] there are positively charged [[proton]]s and electrically neutral [[neutron]]s. Surrounding the nucleus are negatively charged [[electron]]s. The simplest atom is that of [[Hydrogen]]. It has only a single proton and electron and no neutrons.

Protons and neutrons are comprised of [[quark]]s and are contained closely together in the center of an atom, forming the [[nucleus]]. 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 is found in the nucleus. Currently quarks and electrons are considered truly 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.  

Each proton and 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 electrical theory might notice that positively charged particles packed closely together would repel one another. The nucleus stays together because of what is known as the ''strong nuclear force''. The quantization of this force is a particle called a "[[gluon]]".

Those who understand [[electrostatics|electrical theory]] might notice that positively charged particles packed closely together would repel one another. As the protons and neutrons in the nucleus are hadrons, they are affected by another of 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 particle called a "[[gluon]]".

==Atomic Number==

==Atomic Number==

==Ions==

==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 elements fluorine, chlorine, bromine and iodine are known as fluoride, chloride, bromide and iodide (replacing -ine with -ide), respectively.

Atoms are normally electrically neutral; they have no charge. However, electrons 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 [[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]] (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.

Many common substances are made up of ions. For example, [[sodium chloride]] (NaCl), otherwise known as table salt, is made up of [[sodium]] 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==

==Isotopes==

Atoms of the same element that have different numbers of neutrons are known as [[isotope]]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 number of neutrons and protons (number of protons remains constant in a given element) in all naturally occurring isotopes. Many isotopes are [[radioactive]] and [[radioactive decay|decay]] over time.

Atoms of the same element (having the same number of protons) that have different numbers of neutrons are known as [[isotope]]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 number of neutrons and protons (number of protons remains constant in a given element) in all naturally occurring isotopes (isotopes that are not radioactive). For example, [[chlorine]] has two naturally occuring isotopes: chlorine-35 and chlorine-37, with relative abundances of 75.78% and 24.22%.<ref>https://www.webelements.com/chlorine/isotopes.html</ref>. The atomic weight of chlorine is the weighted average and so it is <math>(0.7578 \times 35 )+ (37 \times 0.2422)=35.48</math>. Note that the atomic weight of an element is not necessarily an integer due to the way it is calculated, as is true in this example. Many isotopes are [[radioactive]] and [[radioactive decay|decay]] over time.

==History of the Atomic Model==

==History of the Atomic Model==

The early Greek philosophers [[Democritus]] 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.

The early Greek philosophers [[Democritus]] 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.

[[John Dalton]] developed a realistic atomic theory based on four principles:  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|Joseph Proust's]] [[Law of definite proportions|Law of Definite Proportions]].

[[John Dalton]] developed a realistic atomic theory based on four principles:  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]].

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==

==See also==

*[[Geiger–Marsden experiment]]

*[[Geiger–Marsden experiment]]

==References==

==References==