Editing Chemistry (section)

===Matter===
{{Main|Matter}}
In chemistry, matter is defined as anything that has [[invariant mass|rest mass]] and [[volume]] (it takes up space) and is made up of [[particle]]s. The particles that make up matter have rest mass as well – not all particles have rest mass, such as the [[photon]]. Matter can be a pure [[chemical substance]] or a [[mixture]] of substances.<ref>{{cite book |last=Armstrong |first=James |title=General, Organic, and Biochemistry: An Applied Approach |publisher=[[Brooks/Cole]] |year=2012 |isbn=978-0-534-49349-3 |page=48}}</ref>

====Atom====
{{Main|Atom}}
[[File:Atom Diagram.svg|thumb|upright=0.75|left|A diagram of an atom based on the [[Rutherford model]]]]

The [[atom]] is the basic unit of chemistry. It consists of a dense core called the [[atomic nucleus]] surrounded by a space occupied by an [[electron cloud]]. The nucleus is made up of positively charged [[protons]] and uncharged [[neutrons]] (together called [[nucleon]]s), while the [[electron]] cloud consists of negatively charged [[electron]]s which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons. The nucleus is dense; the mass of a nucleon is approximately 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus.{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=13}}{{sfn|Housecroft|Sharpe|2008|p=2}}

The atom is also the smallest entity that can be envisaged to retain the [[chemical properties]] of the element, such as [[electronegativity]], [[ionization potential]], preferred [[oxidation state]](s), [[coordination number]], and preferred types of bonds to form (e.g., [[metal]]lic, [[ion]]ic, [[covalent]]).

====Element====
[[File:Simple Periodic Table Chart-blocks.svg|thumb|right|upright=1.35|Standard form of the [[periodic table]] of chemical elements. The colors represent different [[block (periodic table)|blocks]] of elements.]]
{{Main|Chemical element}}

A chemical element is a pure substance which is composed of a single type of atom, characterized by its particular number of [[proton]]s in the nuclei of its atoms, known as the [[atomic number]] and represented by the symbol ''Z''. The [[mass number]] is the sum of the number of protons and neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same atomic number, they may not necessarily have the same mass number; atoms of an element which have different mass numbers are known as [[isotope]]s. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element [[carbon]], but atoms of carbon may have mass numbers of 12 or 13.{{sfn|Housecroft|Sharpe|2008|p=2}}

The standard presentation of the chemical elements is in the [[periodic table]], which orders elements by atomic number. The periodic table is arranged in [[Periodic table group|groups]], or columns, and [[period (periodic table)|periods]], or rows. The periodic table is useful in identifying [[periodic trends]].{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=110}}

====Compound====
[[File:Carbon dioxide structure.png|thumb|left|upright=0.6|[[Carbon dioxide]] (CO<sub>2</sub>), an example of a chemical compound]]
{{Main|Chemical compound}}

A ''compound'' is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of its elements.{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=12}} The standard nomenclature of compounds is set by the [[International Union of Pure and Applied Chemistry]] (IUPAC). [[Organic compound]]s are named according to the [[organic nomenclature]] system.<ref>{{cite web |url=http://www.acdlabs.com/iupac/nomenclature/ |title=IUPAC Nomenclature of Organic Chemistry |publisher=Acdlabs.com |access-date=12 June 2011 |archive-date=8 June 2011 |archive-url=https://web.archive.org/web/20110608140820/http://www.acdlabs.com/iupac/nomenclature/ |url-status=live }}</ref> The names for [[inorganic compound]]s are created according to the [[inorganic nomenclature]] system. When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components.<ref name="IUPAC">{{cite book |last1=Connelly |first1=Neil G. |last2=Damhus |first2=Ture |last3=Hartshom |first3=Richard M. |last4=Hutton |first4=Alan T. |title=Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005. |date=2005 |publisher=Royal Society of Chemistry Publishing / IUPAC |location=Cambridge |isbn=0854044388 |url=https://archive.org/details/nomenclatureinor2005conn |access-date=13 June 2022}}</ref> In addition the [[Chemical Abstracts Service|Chemical Abstracts Service (CAS)]] has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as its [[CAS registry number]].

====Molecule====
{{Main|Molecule}}
[[File:Caffeine (1) 3D ball.png|upright=1.05|thumb|right|A ball-and-stick representation of the [[caffeine]] molecule (C<sub>8</sub>H<sub>10</sub>N<sub>4</sub>O<sub>2</sub>)]]

A ''molecule'' is the smallest indivisible portion of a pure [[chemical substance]] that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. However, this definition only works well for substances that are composed of molecules, which is not true of many substances (see below). Molecules are typically a set of atoms bound together by [[covalent bond]]s, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in [[lone pair]]s.

Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named a [[molecular ion]] or a polyatomic ion. However, the discrete and separate nature of the molecular concept usually requires that molecular ions be present only in well-separated form, such as a directed beam in a vacuum in a [[mass spectrometer]]. Charged polyatomic collections residing in solids (for example, common [[sulfate]] or [[nitrate]] ions) are generally not considered "molecules" in chemistry. Some molecules contain one or more unpaired electrons, creating [[Radical (chemistry)|radicals]]. Most radicals are comparatively reactive, but some, such as [[nitric oxide]] (NO) can be stable.

[[File:Benzene-2D-full.svg|thumb|upright=0.7|left|A 2-D [[structural formula]] of a [[benzene]] molecule (C<sub>6</sub>H<sub>6</sub>)]]
The "inert" or [[Noble gas|noble gas elements]] ([[helium]], [[neon]], [[argon]], [[krypton]], [[xenon]] and [[radon]]) are composed of lone atoms as their smallest discrete unit, but the other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various [[pharmaceutical]]s.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as [[ionic compounds]] and [[network solids]], are organized in such a way as to lack the existence of identifiable molecules ''per se''. Instead, these substances are discussed in terms of [[formula unit]]s or [[unit cell]]s as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as [[table salt]]), solids like carbon and diamond, metals, and familiar [[silica]] and [[silicate minerals]] such as quartz and granite.

One of the main characteristics of a molecule is its geometry often called its [[molecular structure|structure]]. While the structure of [[diatomic]], [[triatomic]] or tetra-atomic molecules may be trivial, ([[Linear molecular geometry|linear]], angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.

====Substance and mixture====
{{infobox
| data1 = [[File:Cín.png|100px]] [[File:Sulfur-sample.jpg|100px]]
| data2 = [[File:Diamants maclés 2(République d'Afrique du Sud).jpg|100px]] [[File:Sugar 2xmacro.jpg|100px]]
| data3 = [[File:Sal (close).jpg|100px]] [[File:Sodium bicarbonate.jpg|100px]]
| data5 = Examples of pure chemical substances. From left to right: the elements [[tin]] (Sn) and [[sulfur]] (S), [[diamond]] (an [[allotrope]] of [[carbon]]), [[sucrose]] (pure sugar), and [[sodium chloride]] (salt) and [[sodium bicarbonate]] (baking soda), which are both ionic compounds.
}}
A chemical substance is a kind of matter with a definite [[chemical composition|composition]] and set of [[chemical properties|properties]].<ref>{{Cite book |last1=Hill |first1=J. W. |title=General Chemistry |last2=Petrucci |first2=R. H. |last3=McCreary |first3=T. W. |last4=Perry |first4=S. S. |publisher=Pearson Prentice Hall |year=2005 |edition=4th |location=Upper Saddle River, New Jersey |page=37}}</ref> A collection of substances is called a mixture. Examples of mixtures are [[Earth's atmosphere|air]] and [[alloy]]s.<ref>{{cite book |last1=Avedesian |first1=M. M. |title=Magnesium and Magnesium Alloys |last2=Baker |first2=Hugh |publisher=ASM International |page=59}}</ref>

====Mole and amount of substance====
{{Main|Mole (unit)|l1=Mole}}
The mole is a unit of measurement that denotes an [[amount of substance]] (also called chemical amount). One mole is defined to contain exactly {{val|6.02214076|e=23}} particles (atoms, molecules, ions, or electrons), where the [[particle number|number of particles]] per mole is known as the [[Avogadro constant]].{{sfn|Burrows|Holman|Parsons|Pilling|2009|p=16}} [[Molar concentration]] is the amount of a particular substance per volume of [[Solution (chemistry)|solution]], and is commonly reported in mol/[[decimetre|dm]]<sup>3</sup>.{{sfn|Atkins|de Paula|2009|p=9}}