Editing Geochemistry (section)

==Chemical elements==
The building blocks of materials are the [[chemical element]]s. These can be identified by their [[atomic number]] Z, which is the number of [[proton]]s in the [[Atomic nucleus|nucleus]]. An element can have more than one value for N, the [[neutron number|number of neutrons]] in the nucleus. The sum of these is the [[mass number]], which is roughly equal to the [[atomic mass]]. Atoms with the same atomic number but different neutron numbers are called [[isotope]]s. A given isotope is identified by a letter for the element preceded by a superscript for the mass number. For example, two common isotopes of [[chlorine]] are <sup>35</sup>Cl and <sup>37</sup>Cl. There are about 1700 known combinations of Z and N, of which only about 260 are stable. However, most of the unstable isotopes do not occur in nature. In geochemistry, stable isotopes are used to trace chemical pathways and reactions, while radioactive isotopes are primarily used to date samples.<ref name=McSween/>{{rp|13–17}}

The chemical behavior of an atom – its [[Chemical affinity|affinity for other elements]] and the type of [[chemical bond|bonds]] it forms – is determined by the arrangement of electrons in [[Atomic orbital|orbitals]], particularly the outermost ([[valence electron|valence]]) electrons. These arrangements are reflected in the position of elements in the [[periodic table]].<ref name=McSween/>{{rp|13–17}} Based on position, the elements fall into the broad groups of [[alkali metal]]s, [[alkaline earth metal]]s, [[transition metal]]s, semi-metals (also known as [[metalloid]]s), [[halogen]]s, [[noble gas]]es, [[lanthanide]]s and [[actinide]]s.<ref name=McSween/>{{rp|20–23}}

Another useful classification scheme for geochemistry is the [[Goldschmidt classification]], which places the elements into four main groups. ''Lithophiles'' combine easily with oxygen. These elements, which include [[sodium|Na]], [[potassium|K]], [[silicon|Si]], [[aluminium|Al]], [[titanium|Ti]], [[magnesium|Mg]] and [[calcium|Ca]], dominate in the [[Earth's crust]], forming [[silicate]]s and other oxides. ''Siderophile'' elements ([[iron|Fe]], [[cobalt|Co]], [[nickel|Ni]], [[platinum|Pt]], [[rhenium|Re]], [[osmium|Os]]) have an affinity for [[iron]] and tend to concentrate in the [[Earth's core|core]]. ''Chalcophile'' elements ([[copper|Cu]], [[silver|Ag]], [[zinc|Zn]], [[lead|Pb]], [[sulfur|S]]) form [[sulfide]]s; and ''atmophile'' elements ([[oxygen|O]], [[nitrogen|N]], [[hydrogen|H]] and noble gases) dominate the atmosphere. Within each group, some elements are [[refractory]], remaining stable at high temperatures, while others are [[Volatility (chemistry)|volatile]], evaporating more easily, so heating can separate them.<ref name=Albarede>{{cite book|last1=Albarède|first1=Francis|title=Geochemistry : an introduction|date=2007|publisher=Cambridge Univ. Press|location=Cambridge|isbn=9780521891486|edition=5th|others=Translated from the French.}}</ref>{{rp|17}}<ref name=McSween/>{{rp|23}}