Periodic table (b)

Periodic trends of groups Modern  HYPERLINK "http://en.wikipedia.org/wiki/Quantum_mechanics" \o "Quantum mechanics" quantum mechanical  theories of atomic structure explain group trends by proposing that elements within the same group have the same electron configurations in their  HYPERLINK "http://en.wikipedia.org/wiki/Electron_shell" \l "Valence_shells" \o "Electron shell" valence shell , which is the most important factor in accounting for their similar properties. Elements in the same group also show patterns in their  HYPERLINK "http://en.wikipedia.org/wiki/Atomic_radius" \o "Atomic radius" atomic radius ,  HYPERLINK "http://en.wikipedia.org/wiki/Ionization_energy" \o "Ionization energy" ionization energy , and HYPERLINK "http://en.wikipedia.org/wiki/Electronegativity" \o "Electronegativity" electronegativity . From top to bottom in a group, the atomic radii of the elements increase. Since there are more filled energy levels, valence electrons are found farther from the nucleus. From the top, each successive element has a lower ionization energy because it is easier to remove an electron since the atoms are less tightly bound. Similarly, a group will also see a top to bottom decrease in electronegativity due to an increasing distance between valence electrons and the nucleus. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Periodic_table&action=edit§ion=8" \o "Edit section: Periodic trends of periods" edit ]Periodic trends of periods HYPERLINK "http://en.wikipedia.org/wiki/File:Ionization_energies.png" \o "Periodic trend for ionization energy. Each period begins at a minimum for the alkali metals, and ends at a maximum for the noble gases." INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/commons/thumb/2/27/Ionization_energies.png/300px-Ionization_energies.png" \* MERGEFORMATINET HYPERLINK "http://en.wikipedia.org/wiki/File:Ionization_energies.png" \o "Enlarge" INCLUDEPICTURE "http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png" \* MERGEFORMATINET Periodic trend for  HYPERLINK "http://en.wikipedia.org/wiki/Ionization_energy" \o "Ionization energy" ionization energy . Each period begins at a minimum for the alkali metals, and ends at a maximum for the noble gases. Elements in the same period show trends in  HYPERLINK "http://en.wikipedia.org/wiki/Atomic_radius" \o "Atomic radius" atomic radius ,  HYPERLINK "http://en.wikipedia.org/wiki/Ionization_energy" \o "Ionization energy" ionization energy ,  HYPERLINK "http://en.wikipedia.org/wiki/Electron_affinity" \o "Electron affinity" electron affinity , and  HYPERLINK "http://en.wikipedia.org/wiki/Electronegativity" \o "Electronegativity" electronegativity . Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Similarly, electronegativity will increase in the same manner as ionization energy because of the amount of pull that is exerted on the electrons by the nucleus.  HYPERLINK "http://en.wikipedia.org/wiki/Electron_affinity" \o "Electron affinity" Electron affinity  also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than nonmetals (right side of a period) with the exception of the noble gases. Examples Noble gases All the elements of Group 18, the  HYPERLINK "http://en.wikipedia.org/wiki/Noble_gases" \o "Noble gases" noble gases , have full valence shells. This means they do not need to react with other elements to attain a full shell, and are therefore much less reactive than other groups.  HYPERLINK "http://en.wikipedia.org/wiki/Neon" \o "Neon" Neon  followed by  HYPERLINK "http://en.wikipedia.org/wiki/Helium" \o "Helium" helium  are the most  HYPERLINK "http://en.wikipedia.org/wiki/Inert" \o "Inert" inert  elements among noble gases, since reactivity, in this group, increases with the periods: it is possible to make heavy noble gases react since they have much larger electron shells. However, their reactivity remains very low in absolute terms. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Periodic_table&action=edit§ion=11" \o "Edit section: Halogens" edit ]Halogens In Group 17, known as the  HYPERLINK "http://en.wikipedia.org/wiki/Halogen" \o "Halogen" halogens , elements are missing just one electron each to fill their shells. Therefore, in chemical reactions they tend to acquire electrons (the tendency to acquire electrons is called  HYPERLINK "http://en.wikipedia.org/wiki/Electronegativity" \o "Electronegativity" electronegativity ). This property is most evident for  HYPERLINK "http://en.wikipedia.org/wiki/Fluorine" \o "Fluorine" fluorine  (the most electronegative element of the whole table), and it diminishes with increasing period. As a result, all halogens form acids with hydrogen, such as  HYPERLINK "http://en.wikipedia.org/wiki/Hydrofluoric_acid" \o "Hydrofluoric acid" hydrofluoric acid ,  HYPERLINK "http://en.wikipedia.org/wiki/Hydrochloric_acid" \o "Hydrochloric acid" hydrochloric acid ,  HYPERLINK "http://en.wikipedia.org/wiki/Hydrobromic_acid" \o "Hydrobromic acid" hydrobromic acid  and  HYPERLINK "http://en.wikipedia.org/wiki/Hydroiodic_acid" \o "Hydroiodic acid" hydroiodic acid , all in the form HX. Their  HYPERLINK "http://en.wikipedia.org/wiki/Acidity" \o "Acidity" acidity  increases with higher period, for example, with regard to iodine and fluorine, since a large I?  HYPERLINK "http://en.wikipedia.org/wiki/Ion" \o "Ion" ion  is more stable in solution than a small F?, there is less volume in which to disperse the charge. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Periodic_table&action=edit§ion=12" \o "Edit section: Transition metals" edit ]Transition metals For the  HYPERLINK "http://en.wikipedia.org/wiki/Transition_metal" \o "Transition metal" transition metals  (Groups 3 to 12), horizontal trends across periods are often important as well as vertical trends down groups; the differences between groups adjacent are usually not dramatic. Transition metal reactions often involve coordinated species. [ HYPERLINK "http://en.wikipedia.org/w/index.php?title=Periodic_table&action=edit§ion=13" \o "Edit section: Lanthanides and actinides" edit ]Lanthanides and actinides The chemical properties of the  HYPERLINK "http://en.wikipedia.org/wiki/Lanthanide" \o "Lanthanide" lanthanides  (elements 57?71) and the  HYPERLINK "http://en.wikipedia.org/wiki/Actinide" \o "Actinide" actinides  (elements 89?103) are even more similar to each other than the  HYPERLINK "http://en.wikipedia.org/wiki/Transition_metal" \o "Transition metal" transition metals , and separating a mixture of these can be very difficult. This is important in the chemical purification of  HYPERLINK "http://en.wikipedia.org/wiki/Uranium" \o "Uranium" uranium  concerning  HYPERLINK "http://en.wikipedia.org/wiki/Nuclear_power" \o "Nuclear power" nuclear power .