Why some elements have variable valency?
347007
The valency is determined by how many electrons the atom is using to bond with another atom to form a compound. The electrons in the outermost orbit generally forms the bonds. For materials having multiple electrons in the outermost orbit the element can have different valencies. Whatever happens tends to lower the the total energy of the system to make it stable.
8842
Well adding upto the given answers above, I would like to say it is not the only reason noble elements may form compounds. Compound like XeF4 are formed because Xe has a lot of non bonding electrons. It can donate it to form more compounds. Here are some links where you will find some neon and even helium compounds formed!!
233865
Generally, the elements that form ionic bonds tend to have either very high or very low electronegativity. This means the Alkali metals, Alkaline Earth metals, and the Halogens, especially when reacting with each other.
916600
Ionic compounds try to achieve the same thing...only instead of atoms sharing electrons with other atoms (co-valent) in a chemical bond, Ionic compounds achieve lowest energy configurations by receiving, or giving up, electrons.  For example, the sodium atom has one electron in it's valence "shell" and will give it up (becoming positively charged in the process) in order to have the same electronic configuration as the previous noble gas (neon) and becomes the sodium cation.\nFluorine, having seven electrons in it's valence shell would rather accept and electron (to have the neon electron configuration) and becomes the negatively charged fluoride anion.
12361
i sorta answered that in your other one asking the config for niobium.  There are some exceptions to the electron configuration filling rules, and that is one of them, the element is more stable because the shells are already far out basically, the electron likes having an orbital all to itself. (each orbital can contain up to 2 electrons, the s only has one orbital, and the d has 5)
410413
In the formation of a molecule only the outer shell electrons are involved and they are known as valence electrons.It is, therefore, quite reasonable to consider these valence electrons while discussing chemical bonds and other chemical properties as the case may arise.\n          G.N.Lewis introduced simple symbols to denote valence electrons in an atom.The outer shell electrons are shown as dots surrounding the symbol of the atom. These symbols ignore inner shell electrons. This type of representation is known as Lewis dot representation.Structures of molecules denote by this representation is called Lewis dot structure. \n The common valency of the element is either equal to the dots in Lewis symbol (if less than equal to 4) or 8 minus these dots (if these are more than 4)\n\nFor examples of dot structures click the following link:\nhttp://img106.imageshack.us/img106/3875/untitled1ju.jpg
227254
The general valence shell electronic configuration is\nns2 np3.\nAnd the elements along the group are :-\n1.Nitrogen (N)\n2.Phosphorous (P)\n3.Arsenic (As)\n4.Antimony (Sb)\n5.Bismuth (Bi)\n6.Ununpentium (Uup)\n\nCheck it out in various books above 10th grade
697590
Common valences are the normal number of chemical bonds that must be formed to stabilize an atom. For example, for oxygen its 2-, because O shares 2 electrons to become stable. For hydrogen its 1+, since hydrogen receives one electrons.\n\nSome metals have more than one valence. For instance:\nCu: 2+, 3+\nHg: 2+, 4+ (I think)\n\nAnyway, these valences are the common valences for these atoms. Under extreme conditions the common valences may change.
429543
Filling its valence shell with paired electrons.
175182
The elements in a group on the periodic table all have the same number of valence electrons and therefore generally have similar properties.
99054
Not only sodium rather all elements of s and p orbitals with 1 or 2 valency do not exist as naturally found pure elements.\n\nThis is because of their very high reactivity with majority of substances present in the atmosphere, sea and earth crust.
426762
you know its ionic because Na is a metal while Cl isnt. a metal and nonmetal almost always from an ionic bond. to draw a lewis digram, determine how many valence electons are in each and draw the diagram.. its kinda hard to explain without actually drawing it.. consult a chem bk.
641799
It depends on the octet rule. If the element fulfills its octet, or has 8 valence electrons(such as the noble gases) then it will be very stable. The farther they are from fulfilling the octet rule, the less stable the element is.
285366
well if by terminal you mean, like a hub.  it would probably be due to the larger number of electrons inside the valence shell, not the valence shell itself. the ones give them their strong electro negetivity,combined with the fact that as the atom gets larger there is more distance between the center(+) and the surrounding (-).  these cause coordinate bonds, basicly like static electricity attracts things. the bonds are weaker than covalent or ionic cause the moleceles already stable(this is where the valence shell comes in, stability), a good example of this example is take a hard natural magnet and try sticking a piece of steel note the force that is required to seperate the two. then exert that same amount of force toward breaking the magnet itself.  you'll surely notice that the bonds in the magnet are far stronger than the bonds that the magnet makes with other substances.  ok do you see now how a strong negtive force within a atom has the ability to attract weaker positively charged atoms/molecules?  i noticed one answer had the valence shell mentioned, and true they have a valence configuration which has a deficency of one electron.  this is a factor ivolved in the formation of ionic and covalent bonds.  the coordenate bonds are more like magnets attracting one another. the difference in electro negetivity is what determines the type of bond techniclly.  the ionic bonds are the strongest. now the covalent bonds actually share electrons and these bonds are not as strong, and the coordenate bonds are weaker still.  there are other even weaker bonds such as hydrogen bonds (i.e. in water the hydrogen bonds account for the face that water expands right before it freezes). these bonds are the result of the attraction of the positive charge that is present with the electron of the hydrogen affects the magnetic field within the oxygen atom causing a slightly more negetive charge in a 180 deg. position relevent to the hydrogen. anyway i had a little more time tonight.
746306
Elements consist of a single type of atom.  For example, pure iron has only iron atoms.  Compounds combine more than one type of atom, for example, iron and oxygen form a compound called iron oxide, also known as rust.  Elements and compounds are alike because they cannot be separated into different materials by mechanical means, like filtering, magnetic separation or picking with tweezers.  The are different in that element have only one kinds of atom, while compounds have two or more types of atoms.  Even when you have material which is all one type of atom, such as a hydrogen gas cloud, a distinction is made between "elemental hydrogen", which is extremely reactive, and H2, which might need an outside source of energy to cause it to react with another element.
664669
Yeah, there are some special cases with some transition metals, and you're right with groups 6 and 11.  \nYou'd expect for the Cu (and the other elements in that column) to have the electron configuration [Ar]4s2 3d9.  \nInstead, the configuration is [Ar]4s1 3d10.  This configuration is more stable because the d sublevel is completely filled.  Cr's valence electron configuration (and the rest of the column's) is expected to be 4s2 3d4, but it is actually 4s1 3d5.  There is added stability in the exactly half-filled d sublevel.
333129
An element will become stable when it's outermost electron (valence) shell is full.  In most cases, the valence shell will contain 8 electrons when stable.  Hydrogen and helium only require 2 electrons.  \n\nIn order to achieve a full set, atoms can gain or lose electrons forming ionic compounds.  This normally happens between metals and nonmetals on the periodic table.  Alternatively, atoms can share electrons forming covalent compounds, which generally occurs between two nonmetals.