When atoms of different elements combine together they form compounds.Familiar compounds include common table salt (Sodium Chloride) and water.Table salt is made from a combination of atoms of sodium (Na) and chlorine (Cl)in a ratio of 1:1 forming the compound NaCl. Water is a combination ofhydrogen (H) and oxygen (O) is a ration of 2:1 forming the compound H2O.
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There are different types of chemical bonds. Some bonds involve atransfer of electrons. Others involve a sharing of electrons. Stillother bonds are weak attractions between molecules. Let"s look at eachtype of bond.
1. Ionic Bonds.
Ions are formed by atoms that have non-fulloutermost electron shells in order to become more like the noble gases in Group8 of the Periodic Table (see section on ions). Someatoms add electrons to get a full shell, thus becoming a negative ion.Other atoms subtract electrons from their outermost shell, leaving a full shelland an overall positive charge on the ion. In the previous section, we sawthat atoms with fewer than 4 electrons in their outermost shell tend to formpositive ions, and those with more than 4 electrons tend to form negativeions. Ionic bonds form when atoms transfer electrons between each other,forming ions that are electrically attracted to each other forming a bondbetween them. Sodium chloride (NaCl) is a typical ionic compound.The picture below shows both a sodium and a chlorine ion.
Sodium has 1 electron in its outermost shell, and chlorine has 7electrons. It is easiest for sodium to lose its electron and form a +1ion, and for chlorine to gain an electron, forming a -1 ion. If sodium cantransfer it"s "spare" electron to chlorine (as shown above), bothatoms will satisfy their full outer shell requirements, and an ionic bond willbe formed. If large groups of sodium and chlorine atoms bond this way, theresult is a three-dimensional structure with alternating sodium and chlorineions:
The blue dots are the sodium atoms; the pale green dots are thelarger chlorine atoms. Ionic bonds between each atom forms a relativelystrong bond and a three-dimensional, cubic structure. Below is a look atjust a single layer:
Note that each positive sodium ion is next to a negativechlorine ion. Now imagine this arrangement continuing outward in alldirections with thousands of billions of atoms. Wow!
2. Covalent Bonds.
Sometimes atoms will share electrons instead of transferringthem between the two atoms. This sharing allows both atoms to fill theiroutermost shell while forming a very strong bond between the atoms.Elements such as carbon (C) and Silicon (Si) form strong covalent bonds.Below is a picture showing the electron sharing that occurs in the mineraldiamond. Diamonds are made of pure carbon and its the way that the carbonatoms are bonded that makes diamond the hardest substance.
Each carbon atom has 4 electrons (blue dots) in its outershell. This allows the atom to share electrons with 4 other carbon atomssurrounding it (as the middle carbon atom is doing). Each of these in turnwill share the remaining 3 electrons with adjacent carbon atoms beside, aboveand below it, and those with other carbon atoms, etc., forming a interlocking,three-dimensional network of tightly bonded carbon atoms. Similarly,covalent bonding between silicon and oxygen atoms makes strong bonds that form alarge group of minerals called silicates (more on those later).
3. Metallic and Van der Waals Bonds.
Metallic bonds form when the outer shell electrons are sharedbetween neighboring atoms. Unlike covalent bonding however, there areinsufficient numbers of electrons in most metal atoms (such as copper or silver)to form pure covalent bonds. Therefore, the electrons are shared amongstall the nearest neighbor metal ions, forming a metallic bond. This strangearrangement of "metallic ions is a sea of electrons" gives metalstheir particular physical properties.
Metallic bonds are also explained by band theory. Bandtheory states that closely packed atoms have overlapping electron energy levelsresulting in a conduction "band" wherein the electrons are free toroam between atoms, thus bonding them together. For more information on metallic bonds and bandtheory, see this web site.
Van der Waals bonds are weak bonds that form due to theattraction of the positive nuclei and negative electron clouds of closely packedatoms. This attraction is opposed by the repulsive force of the electronclouds and the repulsive force of neighboring nuclei. However, theattraction is stronger than the total repulsive forces, leaving a residual, weakattraction. Van der Waals bonding is important in minerals such asgraphite and clay minerals.
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