Chemical Bonding & the formation of Compounds

Introduction There are only 118 or so elements listed on the periodic table, however there are obviously many more natural substances in nature.   This is because atoms can react with one another to form new substances called compounds. 

Compounds are different both chemically and physically from its parent atoms.  Example The element sodium is a silver-colored metal that reacts so violently with water that flames are produced when sodium gets wet.  The element chlorine is a greenish-colored gas that is so poisonous that it was used as a weapon in World War I.  However, when chemically bonded together, these two dangerous substances form the compound sodium chloride, a compound so safe that we eat it every day - common table salt!

Chemical Bonding chemical bonds are formed between atoms because electrons from the atoms interact with each other.  elements are most stable when they have full valence shell.  Atoms that do not have full valence shells bond together to share electrons and complete their valence shells. There are three main types of chemical bonds presented here, ionic, covalent & metallic.

Ionic Bonding In ionic bonding, electrons are completely transferred from one atom to another.  In the process of either losing or gaining negatively charged electrons, the reacting atoms form ions.  The oppositely charged ions are attracted to each other by electrostatic forces which are the basis of the ionic bond.  

An example For example, during the reaction of sodium with chlorine: sodium (on the left) loses its one valence electron to chlorine (on the right), resulting in a positively charged sodium ion (left) and a negatively charged chlorine ion (right).   

Formation of Sodium Chloride due to ionic bonding

Notice that when sodium loses its one valence electron it gets smaller in size, while chlorine grows larger when it gains an additional valence electron.  This is typical of the relative sizes of ions to atoms, positive ions tend to be smaller than their parent atoms while negative ions tend to be larger than their parent.  After the reaction takes place, the charged Na+ and Cl-  ions are held together by electrostatic forces, thus forming an ionic bond. 

Ionic compounds share many features in common: Ionic bonds form between metals and non-metals, Ionic compounds dissolve easily in water and other polar solvents, In solution, ionic compounds easily conduct electricity, Ionic compounds tend to form crystalline solids with high melting temperatures.

Covalent Bonding  The second major type of atomic bonding occurs when atoms share electrons.  As opposed to ionic bonding in which a complete transfer of electrons occurs, covalent bonding occurs when two (or more) elements share electrons. 

Covalent bonding occurs because the atoms in the compound have a similar tendency for electrons (generally to gain electrons).  This most commonly occurs when two non-metals bond together.   Because both of the non-metals will want to gain electrons, the elements involved will share electrons in an effort to fill their valence shells.

Formation of covalent bonds

Example A good example of a covalent bond is that which occurs between two hydrogen atoms.  Atoms of hydrogen (H) have one valence electron in their first electron shell.  Since the capacity of this shell is two electrons, each hydrogen atom will 'want' to pick up a second electron.   In an effort to pick up a second electron, hydrogen atoms will react with nearby hydrogen (H) atoms to form the compound H2.  Because the hydrogen compound is a combination of equally matched atoms, the atoms will share each others single electron, forming one covalent bond.  In this way, both atoms share the stability of a full valence shell. 

Unlike ionic compounds, covalent molecules exist as true molecules.  Because electrons are shared in covalent molecules, no full ionic charges are formed.  Thus covalent molecules are not  strongly attracted to one another.  As a result, covalent molecules move about freely and tend to exist as liquids or gases at room temperature.  

Metallic Bonding Metals have low ionization energies, thus they do not have a tight hold on their valence electrons.  These outer electrons easily move around, as they do not "belong" to any one atom, but are part of the whole metal crystal.  The negatively charged electrons act as a "cement" that hold the positively charged metal ions in their relatively fixed positions.

Metallic bonding….

The fact that the electrons flow easily helps to explain some of the characteristics of metals: - Metals are good conductors of heat and electricity. This is directly due to the mobility of the electrons. - The "cement" effect of the electrons determines the hardness of the metal.  Some metals are harder than others; the strength of the "cement" varies from metal to metal. -  Metals are lustrous.  This is due to the uniform way that the valence electrons of the metal absorb and re-emit light energy. - Metals are malleable (can be flattened) and ductile (can be drawn into wires) because of the way the metal cations and electrons can "flow" around each other, without breaking the crystal structure. *Metallic bonds are best characterized by the phrase "a sea of electrons"*

Glossary 1 Atom   The smallest unit of an element that retains the chemical properties of the element. Atoms can exist alone or in combinations with other atoms forming molecules. Chemical Bond   A link between atoms. See ionic bond and covalent bond. Chemical Reaction   A process in which atoms and molecules recombine by forming or breaking chemical bonds. Chemical reactions form new products that have different chemical properties than the initial reacting material. Compound   A material formed by the chemical combination of elements in defined proportions. Compounds can be chemically decomposed into simpler substances. Covalent Bond   A very strong chemical bond formed by the sharing of a pair of electrons. Multiple covalent bonds can be formed when multiple pairs of electrons are shared between atoms. Covalent bonds are generally characterized in two types, polar and non-polar covalent bonds.

Glossary 2 Covalent Molecule   A molecule held together by covalent bonds, that is, pairs of electrons shared between atoms. Covalent molecules are true chemical molecules whose interaction with other molecules is influenced by whether a polar molecule or non-polar molecule is formed Electron   A sub-atomic (ß link to atom) particle with a negative charge of 1.60 × 10-19 coulombs and a mass of 9.11 × 10-31 kg. Electrons are generally found in orbit around the nucleus of an atom, but may be gained or lost during ion formation. Element   One of less than 118 pure chemical substances. An element is a substance composed of atoms with identical atomic number. Force   The result of an interaction that is capable of changing the state of motion of an object. A 'push or pull' on an object resulting from its interaction with another object.  .

Glossary 3 Ion   An atom or molecule that has acquired an electrical charge by either gaining or losing electrons. A cation is an ion that has lost electrons and acquired a positive charge. An anion is an ion that has gained electrons and acquired a negative charge. Ionic Bond   A chemical bond characterized by electrostatic attraction between ions of opposite charge. The formation of an ionic bond involves a complete transfer of electrons between atoms, and can be predicted when one bonding atom has a much higher electronegativity than the other. Ionic Compound   A chemical compound held together by ionic bonds, that is, electrostatic attraction between positive and negative ions. Ionic compounds generally form ordered structures in which each cation is surrounded by several anions and vice versa. Thus ionic compounds commonly form complex lattices rather than true molecules. Parent   the material or source from which something is derived

Glossary 4 Solution   A mixture of more than one substance with properties that do not vary within the sample. Commonly used to describe a solid dissolved in a liquid, solutions of two or more gases, liquids, solid metals or other materials also exist. Also called a homogeneous mixture. Valence   The number of single bonds an atom can form, also measured as the number of hydrogen atoms that typically bond to an atom of an element. For example, in H2O, oxygen has a valence of two; in CH4, carbon has a valence of four. Valence Electron   Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n (electrons in the "outermost" shell of an atom). For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate in chemical bonding. Valence Shell   The outermost electron shell of an atom that contains electrons.

Acknowledgments 207.10.97.102/chemzone/lessons/ 03bonding/mleebonding/ionic_bonds.htm www.visionlearning.com/library/pop_glossary