Chapter 3 Bonding

Chapter 3: BONDING - A molecule is defined as the smallest particle of an element or a compound that retains the characteristics of the original substance. When atoms do combine to form molecules, there is a shifting of valence electrons, the electrons in the outer energy level of each atom. This more stable form may be achieved by the gain or loss of electrons or the sharing pairs of electrons. The resulting attraction of the atoms involved is called a chemical bond. When a chemical bond forms, energy is released; when this bond is broken, energy is absorbed. TYPES OF BONDS Ionic Bonds - When the electronegativity values of two kinds of atoms differ by 1.7 or more (especially differences greater than 1.7), the more electronegative atom will borrow the electrons it needs to fill its energy level, and the either atom will lend electrons until it has a complete energy level. - Because of this the borrower becomes negatively charged and is called an anion; the lender becomes positively charged and is called a cation. They are now referred to as ions, and the bond or attraction b/w them is called an ionic bond. - These ions do not form an individual molecule in the liquid or solid phase but are arranged into a crystal lattice containing many such ions. Ionic solids of this type tend to have high melting points and will not conduct a current of electricity until they are in the molten state. Covalent Bonds Nonpolar Covalent Bond: When the electronegativity difference b/w 2 or more atoms is 0 or very small (not greater than 0.4), the atoms tend to share the valence electrons in their respective outer energy levels. - - These covalent bonded molecules do not have electrostatic charges like those of ionic bonded substances. In general, covalent compounds are gases, liquids having fairly low boiling points, or solids that melt at relatively low temperatures. Unlike ionic compounds, they do not conduct electric currents. When the electronegativity difference is b/w 0.4 and 1.6, there will not be an equal sharing of electrons b/w atoms involved. The shared electrons will be more strongly attracted to the atom of greater electronegativity. As the difference in the electronegativities of the two elements increases above 0.4, the polarity or degree of ionic character increases. At a difference of 1.7 or more, the bond has more than 50% ionic character. Polar Covalent Bond: When the difference is between 0.4 and 1.6 When these non symmetrical polar bonds are placed around a central atom, the overall molecule is polar. Both the bonds and the molecules could be described as polar. Polar molecules are also often referred to as dipoles b/c the whole molecule itself has 2 distinct ends from a charge perspective. However, polar covalent bonds exist in some nonpolar molecules. Examples are 𝐶𝑂2, 𝐶𝐻4 , 𝐶𝐶𝑙4. In the covalent bonds described so far, the shared electrons in the pair contributed one each from the atoms bonded. In some cases, however, both electrons from the shared pair are supplied by only one of the atoms. Two examples are the bonds in 𝑁𝐻4 + and 𝐻2 𝑆𝑂4. Metallic Bonds - In most metals, one or more of the valence electrons become detached from the atom and migrate in a “sea” of free electrons among the positive metal ions. - The attractive force strength varies with the nuclear positive charge of the metal atoms and the number of electrons in this electron sea. - Both of these factors are reflected in the amount of heat required to vaporize the metal. - The strong attraction b/w these differently charged particles forms a metallic bond. Because of this firm bonding, metals usually have high melting points, show great strength, and are good conductors of electricity. INTERMOLECULAR FORCES OF ATTRACTION The term intermolecular forces refers to attractions b/w molecules. It is proper to refer to all intermolecular forces as van der Waals forces. Dipole-Dipole Attraction - One type of van der Waals forces is dipole-dipole attraction. - The asymmetrical distribution of electric charges leads to positive and negative charges in the molecules, referred to as dipoles. - In polar molecular substances, the dipoles line up so that the positive pole of one molecule attracts the negative pole of another. - The force of attraction b/w polar molecules is called dipole-dipole attractions. London Dispersion Forces - Another type of van der Waals force is allied London dispersion forces. - Found in both polar and nonpolar molecules, a molecule/atom that usually is nonpolar sometimes becomes polar b/c the constant motion of its electrons may cause uneven charge distribution at any one instant. - When this occurs, the molecule/atom has a temporary dipole. This dipole can then cause a second, adjacent atom to be distorted and to have its nucleus attracted to the negative end of the first atom. - London dispersion forces are the weakest of all the electrical forces b/w atoms or molecules. - They help to explain why nonpolar substances (nonpolar substances, halogens) condense into liquids and then freeze into solids when the temperature is lowered sufficiently. - They also help to explain why liquids composed of discrete molecules w/ no permanent dipole attraction have low boiling points relative to their molecular masses. - Compounds in the solid state that are bound mainly by this type of attraction have rather soft crystals, are easily deformed, and vaporize easily. - B/c of the low IMF, the melting points are low and evaporation takes place so easily that it may occur at room temperature. Hydrogen Bonds - A hydrogen bond is when a hydrogen atom is bonded to a highly electronegative atom, itss positive charge will have an attraction for neighboring electron pairs. - A proton or hydrogen nucleus has a high concentration of positive charge. - The more strongly polar the molecule is, the more effective the hydrogen bonding is in binding the molecules into a larger unit. - As a result, the boiling points of such molecules are higher than those of similar nonpolar molecules. - It explains why some substances have unexpectedly low vapor pressures, high heats of vaporization, and high melting points. In order for vaporization or melting to take place, molecules must be separated. - Energy must be expended to break hydrogen bonds and thus break down large clusters of molecules into separate molecules. DOUBLE AND TRIPLE BONDS - To achieve the octet structure, an outer energy level resembling the noble gas configuration of 8 electrons, it is necessary for some atoms to share 2 or even 3 pairs of electrons. - Sharing 2 pairs of electrons produces a double bond. - In the line formula, only the shared pair of electrons is indicated by a bond (‒). The sharing of 3 electron pairs results in a triple bond.