Chapter 8 Notes Covalent Bonding
Molecules and Molecular Compounds Helium and Neon are monoatomic, meaning they exist as single atoms Some compounds exist as crystalline solids, such as NaCl Others exist as liquids (H2O) or gases (HCl) at room temperature HCl and H2O bond very differently than NaCl Instead of giving up or taking electrons, they are shared between the atoms Known as a Covalent Bond
Molecules and Molecular Compounds H2O and CO are examples of molecules, meaning that in nature they exist as a neutral group of atoms held together by covalent bonds How does this occur? A diatomic molecule consist of 2 atoms Ex. O2, H2, N2 A compound composed of molecules is called a molecular compound Remember ionic bonds are collections of + and ions held together There is no such thing as a molecule of NaCl See fig. 8.3 p. 214
Molecules and Molecular Compounds Properties of Molecular Compounds 1. Tend to have relatively lower melting and boiling points Boiling point of water is 100C 2. Most molecular compounds are composed of atoms of two or more nonmetals
Molecular Formulas A molecular formula shows how many atoms of each element a molecule contains. Ex. H2O indicates that every molecule of water consists of 2 Hydrogen and 1 Oxygen Ex. C6H12O6 indicates that every molecule of glucose has 6 Carbon, 12 Hydrogen, and 6 Oxygen Unlike formulas for ionic bonds, a molecular formula reflects the actual number of atoms in each molecule
Molecular Formulas Molecular formulas do not tell you how the molecules are arranged or which atoms are sharing electrons CO2 forms a linear structure while H2O forms a triangular structure The structure depends on which atoms are linked and how many electrons they are sharing Fig. 8.6 p. 216
Section 8-2 Notes The Nature of Covalent Bonding
Octet Rule in Covalent Bonding In forming covalent bonds, electron sharing will occur so that atoms attain the electron configuration of a noble gas. Ex. Hydrogen only has 1 e-, so to reach a noble gas configuration Hydrogen forms a diatomic molecule with another Hydrogen atom (H2) Nonmetallic elements in groups 4A, 5A, 6A, and 7A tend to form covalent bonds by sharing electrons
Single Covalent Bonds Two atoms held together by sharing a pair of electrons are joined by a single covalent bond. Ex. H2 Electron dot structures can illustrate which electrons are shared in the covalent bond Ex. H2 = H:H A structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms Ex. H-H A molecular formula only shows the number and kinds of atoms in the bond Ex. H2
Single Covalent Bonds Halogens also form single bonds in their diatomic molecules Show how Cl forms a diatomic bond A pair of valence electrons that is not shared between atoms is called an unshared pair How many unshared pairs do the following molecules have? H2O, CO, CO2, CH4, NH3
Double & Triple Covalent Bonds Atoms form double or triple covalent bonds if they can attain a noble gas structure by sharing 2 or 3 pairs of electrons Double Bond when 2 pairs of electrons are shared CO2 Triple Bond when 3 pairs of electrons are shared Ex. N2
Section 8-4 Notes Polar Bonds and Molecules
Bond Polarity The character of a bond in a given molecule depends on the kind and # of atoms joined together When bonding pairs of e- pull equally (and therefore are shared equally) a nonpolar covalent bond occurs Ex. H 2, O 2, N 2, Cl 2 A polar covalent bond occurs when there is an unequal sharing of electrons The electronegativity of each atom will determine the pull of the e- The higher the electronegativity, the stronger the pull for the electrons being shared
Bond Polarity Polar Bond cont. HCl hydrogen has an electronegativty of 2.1 while chlorine has an electronegativity of 3.0 Cl will have a stronger attraction to the e- and therefore will have a slightly negative charge H has a weaker attraction to the e- and will have a slightly positive charge The polar nature of a bond will often be represented by an arrow pointing in the direction of the more electronegative atom The electronegativity difference between 2 atoms will determine the type of bond that will form See Table 8.3 p. 238
Polar Molecules The presence of a polar bond makes the entire molecule polar Polar molecules have a slightly negative end and slightly positive end A molecule having 2 poles is called a dipole The effect of polar bonds on the polarity of the entire molecule depends on the shape of the molecule and the orientation of the polar bonds Ex. CO 2 is nonpolar, while H 2 O is polar
Attraction Between Molecules Intermolecular attractions are weaker than either ionic or covalent bonds Two kinds of attractions collectively called Van der Waals Forces 1. Dipole interactions occur when polar molecules are attracted to each other The slightly negative end is attracted to the slightly positive end 2. Dispersion Forces weakest attraction caused by the motion of electrons Occurs when e- momentarily move more to one side of a molecule closest to a neighboring molecule
Attraction Between Molecules Hydrogen Bonds are a form of dipole interactions Occurs when a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom Hydrogen bonds are the strongest of the intermolecular forces
Molecular Properties The physical properties of a compound depend on the type of bonding it displays In most solids formed from molecules, only weak attractions need to be broken resulting in lower melting points A few solids consists of molecules that do not melt until temperatures reach 1000 C or higher Known as network solids which are very stable substances Ex. Diamond