Chemical Bonding Chapter 12
Ionic and Covalent Bonds Who does what Ionic Bonds occur between a metal cation and a non-metal anion, a metal gives electron(s) to a non-metal. Example: Na Cl Fe O Covalent Bonds occur between 2 nonmetals, electrons are shared by 2 nuclei. Example: CH 4 O = O
Figure 12.1: The formation of a bond between two atoms. In an H 2 molecule, the electrons reside primarily in the space between the 2 nuclei. Each H atom in the H 2 molecule pulls the electrons equally based on the electronegativity of H. We call this type of bond nonpolar covalent Company 12-3
Figure 12.2: Probability representations of the electron sharing in HF. When we look at two different atoms, we see that the electrons are not shared equally, such as between H and F. The fluorine has a stronger attraction than the hydrogen for the shared electrons. We call this bond polar covalent. Company 12-4
Dipole Moments Any diatomic (2 atom) molecule that has a polar bond has a dipole moment. When a molecule has a center of positive charge and a center of negative charge, we call this a dipole moment Figure 12.5: Charge distribution in the water molecule. Company 12-5
Figure 12.6: Polar water molecules are strongly attracted to positive ions by their negative ends. Company 12-6
Figure 12.6: Polar water molecules are strongly attracted to negative ions by their positive ends. Company 12-7
Figure 12.4: The three possible types of bonds. a) nonpolar covelant (equal sharing of electrons b) polar covelant (unequal sharing of electrons c) ionic (giving away and taking of electrons) Company 12-8
Table 12.2 When we look at the electron configuration of atoms becoming ions, we see that they are trying to achieve a noble gas configuration. Company 12-9
General Rules Ionic bonds: Metals will become positive ions, they will lose electrons to achieve the electron configuration of the previous noble gas before them. Non-metals will become negative ions, they will gain electrons to achieve the electron configuration of the next noble gas. Covalent bonds: Atoms in covalent bonds are sharing electrons, but all the same, they want to have noble gas configurations in their outer valence shells so they share electrons to achieve this. Company 12-10
Practice Determine the electron configuration for the following elements, and then determine who will gain and who will lose electrons in an ionic bond. S and Mg Al and O Company 12-11
Ion Size vs Atom Size When atoms become ions, the size of each changes. A cation(+) is always smaller than the parent atom. An anion (-) is always larger than the parent atom. To explain this, think about who has lost electrons and who has gained electrons. Company 12-12 Figure 12.9: Relative sizes of some ions and their parent atoms.
Lewis Structures We must look at how many valence electrons an atom has to determine the Lewis structure of an atom. Example: Fluorine How many valence electrons? Draw 1 dot for each valence electron around the symbol for the element. We draw the electrons in pairs on each of the 4 sides of the element symbol. F Company 12-13
Practice Draw the Lewis structures for the following elements: He Al H Mg O K C N S Ar Company 12-14
Lewis Dot Diagrams and Bonding Ionic Bonds let s look at KBr How many valence electrons do K and Br have? Who is going to gain and lose electrons in this bonding interaction? Once these 2 atoms have bonded in an ionic bond, each has gained or lost electrons, we would write the Lewis dot diagram for these as following K + [::Br::] - Now you try, write the Lewis dot diagram for NaCl Company 12-15
Lewis Dot Diagrams and Bonding Covalent bonding atoms have to share electrons to make them happy The most important requirement for the formation of a stable compound is that the atoms achieve noble gas electron configurations. Let s look at OCl 2 How many valence e -? That s not enough to gain or steal, so what if they share 2 pairs of electrons. Company 12-16
Bond Types Single bond involves two atoms sharing one electron pair. Example: F-F Double bond involves two atoms sharing two pairs of electrons. Example: O=O Triple bond involves two atoms sharing 3 pairs of electrons. Example: N N Remember in each case, each atom wants a noble gas configuration with 8 outer valence electrons. Not all the electrons involved maybe bonded, you may find you have a lone pair of electrons, a pair of electrons that are not bonded. Company 12-17
Practice Use Lewis dot diagrams for the following atoms to determine the bonds: CH 4 HF NH 3 N 2 O 2 SO 2 HINT: Some may have multiple bonds. Company 12-18