Chemistry 20 notes molecular compounds BIG IDEA: A covalent bond forms when nonmetal atoms share one or more pairs of electons with one another Can be solid, liquid or gas at SATP (Standard Ambient Temperature and Pressure: 25 o C and 100 kpa) Atoms in molecular compounds are held together by covalent bonds (meaning 2 valence energy levels are involved) Covalent bonds are formed by the sharing of valence electrons between two nonmetals. A covalent bond results from the simultaneous attraction of two nuclei for a shared pair of valence electrons. o Example: Cl 2 The electrons involved in the covalent bond spend part of their time around one atom and the remainder of the time around the other atom. As some non-metals have more than one single (unpaired) valence electron, there can exist more than one covalent bond between some nonmetals. You could also have more than one kind of nonmetal covalently bonded together. Double bonds form when two nonmetals share 2 pairs of bonding electrons (4 e - total) Triple bonds form when two nonmetals share 3 pairs of bonding electrons (6 e - total) A molecular compound, if large enough, may contain a combination of single, double or triple bonds. o Example 1: single bond o Example 2: double bond o Example 3: triple bond Chemistry 20 Lesson 1-03 Page 1
The above lewis diagrams can be drawn as structural diagrams, showing only the bonds between atoms, where each shared pair of electrons is represented by a line. o Examples: The bonding capacity of an atom is the total number of single covalent bonds that it can form. This is equal to the number of single valence electrons o Examples Lewis structure Example Bonding capacity ydrogen atom Group 14 atoms Group 15 atoms Group 16 atoms Group 17 atoms When drawing diagrams (lewis or structural) of molecules, the atom(s) with the highest bonding capacity are placed in the center. These are called the central atom(s) and are single bonded together to start. All remaining atoms are then bonded to the central atom(s). If bonding capacities are not filled (single electrons remain) you need to rearrange the atoms. It is usually easiest to start with Lewis diagrams. o Example 1: ammonia, N 3 Proper Lewis diagrams help show the correct molecular shape of a molecule Atoms are arranged around the central atom in such a way as to maximize the space around each atom. Note: lone pairs occupy the same amount of space as an atom. Chemistry 20 Lesson 1-03 Page 2
Coordinate covalent bonds. Both oxygen and nitrogen have lone pairs of electrons: These non-bonding electron pairs can, in acidic conditions, form an additional covalent bond with a hydrogen ion ( + ). Remember that acids produce + in solution. A coordinate covalent bond will form because + is just a proton. ydrogen atoms have only 1 electron, which is lost to form hydrogen ion, and no neutrons. This makes a hydrogen ion a very concentrated positive charge. The non-bonding electron pairs on the very electronegative atoms of oxygen and nitrogen only are strong concentrated negative charges. The two opposite charges attract each other allowing a hydrogen ion to share the non-bonding electron pair. Lesson Summary ow does one nonmetal atom bond to another nonmetal atom in a molecular compound? Molecular compounds are made of nonmetal atoms that are covalently bonded. A covalent bond is a bond in which a pair of valence electrons is shared by two atoms. The valence electrons that are involved in bonding are called a bonding pair. The valence electrons that are paired up in a molecule, but do not take part in bonding, are called lone pairs (or nonbonding pairs). Key Terms Covalent bond Bonding pair Lone pair Double bond Triple bond Lewis diagram Structural diagram Bonding capacity Central atom Chemistry 20 Lesson 1-03 Page 3
Chemistry 20 Formative Problem Set Lewis Diagrams for Molecules Consistent with the octet rule, atoms share an appropriate number of electrons to attain eight valence electrons the stable electron configuration characteristic of the noble gases (ydrogen, an important exception to the octet rule, tends to attain two valence electrons the structure characteristic of the noble gas helium). Rules for Drawing Lewis Diagrams of Molecules. 1. Draw the Lewis electron-dot diagram for one of each of the atoms in the molecule. 2. Upaired electrons (called bonding electrons) are available for sharing to form a covalent bond. 3. Paired electrons (called either nonbonding electron pairs or lone pairs) do not partake in bonding. They still occupy space, however. nonbonding e - (lone pair) N 4. The atom with the most bonding electrons (called the central atom) is placed in the centre with the other atoms bonded to it. 5. All atoms (and lone pairs) must be arranged around central atoms so that they are as far away from each other as possible. a. Note: lone pairs of electrons occupy space just like an atom. bonding e - 6. In the resulting diagram all electrons must be paired and each atom, except, must be surrounded by an octet of electrons. Electrons shared between atoms are drawn between the two atomic symbols and thus may appear to be out of place. Example: C 3 Cl atom C C atom central atom Cl Cl atom become C Cl Bonding representation is often simplified by omitting the nonbonding electrons pairs and by substituting a line for each bonding pair in the molecule. The resulting drawing is called a structural diagram. Structural diagrams are used to visualize the bonding that occurs within a molecule and also to give some idea about the shape of the molecule. Example: NI 3 I N I I becomes I N I I Atoms which have two or more bonding electrons may share two or three of these bonding electrons with the same atom. If two electrons pairs are shared between two atoms, the bond is called a double bond. Three electron pairs shared between two atoms are called a triple bond. Example: 2 CO becomes C O C O with a structural diagram of Chemistry 20 Lesson 1-03 Page 4
Complete the following table. Molecular Lewis diagrams of formula individual atoms Lewis diagram of molecule (start with central atom) Structural diagram of molecule EXAMPLE CO 2 CN C 2 F 4 C 2 Cl 2 2 O Cl Cl 2 C 2 6 Chemistry 20 Lesson 1-03 Page 5
Complete the following table by predicting the simplest molecule (fewest central atoms) possible using only the listed atoms. Atoms may appear more than once in the molecule. Atoms to be combined Lewis diagrams of single individual atoms Lewis diagram of molecule (start with central atom) Structural diagram of molecule C & atoms N & I atoms O & F atoms P & Br atoms Chemistry 20 Lesson 1-03 Page 6