Unit 11 Bonding INTRODUCTION Within molecules, there are forces that hold atoms together These forces are called bonds There are different types of bonds, or more correctly, variations Identifying the type of bonding involved in a molecule will allow us to predict certain general properties of a compound The making and breaking of bonds is also an important aspect of biochemistry, the study of the compounds and their reactions that are parts of all living things A working knowledge of bonding is essential in any study of chemistry OBJECTIVES 1 The student will describe the different types of bonds and the forces that make them up 2 The student will predict the type of bond holding atoms together in a molecule 3 The student will draw the Lewis Dot structures for atoms and simple molecules, employing the Octet rule and the Lewis Dot Theory DISCUSSION A The Bond A bond is the force that holds atoms together in a molecule This force is electrostatic It is the result of the attraction between electrons and the nuclei of the atoms involved in the bond Electrons are negatively charged while the nucleus has a positive charge due to the protons The electrons involved in the bond are only the outer shell or valence electrons These electrons are less tightly held by the nucleus and are available to be shared by, or transferred to another atom In bonding we are only concerned with the atom and the number of valence electrons it has We will deal only with compounds whose atoms come from groups 1a through 7a and 0, and generally with only the first four periods The transition metals, Lanthanides and Actinides, present special considerations in bonding that will be dealt with in Chemistry 102
B Lewis Dot Theory An early bond theory that is still useful today is the Lewis Dot Theory In this theory, bonds are formed as the result of the sharing of valence electrons of the atoms involved in the bond Enough electrons are shared between atoms so that each atom will have eight electrons in its outer shell This criterion for a bond of having eight electrons in the outer shell after the bond or bonds are formed is called the Octet Rule The rules of the Lewis Dot Theory and the Octet Rule are the following 1 The number of valence electrons is determined by the group number the atom is in Groups 1a through 7a have 1 through 7 valence electrons respectively 2 The Lewis Dot Structure for an atom is the symbol for the element with dots (or x s or o s) representing the number of valence electrons The atoms of the 2 nd period would have the following Lewis Dot Structures Group 1a 2a 3a 4a 5a 6a 7a 0 Lewis Dot Structure Li Be: B: C N: O: F: :Ne: 3 In a compound the atoms are first written out to show the number of valence electrons each has 4 The Lewis Dot Structure for the molecule is determined by sharing or transferring sufficient electrons from each atom so that in the resulting molecule each atom has an octet or eight electrons in its outermost shell 5 A pair of electrons represents one single bond Two pairs of electrons represent a double bond and three pairs of electrons represent a triple bond The electrons involved in a bond must always be written between the two elements involved A dash may be used to represent a pair of electrons as a shorthand notation Thus: single bond :: = double bond MM triple bond 6 There may be electrons on each atom not involved in the bond, but still they are valence electrons They are recorded as dots, but not between atoms They are shown at either end of the molecule The following sections will demonstrate how the Lewis Dot Theory is used to show bonding for some different types of bonds
C Covalent Bonds A covalent bond is the result of the equal sharing of valence electrons in the bond between two atoms The only time that the electrons involved in a bond can be equally shared is when the two atoms are the same In this case both atoms will have the same electronegativity or pull for electrons in the bond A covalent bond of this type will have electrons being attracted or pulled to both nuclei equally The following compounds have bonds that are completely covalent H 2, N 2, O 2, F 2, Cl 2 and Br 2 Let us look at the bonding in the molecules Hydrogen (H 2 ) This molecule violates the Octet Rule, because there is only one valence electron per atom Writing the Lewis Dot Structure is approached in the following manner a) Lewis Dot Structures a written for both atoms b) Atoms are combined to form a bond c) The shorthand notation is used for the bond A single bond holds a molecule of Hydrogen together
Fluorine (F 2 ) The Lewis Dot Structures for Cl 2 and Br 2 would be identical because they are elements of the same group The Lewis Dot Structure would be determined in the following fashion a) Lewis Dot Structures for the atoms b) Atoms are combined to form a bond and obey the Octet Rule The two electrons between the atoms that form the bond are counted with both atoms because they are shared The six electrons on each atom not in the bond are involved called non bonding electrons c) The formula is written in shorthand A molecule of Fluorine is held together by a single bond Oxygen (0 2 ) The Lewis Dot Structure for molecular Oxygen (the Oxygen necessary for life) is determined in the following fashion a) Lewis Dot for the atoms b) The only way in which the Octet Rule can be obeyed is to have each atom contribute two electrons to the bond c) The shorthand notation is A molecule of Oxygen is held together by a double bond Each atom of Oxygen has two pairs of non bonding electrons
Nitrogen (N 2 ) a) Lewis dots for atoms b) The Octet Rule is obeyed if each atom contributes three electrons to a triple bond c) The shorthand notation is A molecule of Nitrogen is bound together by a triple bond A triple bond is stronger and shorter than double bonds, that are stronger and shorter than single bonds The molecules that we just encountered are all said to be non polar This means that there is no build-up of electrons on either end of the molecule The reason they are non polar is because they are all homonuclear diatomic molecules A homonuclear diatomic molecule is just one with two atoms bound to one another, where the two atoms are the same D Polar Covalent Bonds A covalent bond in which the two atoms bound together are not the same as a polar covalent bond In molecules of this type the two elements (or more) are from groups 3a through 7a and Hydrogen These elements have very similar electro negativities We treat their Lewis Dot Structure in the same fashion as we did for the non polar covalents but recognize that these bonds are polar due to the difference in electronegativity between the atoms The more electronegative atom has a greater share in the electrons resulting in a slight build-up of negative charge on it The other atom looses control of the electrons to some extent and becomes positive because electrons are pulled from it stripping the nucleus of negative electrons and exposing some of its positive charge Polar covalent compounds are much more numerous that non polar covalent compounds, because of the greater number of possibilities
Example Problem (1) Solution Draw the Lewis Dot Structure for Carbon Monoxide (CO) Indicate which atom bears a partial negative charge and which a partial positive due to differences in electronegativity a) Lewis Dot for the atoms b) The Octet Rule is obeyed by sharing three electrons from each atom c) The shorthand notation is d) Polarity of the bond is determined by noting which element is more electronegative The Oxygen atom carries a partial negative charge because it is more electronegative and has more of pull for the electrons in the triple bond + end end
Example Problem (2) Solution Draw the Lewis Dot Structure for Water (H 2 O) a) Lewis Dot for the atoms b) Hydrogen needs only one electron to complete its outer shell, while Oxygen can obey the Octet Rule by sharing one from each Hydrogen c) The shorthand is E Ionic Bonds The two O-H bonds in water are polar covalent bonds because of the difference in electronegativity between O and H Oxygen is more electronegative, therefore is bears a small negative charge In an ionic bond electrons are said to be transferred from one atom to another This is actually saying they are unequally shared Ionic bonds result when elements from groups 1a or 2a are bound to elements from groups 5a, 6a and 7a In these instances there is a large difference in electronegativity This is a condition for an ionic bond Ionic compounds may contain any of the radicals we encountered in the previous unit The elements of groups 1a and 2a always lose electrons to elements of groups 5a, 6a and 7a This results in a compound that contains a cation and an anion, thus the same ionic compound By loosing electrons the elements of groups 1a and 2a obey the Octet Rule This loss results in the outer shell being the next lower level that was filled anyway The elements of groups 5, 6 and 7a gain 3, 2 and 1 electrons respectively to achieve their oclets
Example Problem (3) Solution Draw the Lewis Dot Structure for Sodium Chloride (NaCl) a) Lewis Dot for the atoms b) Sodium losses its one electron to Chlorine c) Sodium becomes a cation, Chlorine becomes the Chloride anion This is the Lewis Dot Structure for Sodium Chloride Example Problem (4) Solution Draw the Lewis Dot Structure for Calcium Fluoride (Ca F 2 ) a) Lewis Dots for atoms b) Calcium looses one electron each to the Fluorines c) Calcium becomes a cation the Fluorines become two Fluoride anions This is the Lewis Dot Structure for Calcium Fluoride There is no formula for determining Lewis Dot Structure You must follow the guidelines given and try variations until the Octet Rule is fulfilled, except in the case of Hydrogen
PROBLEMS 1 Draw Lewis Dot Structure for the following atoms: a) I b) Se c) As d) Si e) Ga f) Ba g) Cs 2 Draw Lewis Dot Structures for the following non polar compounds a) P 3 b) Cl 2 c) P 4 3 Draw Lewis Dot Structures for the following polar covalent compounds a) NH 3 b) CO 2 (carbon between two Oxygens) c) H 2 Se d) HF