Covalent Bonding. a. O b. Mg c. Ar d. C. a. K b. N c. Cl d. B

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1 Covalent Bonding 1. Obtain the number of valence electrons for each of the following atoms from its group number and draw the correct Electron Dot Notation (a.k.a. Lewis Dot Structures). a. K b. N c. Cl d. B a. O b. Mg c. Ar d. C Review: Valence electrons (the outer most electrons) are responsible for the interaction between atoms when forming chemical compounds. Another way to say that is that valence electrons are the electrons that participate in chemical bonding. The Octet Rule explains that every atom seeks a full valence shell. It is the attaining or loss of valence electrons that will satisfy the octet rule. The question now becomes: How do atoms gain a noble gas electron arrangement, or an octet? Answer: Through Chemical Bonding! A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds them together. Ultimately resulting in a noble gas electron arrangement for all atoms involved. Matter Exists as Atoms, Metals, Ions, or Molecules Nobel gases actually exist as individual atoms. Remember, it is the interactions between the valence electrons of different elements to gain an octet that determine how the element will react with other elements. Nobel gases have an octet, therefore, they do not need to interact with other atoms to be energetically favorable. Matter Exists as Atoms, Metals, Ions, or Molecules We commonly work with pure metals. How is it then that only noble gases exist as atoms when pure metals are very common? Do they not exist simply as atoms? Unfortunately, no; they must shift their electrons around endlessly to gain a pseudo-octet called Metallic Bonding. 1

2 When metal atoms interact, they delocalize their electrons to attain a pseudooctet. Electron-Sea Model describes what is known as metallic bonding: Metals can be thought of as nuclei with core electrons suspended in sea of valence electrons. Attractions hold valence electrons near nucleus, but not so tightly as to impede their flow. Electron-Sea Model This explains properties of metals- Conductivity of heat and electricity Malleability Ductility Extremely strong attractions, or high bond energy Matter Exists as Atoms, Metals, Ions, or Molecules While metal atoms can achieve a pseudo-octet with delocalized valence electrons, allowing for a stable elemental form, metals rarely exist as pure substances. Rather, they exist as ions by completely giving away their valence electrons to non-metal atoms. The transfer of electrons results in the formation of Ions; cations (+) and anions (-). The resulting electrical attraction that then pulls cations and anions together is called ionic bonding forming ionic compounds. Ionic Bonds Here we see the formation of sodium chloride from sodium and chlorine. The ionic compound is formed as the electronegative chlorine atom takes an electron from the sodium atom. Metals vs. Nonmetals The simplest whole-number ration of ions in a compound is called a formula unit. Since the sodium is +1 and the chlorine is -1, the ratio of cations to anions in sodium chloride is 1:1. Therefore, the formula unit for sodium chloride is NaCl; or, one sodium per one chloride ion. When atoms lose or gain electrons, they become ions. Cations are positive and are formed by elements on the left side of the periodic chart (metals). Anions are negative and are formed by elements on the right side of the periodic chart (non-metals). But, not all ionic compounds are formed between ions of equal but opposite charge. 2

3 Observe magnesium in group 2. The octet rule states that all atoms seek the electron configuration of the nearest noble gas, which is neon for magnesium. 12 Mg 2 p y To obtain an octet, magnesium gives up its two valence electrons to be like neon which has an octet in the next lowest energy level. 12 Mg 2 Mg Mg e - The result is a cation with a 2 + charge. 3s 3s Fluorine also wants to be like neon; however, fluorine is electronegative must gain an electron to be like neon forming a 1- anion to obtain its octet. F + 1e - F F F - So, given magnesium forms a 2+ ion and fluorine forms a 1- ion what would be the formula unit for a compound created between magnesium and fluorine? Properties of Ionic Compounds Ionic compounds exist in a regularly repeating geometric arrangement called a crystal lattice. While their crystalline structure is hard, they can be cleaved along plains making them brittle. Properties of Ionic Compounds Ionic compounds have high melting points and are solids at room temperature. MgO, Mg 2+ and O 2- m.p o C NaCl, Na + and Cl -, m.p. 804 o C Properties of Ionic Compounds Some ionic compounds can dissociate in water. Those that do dissolve, conduct electrical current and are called electrolytes. All ionic compounds are solids at room temperature Not all ions are monatomic. Sometimes a group of atoms that are bonded together can gain or loose electrons collectively. The result is a group of bonded atoms with a charge, known as a polyatomic ion. There are 11 polyatomic ions for which you will need to be able to identify the chemical formula, charge and name. Look at your Common Ion Reference and identify the eleven ions you are responsible for We will eventually learn where polyatomic ions come from, but for now, all you need to do is be able to recognize them. 3

4 CH 3 CO 2 - acetate ion Examples of Polyatomic Ions NH 4 + ammonium ion One of the few common polyatomic cations CO 3 2- carbonate ion Matter Exists as Atoms, Metals, Ions, or Molecules While non-metal atoms prefer to take electrons from metal atoms forming ions, this is not always an option. Given that all non-metal atoms seek to gain electrons in order to gain an octet, when non-metal atoms interact with only other non-metal atoms, they are forced to share electrons between them forming a covalent bond resulting in the formation of molecules. Covalent Bonding All atoms seek an octet, or a full valence shell. Non-metal atoms always want to take electrons to fill their valence shells. If a non-metal atom can not take electrons from other elements to fill its octet, it must arrange itself in such a way as to share electrons to fill its valence shell. Each pair of electrons shared by atoms constitutes a single bond. Essentially, the concentration of electron density between the two atoms sharing the electrons holds the atoms together by electrostatic attraction. Non-metals overlap their atomic orbitals in order to share electrons with other non-metals. 9 F 1H Each atom has an octet by sharing The sharing of an electron pair in overlapping atomic orbitals known as a covalent bond. Covalent Bond Formation H + Cl H Cl Overlap of H () and Cl (2p) Note that each atom participating in the formation of the covalent bond has a single, unpaired electron it contributes to the sharing. Covalent Bonding In these bonds atoms share electrons. There are several electrostatic interactions in these bonds: Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei 4

5 Examples of Molecules of More Than One Atom Type Methane Aspirin Some elements can only exist in the elemental form with covalent bonds between atoms. These are called molecular elements: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2, P 4, S 8. Carbon dioxide Boron triflouride Ammonia Or, H-7, P. S. Remember us! Anytime we are describing one of these elements, we must write them as molecules. Let s put this on one of our periodic tables. What determines the bond type between atoms? Electronegativity: The ability of atoms in a molecule to attract electrons to itself. On the periodic chart, electronegativity increases as you go from left to right across a row. from the bottom to the top of a column. When atoms have only low electronegativites neither can give electrons to the other and they must delocalize their electrons to obtain an octet (metallic bonding) forming metals. If the difference in electronegativities is large enough, one atom to remove electrons from another atom (ionic bonding) forming ionic compounds. When atoms have only high electronegativites neither atom can gain an electron from another, they must share electrons in order to obtain a full valence shell (covalent bonding) forming molecules. Covalent Bond Strength Most simply, the strength of a bond is measured by determining how much energy is required to break the bond. This is the energy. The bond energy for a Cl Cl bond, D (Cl Cl), is measured to be 242 kj/mol. Once atoms arrange themselves in a manner that will complete their valence shells, they position themselves in such a way as to achieve the lowest possible energy. The distance between atoms where the energy is minimal is called the bond length. This is why it requires energy to break bonds Bond Energy (kj/mol) H-F 565 H-Cl 427 H-Br 363 H-I 295 Reference 5

6 Double and even triple bonds are commonly observed for C, N, P, O, and S H 2 CO Bond Length Bond length depends on the number of covalent bonds between two atoms. C 2 F 4 SO 3 The GREATER the number of bonds the HIGHER the bond energy and the SHORTER the bond. Bond distances measured using Angstrom units where 1 A = 10-2 pm. Notice, the greater the number of bonds, the shorter the distance Polar Covalent Bonds Although atoms often form compounds by sharing electrons, the electrons are not always shared equally. Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. H F Due to F being so E.N., the electrons of the covalent bond spend more time in the shared orbital near the fluorine atom than the hydrogen atom. This results in the hydrogen having positive character and the fluorine having negative character. Because of the partial charges (γ) on both hydrogen (γ + ) and fluorine(γ - ), the H-F molecule is said to have two poles, or a dipole moment (dipole force). + - Center of Dipole Moment H - F Center of γ + γ - Polar Covalent Bonds The greater the difference in electronegativity, the more polar is the bond. 6

7 Determining Principal Character of Bond Ionic Bond Characters EN covalent ionic ~0 1.7 ~4 The greater the difference in electronegativity between two atoms, the greater the ionic character, or the more likely to be ionic. F -- F H F Li - F + Non-polar covalent even electron distribution Polar Covalent uneven electron distribution Ionic loss and gain of electron The Chemical Formula for molecules, called Molecular Formula, describe the types and numbers of atoms combined in a single Molecule for Molecular Compounds. According to Lewis Theory, there are two types of valence electrons: Non-bonding (or unshared) pairs Bonding single (or unpaired) electrons To determine the number of each type of atom in a molecule, we must investigate the number of valence electrons an atom can share. Boron has three unpaired electrons therefore it can form three covalent bonds Bromine has three unshared pairs and one unpaired electron, therefore it can only form one covalent bond. What about nitrogen? The shared electrons of the covalent bond can be shown using Lewis structures. To draw elementary Lewis structures, the Lewis symbols can be used. If you know a molecule can be made from one carbon and 2 oxygen atoms, Lewis symbols can be used to determine the Lewis structure. C O O Carbon needs four electrons and each oxygen needs two. The only possible arrangement is: O = C = O Use electron dot symbols (Lewis Symbols) to build covalently bonded molecules for the following sets of atoms. 1. Carbon and hydrogen 2. Fluorine and sulfur 3. Chlorine and chlorine 4. Nitrogen and fluorine 5. Oxygen and oxygen 7