7.2 Valence Electrons and Bonding Patterns

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1 7.2 Valence Electrons and Bonding Patterns Only valence electrons form bonds When a chemical bond forms some valence electrons are either shared or transferred between atoms. Only the unpaired valence electrons in an atom participate in chemical bonds. for complex reasons, the 5 th, 6 th, and 7 th valence electrons pair up and reduce the number of electrons available for bonding. The diagram below shows the main group elements along with their paired and unpaired valence electrons. Each unpaired valence electron can form ONE covalent bond In a molecular compound, each unpaired valence electron can form one covalent chemical bond. For example, both nitrogen (N) and phosphorous (P) atoms each have three unpaired valence electrons. In molecular compounds, these elements both form three covalent bonds. In a molecular compound, each unpaired valence electron forms one covalent bond Ion charge and valence electrons When forming a positive ion, each valence electron can contribute 1 unit of positive charge to the ion. This occurs when an atom loses its valence electron(s) to another atom, leaving a charge of +1 for every valence electron lost. For example, sodium (Na) and potassium (K) have a single valence electron and form singly charged ions (+1). Magnesium (Mg) and calcium (Ca) have two valence electrons each. These elements form ions with a charge of +2. In positive ions, each valence electron can contribute 1 unit of positive charge A NATURAL APPROACH TO CHEMISTRY 207

2 Section 7.2 Valence Electrons and Bonding Patterns The octet rule eight valence electrons = chemical stability In Chapter 6 you learned that the noble gasses have eight valence electrons (except for He which has two). These elements form no bonds because they are already at the most stable configuration of electrons. From the noble gasses we infer that elements with eight valence electrons are chemically stable. The observed behavior of the rest of the elements confirms that elements form chemical bonds to achieve the magic configuration of 8 valence electrons. This observation is known as the octet rule. The octet rule states that elements transfer or share electrons in chemical bonds to reach a stable configuration of 8 valence electrons. Elements form bonds to reach 8 valence electrons 2 valence electrons are most stable for H, Li, Be, and B The elements hydrogen, lithium, beryllium, and boron have so few electrons that their version of the octet rule is really based on helium as the closest noble gas. Helium fills the first energy level with its 2 electrons. That means either 0 or 2 valence electrons are also a noble gas electron configuration. The octet rule for hydrogen, lithium, beryllium, and boron is more accurately the duet rule or the rule of 2. These elements form chemical bonds to achieve 2 valence electrons. H, Li, Be, and B form bonds to reach 2 valence electrons Shared electrons are counted by both atoms In a covalent bond, a shared electron gets counted as a valence electron by both atoms. For example, molecular hydrogen (H 2 ) shares electrons to get 2 valence electrons - like helium. In water (H 2 O) each of the two hydrogen atoms shares 1 electron with oxygen giving each hydrogen 2 valence electrons and oxygen 8 valence electrons. octet rule - elements transfer or share electrons in chemical bonds to reach a stable configuration of 8 valence electrons. The light elements H, Li, Be, and B have helium as the closest noble gas so the preferred state is 2 valence electrons instead of A NATURAL APPROACH TO CHEMISTRY

3 Valence electrons and ion formation ionic charge and the periodic table There are two ways in which elements satisfy the octet rule: by sharing electrons in a covalent bonds, or by transferring electrons in an ionic bond. When the difference in electronegativity is larger than 2.1, the chemical bond is ionic. Most elements form some ionic compounds. The table below shows the most common charges for ions of the main group elements. Ion patterns You should see several patterns: 1. Elements on the left side tend to form positively charged ions, and elements on the right side form negatively charged ions; 2. Elements in the middle sometimes form positive and sometimes form negative ions and are marked as variable charge; 3. The alkali metals (group 1) form +1 charged ions because they have one valence electron; 4. The alkali earth metals (group 2) form +2 charged ions because they have two valence electrons; 5. Boron-like elements (group 13) form +3 charged ions because they have three valence electrons; 6. Oxygen-like elements (group 16) form -2 charged ions. These elements have 6 valence electrons. Their easiest path to the octet rule is to gain 2 electrons; 7. The halogens (group 17) form -1 charged ions. These elements have 7 valence electrons. Their easiest path to the octet rule is to gain one electron. A NATURAL APPROACH TO CHEMISTRY 209

4 Section 7.2 Valence Electrons and Bonding Patterns Electron configuration of ions Why sodium forms a +1 ion Why oxygen forms a -2 ion Sodium is an alkali metal with one valence electron. Sodium usually loses its one valence electron to become Na + in order to satisfy the octet rule and have an electron configuration like that of a noble gas. Theoretically, sodium could also gain 7 electrons to become Na -7, but losing one is so much more likely that we do not ever observe Na -7 ions in nature. The electron configuration of Na, Na +, and Ne are shown below. Note that Na + has the same electron configuration as neon. Sodium tends to form Na + ions because this is the lowest energy path by which sodium can satisfy the octet rule and reach a noble gas electron configuration. Oxygen atoms have six valence electrons. In order to have an electron configuration like that of a noble gas, oxygen could either gain two electrons to have the same electron configuration as neon, or lose six electrons to have the same electron configuration as helium. Considering that oxygen has high ionization energy, it is not likely to lose six electrons. Also, oxygen has high electronegativity, so it has the ability to grab electrons from other atoms. For oxygen it makes sense that it will gain two electron rather than lose six. Write the electron configuration for a magnesium ion (Mg 2+ ) Asked: electron configuration of Mg 2+ Given: Mg, atomic #12, charge of +2. Relationships: The electron configuration of magnesium is 1s 2, 2s 2, 2p 6, 3s 2 Solve: Answer: Mg must lose 2 electrons to become Mg 2+. Therefore it loses the pair of 3s 2 electrons. The electron configuration of Mg 2+ is 1s 2, 2s 2, 2p 6, which is identical to neon. 210 A NATURAL APPROACH TO CHEMISTRY

5 Simple ionic formulas Ionic crystals Why calcium chloride has the formula CaCl 2 Ionic substances typically form a crystal, which is a large group of oppositely charged ions arranged in a regular pattern. The calcium chloride (CaCl 2 ) crystal in the diagram is a good example. Calcium chloride is often used to melt ice on roads because it is better for the environment than sodium chloride (also used to melt ice). Ionic crystals are neutral even though they are formed through the attractions of trillions of charged ions. You can have any number of ions in the crystal as long as the positive charges exactly balance the negative charges. To determine the formula of an ionic compound, you need to balance the positive and negative charges. For example, calcium makes a Ca 2+ ion. Chlorine makes a Cl - ion. Each calcium atom loses two electrons and each chloride ion gains only one. This means the compound requires two chloride ions to have the same amount of negative charge as one calcium ion. This makes the ratio of calcium to chlorine 1:2, and the formula is therefore CaCl 2. What is the correct formula for calcium oxide, a compound used in making paper, pottery, and adjusting the ph of soils? Asked: Given: What is the formula for the ionic compound calcium oxide? Calcium oxide is made from calcium and oxygen ions. Calcium forms +2 ions and oxygen forms -2 ions. Relationships: Ca +2 and O -2 must combine in a ratio that will balance out the positive and negative charges. Solve: The charge on one Ca +2 ion will balance out with the charge on one O -2 ion. Therefore the ratio is 1:1 and the formula is CaO. A NATURAL APPROACH TO CHEMISTRY 211

6 Section 7.2 Valence Electrons and Bonding Patterns Covalent bonds Covalent bond formation In covalent bonds, electrons are shared between atoms, not transferred. The number of covalent bonds is equal to the number of unpaired valence electrons. The diagram below shows the number of covalent bonds by the main group elements. Covalent bond patterns Paired and unpaired electrons 1. Only non-metals and hydrogen are commonly found as covalently bonded parts of molecules. 2. Carbon-like elements (group 14) form four covalent bonds. 3. Nitrogen-like elements (group 15) form three covalent bonds. 4. Oxygen-like elements in (group 16) form two covalent bonds. 5. Halogens (group 17) form one covalent bond. 6. The number of covalent bonds is equal to the number of unpaired valence electrons. Going from left to right across the periodic table, notice that nitrogen has three unpaired electrons and one pair. The 8 electrons in s and p orbitals act like 8 strangers filling up four bench seats on a bus. Everyone prefers their own seat so up to four, each person sits solo. The fifth person must pair up with someone. The same is true of electrons. The fifth valence electron pairs up and is no longer available for bonding! For reasons of quantum mechanics, only unpaired electrons form bonds. 212 A NATURAL APPROACH TO CHEMISTRY

7 Atoms form bonds with every unpaired electron Every unpaired electron forms a bond Take a look at the molecule of vitamin C to the right. This molecule is made from several carbon, oxygen, and hydrogen atoms. Take a close look at each carbon atom. You will see that every carbon atom has four covalent bonds to other atoms. The oxygen atoms all from two covalent bonds, and the hydrogen atoms all form one covalent bond. Covalent bonds and noble gases Carbon has four valence electrons and they are all unpaired. Therefore carbon forms four chemical bonds. Oxygen has six valence electrons. Four of these electrons are paired and two are unpaired. Oxygen always forms two covalent bonds because it has two unpaired electrons. Hydrogen has one valence electron, and its closest noble gas is helium with two valence electrons. Hydrogen needs one more electron to be like helium. It gets this by forming one covalent bond with another atom. Unpaired electrons create very reactive atoms or molecules One quick way to tell how many covalent bonds an atom will form is to look at its Lewis dot structure. Atoms will form one covalent bond for each unpaired valence electron. Atoms or molecules that have unpaired electrons are highly reactive and are known as free radicals. These are the kind of molecules which can be responsible for aging and diseases like cancer. Sometimes free radicals are created as part of our own natural metabolism, and sometimes they are caused by outside sources like ultraviolet radiation from the sun. Antioxidants are considered an important part of a person s diet because of their role in preventing free radicals from reacting with and damaging DNA. free radical - a molecule or atom that is highly reactive due to its having one or more unpaired valence electrons. antioxidant - a molecule that reacts easily with free radicals. Some sources of antioxidants include brightly colored fruits and vegetables, vitamin E, and chocolate. A NATURAL APPROACH TO CHEMISTRY 213