Chemical Bonding. Chemical Bonds. Metals, Ions, or Molecules. All Matter Exists as Atoms,

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1 Chemical Bonding 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 like that of the nearest noble gas. It is the rearranging of valence electrons that will satisfy the octet rule. Chemical Bonds Atoms acquire octets to become more stable by forming chemical bonds Metallic Bonding involves delocalizing metal electrons Ionic bonding involves loss of electrons by a metal gain of electrons by a nonmetal Covalent bonding involves a sharing of electrons All 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 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. All 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? Unlike noble gases, metals do not have an octet, therefore, multiple atoms must interact to be energetically favorable. 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. 1

2 All Matter Exists as Atoms, Metals, Ions, or Molecules We have seen how metal atoms can share delocalized electrons to achieve a pseudo-octet with delocalized valence electrons, allowing for a stable elemental form. However, metals rarely exist as pure substances. Rather, they exist as ions by completely giving away their valence electrons to non-metals. Metals Form Positive Ions Metals form positive ions because of their low ionization energy by a loss of their valence electrons with the electron configuration of their nearest noble gas that have fewer electrons than protons. Group 1A (1) metals ion + Group 2A (2) metals ion 2+ Group 3A (13) metals ion 3+ 8 Positive Ions: Loss of Electrons A sodium atom (Na) will lose its 3s 1 electron to form a sodium ion (Na + ). Formation of Negative Ions In ionic compounds, nonmetals because of their high ionization energy by a gain of valence electrons achieve an octet arrangement with the electron configuration of their nearest noble gas form negatively charged ions with 3, 2, or 1 charges Positively charged ions of metals are called cations. 10 Negative Ions: Gain of Electrons An atom of chlorine with seven valence electrons gains one electron to form an octet. Because it now has 18 electrons and not 17 electrons, it becomes a chloride ion (Cl ) with a charge of 1. Negatively charged ions of nonmetals are called anions. However, Ions can not exist independently as ions. They must be electrically balanced by an ion(s) of opposite charge. Ionic compounds are compounds composed of metal cations and non-metal anions that are electrically neutral. The electrostatic force of attraction between ions in an ionic compound are called ionic bonds. 2

3 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. Ionic Compounds Ionic compounds consist of positive and negative ions have high melting and boiling points are solids at room temperature are composed of ions in a regularly repeating pattern of ions called a crystal lattice 14 Since the sodium is +1 and the chlorine is -1, the ratio of cations to anions in sodium chloride is 1:1. But, not all ionic compounds are formed between ions of equal but opposite charge. Charge Balance In MgCl 2 In MgCl 2 a Mg atom loses two valence electrons two Cl atoms each gain one electron subscripts indicate the number of ions needed to give charge balance The simplest whole-number ration of ions in a compound is called a formula unit. Therefore, the formula unit for sodium chloride is NaCl; or, one sodium per one chloride ion. 16 Charge Balance In Na 2 S In Na 2 S, two Na atoms lose one valence electron each one S atom gains two electrons subscripts show the number of ions needed to give charge balance Formula from Ionic Charges Write the ionic formula of the compound containing Ba 2+ and Cl. Write the symbols of the ions (cation is always first). Ba 2+ Cl Balance the charges. Ba 2+ Cl Cl two Cl needed Write the ionic formula using a subscript 2 for two chloride ions that give charge balance BaCl 2 3

4 Naming of Ionic Compounds In the name of an ionic compound, the positive ion (first ion) is named as the element the negative ion (second ion) is named by changing the end of the element name to ide Examples of Ionic Compounds with Two Elements Formula Ions Name Cation Anion NaCl Na + Cl sodium chloride K 2 S K + S 2 potassium sulfide MgO Mg 2+ O 2 magnesium oxide CaI 2 Ca 2+ I calcium iodide Al 2 O 3 Al 3+ O 2 aluminum oxide Transition Metals That Form Two or More Positive Ions Most transition metals and Group 4 (14) metals form two or more positive ions Zn 2+, Ag +, and Cd 2+ form only one ion Examples: Copper forms Cu + and Cu 2+ Iron forms Fe 2+ and Fe 3+ Gold forms Au + and Au 3+ Metals with Variable Charge Periodic Table and Typical Ions Examples of Names of Compounds with Variable Charge Metals Transition metals with two different ions use a Roman numeral after the name of the metal to indicate the ionic charge only zinc, silver, and cadmium do not use a Roman numeral because they form only one ion (Zn 2+, Ag +, and Cd 2+ )

5 Naming FeCl 2 STEP1 Determine the charge of the cation from the anion. Fe (?) + 2Cl = Fe (?) + 2(1 ) = 0 Fe (?) = 2+ = Fe 2+ STEP 2 Name the cation by its element name and use a Roman numeral in parentheses for the charge. Fe 2+ = iron(ii) Naming FeCl 2 (continued) STEP 3 Name the anion by using the first syllable of its element name followed by ide. chloride STEP 4 Write the name of the cation first and the name of the anion second. iron(ii) chloride = FeCl Naming Cr 2 O 3 STEP1 Determine the charge of the cation from the anion. 2Cr(?) + 3O 2 = 2Cr (?) + 3(2 ) = 0 2Cr(?) = 6+ Cr(?) = 3+ = Cr 3+ STEP 2 Name the cation by its element name and use a Roman numeral in parentheses for the charge. Cr 3+ = chromium(iii) Naming FeCl 2 (continued) STEP 3 Name the anion by using the first syllable of its element name followed by ide. oxide STEP 4 Write the name of the cation first and the name of the anion second. chromium(iii) oxide Writing Formulas Write the formula of potassium sulfide. STEP1 STEP 2 STEP 3 Identify the cation and anion. potassium = K + sulfide = S 2 Balance the charges. K + K + S 2 2(1+) + 2(1 ) = 0 Write the formula, cation first, using the subscripts from the charge balance. 2 K + and 1 S 2 = K 2 S Writing Formulas Write the formula of cobalt(iii) chloride. STEP1 STEP 2 STEP 3 Identify the cation and anion. cobalt (III) = Co 3+ (III = charge of 3+) chloride = Cl Balance the charges. Co 3+ and 3Cl = (3+) + 3(1-) = 0 Write the formula, cation first, using the subscripts from the charge balance. 1 Co 3+ and 3 Cl = CoCl

6 Polyatomic Ions A polyatomic ion is a group of atoms has an overall ionic charge Examples: NH 4 + ammonium OH hydroxide NO 3 nitrate NO 2 nitrite CO 2 3 carbonate PO 3 4 phosphate HCO 3 hydrogen carbonate (bicarbonate) Names and Formulas of Common Polyatomic Ions Prefixes for Names of Polyatomic Ions of Halogens Some polyatomic ions of the halogens require prefixes. ClO 4 perchlorate one oxygen more ClO 3 chlorate most common form ClO 2 chlorite one oxygen less ClO hypochlorite two oxygens less Writing Formulas with Polyatomic Ions The formula of an ionic compound containing a polyatomic ion must have a charge balance that equals zero(0) Na + and NO 3 NaNO 3 with two or more polyatomic ions encloses the polyatomic ions in parentheses Mg 2+ and 2NO 3 Mg(NO 3 ) 2 subscript 2 for charge balance Name the following compounds: A. Ca 3 (PO 4 ) 2 B. FeBr 3 C. Al 2 S 3 D. Mn(NO 2 ) 2 E. NaHCO 3 Write the formulas for the following: A. calcium nitrate B. iron(ii) hydroxide C. aluminum carbonate D. copper(ii) hypobromite E. lithium phosphate

7 All Matter Exists as Atoms, Metals, Ions, or Molecules So: Nobel gases have an octet and can exist as atoms in nature. Metal atoms can share delocalized electrons to achieve a pseudo-octet with delocalized valence electrons, allowing for a stable elemental form. Metals can also loose electrons to highly electronegative non-metals forming ions resulting in an ionic compound. But, what happens when non-metals bond with other non-metal atoms and not metal atoms? Things to consider: Since all non-metals are electronegative, no one nonmetal atom can remove electrons from another; Therefore, no ions will form. All atoms still require a full valence shell, or an octet. If the difference in electronegativities is not large enough for one atom to remove electrons from another atom (ionic bonding), and they can not delocalize their electrons to obtain an octet (metallic bonding), the atoms will have to share electrons in order to obtain an octet. H 2, A Covalent Molecule In a hydrogen (H 2 ) molecule, two hydrogen atoms share electrons to form a covalent single bond each H atom acquires two (2) electrons each H becomes stable like helium (He) The sharing of electrons in order to obtain a full valence shell in known as covalent bonding. 40 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 Each atom seeks and octet, or a full valence shell. If an 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. 7

8 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 one oxygen, 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 According to Lewis Theory, there are two types of valence electrons: Non-bonding (or unshared) pairs Bonding single (or unpaired) electrons 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? 1. Use electron dot symbols (Lewis Symbols) to build covalently bonded molecules for the following sets of atoms. N and 3 Cl s O and S N and N C and 4 H s Diatomic Elements These elements share electrons to form diatomic, covalent molecules. H-7, P. S. Remember us! 46 Because molecules do not contain ions, there are no charges to determine the formulas for the compounds. No Ions! Therefore, in order to name molecules we must use prefixes to denote the number of atoms in a molecular compound. Names of Covalent Compounds Prefixes are used in the names of covalent compounds because two nonmetals can form two or more different compounds Examples of compounds of N and O: NO nitrogen oxide NO 2 nitrogen dioxide N 2 O dinitrogen oxide N 2 O 4 dinitrogen tetroxide N 2 O 5 dinitrogen pentoxide 48 8

9 Naming Molecular Compounds List atoms in order of increasing electronegativity Write the name of each covalent compound: Use prefixes to denote the number of atoms in the molecule. (mono-, di-, etc ) CO CO The last atom is given an ide suffix 2 Drop o or a from prefix PCl 3 Molecular Prefixes: for atoms beginning with a CCl 1 mono- 7 hepta- 4 vowel 2 di- 8 octa- N 2 O Omit the prefix mono- when 3 tri- 9 nonathere is only one of the first (least electronegative) atom. 4 tetra- 10 deca- Mono- is sometimes omitted 5 penta- 11 undecafrom the second atom in 6 hexa- 12 dodeca- 50 diatomic molecules. Write the correct formula for each of the following: A. phosphorus pentachloride B. dinitrogen trioxide Identify each compound as ionic or covalent and give its correct name. A. SO 3 B. MnCl 2 C. (NH 4 ) 3 PO 4 D. Cu 2 CO 3 C. sulfur hexafluoride E. N 2 O Name each of the following ionic or molecular compounds. A. BF 3 B. OCl 2 C. Al 2 O 3 D. Ni 3 PO 4 9