IV. Chemical Bonding

Dots represent valence electrons Dots represent valence electrons. Everything else (inner shell electrons and nucleus) is called the Kernel and is represented by the symbol. Phosphorous has 5 valence electrons so we draw 5 dots around the symbol for phosphorous. J Deutsch 2003

It does not matter which side you start from. The correct order to fill in the dots is to make a pair from the first 2 electrons then we fill in one on each side before we pair up. 1 2 Ne 5 3 8 6 4 7 It does not matter which side you start from. J Deutsch 2003

The correct order to fill in the dots is to make a pair from the first 2 electrons then we fill in one on each side before we pair up. 5 8 Ne 1 7 4 2 6 3 This will let you know how many electron pairs and how many unpaired electrons are in the atom’s valence shell J Deutsch 2003

Draw the Lewis Dot Structures of the first 18 elements. J Deutsch 2003

What is a chemical bond? When an atom gains, loses or shares electrons to complete their outer shell (octet) J Deutsch 2003

Atoms attain a stable valence electron configuration by bonding with other atoms. Noble gases have stable valence configurations and tend not to bond. J Deutsch 2003

Metals tend to react with nonmetals to form ionic compounds Metals tend to react with nonmetals to form ionic compounds. Nonmetals tend to react with other nonmetals to form molecular (covalent) compounds. Ionic compounds containing polyatomic ions have both ionic and covalent bonding. J Deutsch 2003

Two major categories of compounds are ionic and molecular (covalent) compounds. Ionic compounds are formed when a metal combines with a nonmetal. Ionic compounds have ionic bonds. Molecular compounds are formed between two or more nonmetals. Molecular compounds have covalent bonds. J Deutsch 2003

Chemical bonds are formed when valence electrons are : Ionic: transferred of electrons from a metal to a non metal Covalent: shared between two or more non metals Metallic: mobile electrons within a metal J Deutsch 2003

Polyatomic ions are groups of atoms covalently bonded together that have a negative or positive charge. J Deutsch 2003

Polyatomic ions are held together by covalent bonds but form ionic bonds with other ions. + Ionic bond H - Covalent bonds H N H Cl H J Deutsch 2003

The bonds holding metals together in their crystal lattice are called metallic bonds. All metals have metallic bonds “Positive ions immersed in a sea of mobile electrons” Bonds are between Kernels, leaving the valence electrons free to move from atom to atom Mobile electrons give metals the ability to conduct electricity J Deutsch 2003

Magnesium atom Magnesium ion When metals lose electrons to form ions, they lose all their valence electrons. The Lewis Dot Structure of a metal ion has no dots. The charge indicates how many electrons were lost. Magnesium atom Magnesium ion J Deutsch 2003

When nonmetals gain electrons, they fill up their valence shell with a complete octet (except hydrogen.) The ion is placed in brackets with the charge outside the brackets. J Deutsch 2003

When a bond is formed, energy is released When a bond is formed, energy is released. Energy is needed to break a bond J Deutsch 2003

The desire for eight valence electrons in the outer shell What is the octet rule? The desire for eight valence electrons in the outer shell J Deutsch 2003

Regents Question: 06/02 #3 þ Which compound contains ionic bonds? NO CaO (4) CO2 Nitrogen – nonmetal Oxygen – nonmetal Calcium – metal Carbon – nonmetal þ J Deutsch 2003

Regents Question: 08/02 #11 Which formula represents an ionic compound? (1) NaCl (2) N2O (3) HCl (4 )H2O þ J Deutsch 2003

What type of bonding would sodium and chlorine have? IONIC How do you know? Sodium is a metal and chlorine is a non metal J Deutsch 2003

Compounds with Ionic bonds have the following properties. - Solids with high melting and boiling points (strong attraction between ions) Crystalline solids that form regular geometric patterns Hard Electrolytes: able to conduct electricity Do not conduct electricity as solids but do when dissolved or molten – ions are charged particles that are free to move J Deutsch 2003

Ionic solids conduct electricity when dissolved or molten Ionic solids conduct electricity when dissolved or molten. Molecular solids do not. Solution conducts electricity Solution doesn’t conduct electricity Ionic Solid dissolved in water Molecular Solid dissolved in water J Deutsch 2003

Fluorine is assigned the value 4.0 – the highest of any element Electronegativity values are assigned according to arbitrary scales. (5.2j) Fluorine is assigned the value 4.0 – the highest of any element Nonmetals have high electronegativity – they want to attract electrons so they can fill their valence shell Metals have low electronegativity – they want to lose electrons to get rid of their valence shell J Deutsch 2003

Electronegativity indicates how strongly an atom of an element attracts electrons in a chemical bond. J Deutsch 2003

Ionic bonds are formed when metals transfer their valence electrons to nonmetals. The oppositely charged ions attract each other to form an ionic bond. Sodium has one valence electron and chlorine has seven. Sodium want to lose 1 electron and chlorine needs to gain 1. Forming an Na+ and a Cl- ion – sodium chloride NaCl Sodium transfers its valence electron to chlorine J Deutsch 2003

The formula for magnesium fluoride is MgF2 A + metal ion is attracted to a – nonmetal ion (opposites attract) forming an ionic compound. We can use Lewis dot structures to represent ionic compounds. The formula for magnesium fluoride is MgF2 J Deutsch 2003

Additional atoms may be necessary to insure that all the ions formed have a stable (noble gas) electron configuration. What happens when aluminum combines with oxygen to make aluminum oxide? J Deutsch 2003

Aluminum want to lose its 3 valence electrons Aluminum want to lose its 3 valence electrons. Oxygen needs to gain 2 electrons. The number of electrons lost by the metal must equal the number of electrons gained by the nonmetal J Deutsch 2003

The formula of aluminum oxide is Al2O3. J Deutsch 2003

Regents Question: 06/02 #51-53 Draw the electron-dot (Lewis) structure of an atom of calcium. Draw the electron-dot (Lewis) structure of an atom of chlorine. Draw the electron-dot (Lewis) structure of calcium chloride. J Deutsch 2003

Covalent bonds can be divided into two categories molecular and network J Deutsch 2003

Properties of molecular compounds Low melting and boiling points (weak attraction between molecules) Nonelectrolytes: Do not conduct electricity as solids or when dissolved or molten – no charged particles (ions) to move Solids are soft Examples: Wax and sugar C6 H12 O6 J Deutsch 2003

Network solids High melting and boiling points Hard Non electrolytes Examples: sand SiO2 and diamond (carbon compound) J Deutsch 2003

The stronger the intermolecular forces, the higher the boiling points and melting points. Strongest Ionic Solids Molecules with Hydrogen bonds Polar molecules Nonpolar molecules Weakest For nonpolar molecules, the greater the mass, the greater the force of attraction. J Deutsch 2003

Regents Question: 08/02 #33 The table below shows the normal boiling point of four compounds. Compound Normal Boiling Point (°C) HF (l) 19.4 CH3Cl (l) –24.2 CH3F (l) –78.6 HCl (l) –83.7 Which compound has the strongest intermolecular forces? (1) HF(l) (2) CH3Cl(l) (3)CH3F(l) (4)HCl(l) J Deutsch 2003 þ

When nonmetals combine with nonmetals, they share electrons When nonmetals combine with nonmetals, they share electrons. The attraction of two atoms for a shared pair makes a covalent bond. Electrons are always shared in pairs. Sharing electrons fills the valence shell with 8 electrons (2 for hydrogen.) Hydrogen chloride water ammonia methane J Deutsch 2003

Regents Question: 06/02 #61-63 Testing of an unknown solid shows that it has the properties listed below. (1) low melting point (2) nearly insoluble in water (3) nonconductor of electricity (4) relatively soft solid State the type of bonding that would be expected in the particles of this substance. Explain in terms of attractions between particles why the unknown solid has a low melting point. Explain why the particles of this substance are nonconductors of electricity. Covalent The attraction between particles is weak because there are no charged particle. Molecular substances are non-electrolytes – they do not form ions. J Deutsch 2003

Regents Question: 01/03 #35 Which of the following solids has the highest melting point? (1) H2O(s) (2) Na2O(s) (3) SO2 (s) (4) CO2 (s) þ J Deutsch 2003

Regents Question: 08/02 #53 Draw an electron-dot diagram for each of the following substances: A calcium oxide (an ionic compound) B hydrogen bromide C carbon dioxide J Deutsch 2003

Regents Question: 06/03 #12 Which type of chemical bond is formed between two atoms of bromine? Metallic (2) Hydrogen (3) ionic (4) covalent þ J Deutsch 2003

In a multiple covalent bond, more than one pair of electrons are shared between two atoms. (5.2e) Diatomic oxygen has a double bond O=O (2 shared pairs) because oxygen needs 2 electrons to fill its valence shell Diatomic nitrogen has a triple bond NN (3 shared pairs) because nitrogen needs 3 electrons to fill its valence shell Carbon dioxide has two double bonds J Deutsch 2003

Regents Question: 08/02 #17 Which molecule contains a triple covalent bond? (1) H 2 (2) N 2 (3) O 2 (4) Cl 2 þ J Deutsch 2003

The electronegativity difference between two bonded atoms is used to assess the degree of polarity in the bond. (5.2k) Polar covalent bonds form between two different nonmetals Polar bonds have a negative side and a positive side The electrons are attracted more to the atom with the higher electronegativity. The atom with the higher electronegativity is the negative side of the bond. J Deutsch 2003

Symmetrical molecules are not dipoles. Symmetrical (nonpolar) molecules include CO2 , CH4 , and diatomic elements. .. Symmetrical molecules are not dipoles. J Deutsch 2003

Asymmetrical (polar) molecules include HCl, NH3 , and H2 O. (5.2l) The negative side of the molecule is the side that has the atom with the higher electronegativity. J Deutsch 2003

Three types of bonds Ionic bonds– transfer of electrons- occur between a metal and a nonmetal Polar bonds– unequal sharing- occur between two different nonmetals Nonpolar bonds– equal sharing- occur between two of the same nonmetals J Deutsch 2003

Molecular polarity can be determined by the shape of the molecule and the distribution of charge. Possible shapes Linear (X2 HX CO2) Bent (H2O) Pyramidal (NH3) Tetrahedral (CH4 CCl4) A polar molecule is called a dipole. It has a positive side and a negative side – uneven charge distribution. J Deutsch 2003

Physical properties of substances can be explained in terms of chemical bonds and intermolecular forces. These properties include conductivity, malleability, solubility, hardness, melting point, and boiling point. (5.2n) J Deutsch 2003

Regents Question: 01/03 #10 The strength of an atom’s attraction for the electrons in a chemical bond is the atom’s (1) electronegativity (2) ionization energy (3) heat of reaction (4) heat of formation þ J Deutsch 2003

Regents Question: 06/03 #13 Which of these formulas contains the most polar bond? (1) H–Br (2) H–Cl (3) H–F (4) H–I þ J Deutsch 2003

Regents Question: 06/02 #8 þ Metallic bonding occurs between atoms of (1) sulfur (2) copper (3) Fluorine (4) carbon þ J Deutsch 2003

Regents Question: 01/03 #15 The high electrical conductivity of metals is primarily due to (1) high ionization energies (2) filled energy levels (3) mobile electrons (4) high electronegativities þ J Deutsch 2003

Regents Question: 06/03 #33 þ Which substance contains metallic bonds? (1) Hg(l) (2) H2O(l) (3) NaCl(s) (4)C6H12O6(s) þ J Deutsch 2003

Regents Question: 01/03 #39 A chemist performs the same tests on two homogeneous white crystalline solids, A and B. The results are shown in the table below. The results of these tests suggest that (1) both solids contain only ionic bonds (2) both solids contain only covalent bonds (3) solid A contains only covalent bonds and solid B contains only ionic bonds (4) solid A contains only ionic bonds and solid B contains only covalent bonds þ J Deutsch 2003

Regents Question: 01/03 #57-60 Each molecule listed below is formed by sharing electrons between atoms when the atoms within the molecule are bonded together. Molecule A: Cl2 Molecule B: CCl4 Molecule C: NH3 Draw the electron-dot (Lewis) structure for the NH3 molecule. Explain why CCl4 is classified as a nonpolar molecule. Explain why NH3 has stronger intermolecular forces of attraction than Cl2 . Explain how the bonding in KCl is different from the bonding in molecules A, B, and C. It is symmetrical. There is an even charge distribution around the molecule. Ammonia has hydrogen bonds, chlorine is nonpolar. J Deutsch 2003 KCl is ionic and molecules A, B and C are molecular with covalent bonds.

The oxidation states of the elements are available on the Periodic Table of the Elements J Deutsch 2003

To find the formula of a compound, use the criss-cross method Write the symbol and oxidation state of the metal (lower electronegativiy if both are nonmetals) in the upper right hand corner of the symbol. Positive oxidation state is written first. Repeat for nonmetal (-) C4+ O2- Drop the sign C4 O2 Criss Cross C2 O4 Reduce if possible CO2 Although Carbon and Oxygen are both nonmetals, Carbon has a lower electronegativity and therefore uses a positive oxidation number J Deutsch 2003

To find the formula of a compound, use the criss-cross method Write the symbol and oxidation state of the metal (lower electronegativiy if both are nonmetals) in the upper right hand corner of the symbol. Positive oxidation state is written first. Repeat for nonmetal (-) C4+ O2- Drop the sign C4 O2 Criss Cross C2 O4 Reduce if possible CO2 Although Carbon and Oxygen are both nonmetals, Carbon has a lower electronegativity and therefore uses a positive oxidation number J Deutsch 2003

Naming Ionic Compounds Compounds between a metal and a nonmetal Name the metal Name the nonmetal with an IDE ending Ammonium chloride Aluminum oxide Barium sulfide Zinc fluoride If the negative ion is a polyatomic ion Name the polyatomic ion Sodium Nitrate Calcium phosphate J Deutsch 2003

Finding the oxidation state of a polyvalent metal The net charge of a compound must be zero The number of + charges must equal the total number of – charges Multiply the oxidation state of the nonmetal by the subscript for the nonmetal Divide that number by the subscript of the metal PbO2 the oxidation state of oxygen is –2 we multiply the 2 by the subscript 2 and get 4. Divide the 4 by the subscript for the lead (1) and get 4. The oxidation state for the lead is 4 and the name is Lead(IV)oxide J Deutsch 2003

Regents Question: 06/03 #19 Which formula correctly represents the compo-sition of iron (III) oxide? (1) FeO3 (2) Fe2O3 (3) Fe3O (4) Fe3O2 þ J Deutsch 2003

The Stock System - Naming ionic compounds in which the metal has more than one oxidation state (polyvalent) Check the Periodic Table to see if the metal has more than one oxidation state if it does… Determine the oxidation state of the metal Use the oxidation state of the nonmetal and the subscripts The oxidation state of the metal is written as a roman numeral in parenthesis after the name of the metal CuSO4 Copper(II) Sulfate J Deutsch 2003

Naming molecular compounds – compounds between nonmetals Use a prefix to indicate how many of each element mono 1 (never used for first element) di 2 tri 3 tetra 4 CO2 Carbon dioxide CO Carbon monoxide N2O4 Dinitrogen tetraoxide BCl3 Boron trichloride J Deutsch 2003

When using the criss-cross method with polyatomic ions, use parenthesis ( ) Write the symbol and oxidation state of the metal (+ ion) in the upper right hand corner of the symbol. Repeat for nonmetal (- ion) (NH4) 1+ (PO4) 3- Drop the sign (NH4) 1 (PO4) 3 Criss Cross (NH4) 3 (PO4) 1 Reduce if possible (numbers outside parenthesis only) If there is no number following the parenthesis, drop the parenthesis. (NH4)3 PO4 J Deutsch 2003

Polyatomic ions are found on Reference Table E J Deutsch 2003