BONDING Ch 8 9 Honors Chemistry General Rule

BONDING Ch 8 & 9 – Honors Chemistry General Rule of Thumb: metal + nonmetal = ionic polyatomic ion + metal or polyatomic ion = ionic (both) nonmetal + nonmetal(s) = covalent

Ionic Bonds Isn’t it ionic that opposites attract?

Valence Electrons ¡ ¡ Knowing electron configurations is important because the number of valence electrons determines the chemical properties of an element. Valence Electrons: The e- in the highest occupied energy level of an element’s atoms.

Valence Electrons ¡ All elements in a particular group or family have the same number of valence electrons (and this number is equal to the group number of that element) ¡ Examples: l l l Group 1 elements (Na, K, Li, H): 1 valence electron. Group 2 elements (Mg, Ca, Be): 2 valence electrons. Group 17 elements (Cl, F, Br): 7 valence electrons.

Lewis Structures ¡ ¡ ¡ ¡ Electron dot structures show the valence electrons as dots around the element’s symbol: Li B Si N O F Ne

Octet Rule Noble gas atoms are very stable; they have stable electron configurations. In forming compounds, atoms make adjustments to achieve the lowest possible (or most stable) energy. ¡ Octet rule: atoms react by changing the number of electrons so as to acquire the stable electron structure of a noble gas. ¡

Octet Rule ¡ ¡ ¡ ¡ ¡ Atoms of METALS obey this rule by losing electrons. Na: Na+: Atoms of NONMETALS obey this rule by gaining electrons. Cl: Cl-: Transition metals are exceptions to this rule. Example: silver (Ag) By losing one electron, it acquires a relatively stable configuration with its 4 d sublevel filled (pseudo noble-gas)

Ionic Bonds ¡ Anions and cations have opposite charges; they attract one another by electrostatic forces (IONIC BONDS)

Ionic Bonds ¡ Ionic compounds are electrically neutral groups of ions joined together by electrostatic forces. (also known as salts) l l the positive charges of the cations must equal the negative charges of the anions. use electron dot structures to predict the ratios in which different cations and anions will combine.

Examples of Ionic Bonds ¡ Na Cl Na+Cl- = Na. Cl ¡ Al Br Al 3+Br- = Al. Br 3 ¡ K O K+O 2 - = K 2 O ¡ Mg N Mg 2+N 3 - = Mg 3 N 2 ¡ K P K+P 3 - = K 3 P

Criss Cross Method Criss-cross ionic charges down as subscripts (without +/-) and reduce (to determine lowest whole # ratio of cation: anion) ¡ ANY TIME YOU ADD A SUBSCRIPT TO A POLYATOMIC ION, YOU MUST FIRST PUT THAT ION IN PARENTHESES. ¡

Covalent Bonds The joy of sharing!

Covalent Bonds ¡ ¡ Covalent bonds: occur between two or more nonmetals; electrons are shared not transferred (as in ionic bonds) The result of sharing electrons is that atoms attain a more stable electron configuration.

Covalent Bonds ¡ Most covalent bonds involve: l l l 2 electrons (single covalent bond), 4 electrons (double covalent bond, or 6 electrons (triple covalent bond).

¡ Lewis structures (electron dot structures) show the structure of molecules. (Bonds can be shown with dots for electrons, or with dashes: 1 dash = 2 electrons) ¡ H 2 HBr ¡ CCl 4 O 2 ¡ N 2 CO

Octet Rule ¡ Octet Rule: The representative elements achieve noble gas configurations (8 electrons) by sharing electrons.

Writing Lewis Structures 1) 2) 3) 4) 5) 6) Select a skeleton for the molecule (the least electronegative element is usually the central element). Calculate N (the # of valence e- need by all atoms in the molecule of polyatomic ion. Calculate A (the # of electrons available). Calculate S (the # of electrons shared in the molecule) S = N – A Place the S electrons as shared pairs in the skeleton. Place the additional electrons as unshared pairs to fill the octet of every representative elements (except hydrogen!).

Lewis Structure Examples: ¡ CO 2 - ¡ CBr 3 - ¡ OH- ¡ N 22 - ¡ NO 3 - ¡ CO 32 - ¡ SO 42 - ¡ NH 4+

Electronegativity ¡ We’ve learned how valence electrons are shared to form covalent bonds between elements. So far, we have considered the electrons to be shared equally. However, in most cases, electrons are not shared equally because of a property called electronegativity.

Electronegativity The ELECTRONEGATIVITY of an element is: the tendency for an atom to attract electrons to itself when it is chemically combined with another element. ¡ The result: a “tug-of-war” between the nuclei of the atoms. ¡

Electronegativity ¡ ¡ Electronegativities are given numerical values (the most electronegative element has the highest value; the least electronegative element has the lowest value) **See Figure 6 -18 p. 169 (Honors) l l Most electronegative element: Fluorine (3. 98) Least electronegative elements: Fr (0. 70), Cs (0. 79)

Electronegativity ¡ Notice the periodic trend: l l ¡ As we move from left to right across a row, electronegativity increases (metals have low values nonmetals have high values – excluding noble gases) As we move down a column, electronegativity decreases. The higher the electronegativity value, the greater the ability to attract electrons to itself.

Nonpolar Bonds When the atoms in a molecule are the same, the bonding electrons are shared equally. ¡ Result: a nonpolar covalent bond ¡ l Examples: O 2, F 2, H 2, N 2, Cl 2

Polar Bonds ¡ ¡ ¡ When 2 different atoms are joined by a covalent bond, and the bonding electrons are shared unequally, the bond is a polar covalent bond, or POLAR BOND. The atom with the stronger electron attraction (the more electronegative element) acquires a slightly negative charge. The less electronegative atom acquires a slightly positive charge.

Polar Bonds ¡ Example: HCl ¡ Electronegativities: l l H = 2. 20 Cl = 3. 16 + H Cl -

Polar Bonds ¡ Example: H 2 O ¡ Electronegativities: l l H = 2. 20 O = 3. 44

Predicting Bond Types ¡ Electronegativities help us predict the type of bond: Electronegativity Difference 0. 00 – 0. 40 0. 41 – 1. 00 1. 01 – 2. 00 2. 01 or higher Type of Bond covalent (nonpolar) covalent (slightly polar) covalent (very polar) ionic Example H-H H-Cl H-F Na+Cl-

Polar Molecules ¡ ¡ A polar bond in a molecule can make the entire molecule polar A molecule that has 2 poles (charged regions), like H-Cl, is called a dipolar molecule, or dipole.

Polar Molecules ¡ The effect of polar bonds on the polarity of a molecule depends on the shape of the molecule. ¡ Example: O=C=O CO 2 shape: linear *The bond polarities cancel because they are in opposite directions; CO 2 is a nonpolar molecule.

Polar Molecules ¡ ¡ The effect of polar bonds on the polarity of a molecule depends on the shape of the molecule. Water, H 2 O, also has 2 polar bonds: l l But, the molecule is bent, so the bonds do not cancel. H 2 O is a polar molecule.

Resonance ¡ A molecule or polyatomic ion for which 2 or more dot formulas with the same arrangement of atoms can be drawn is said to exhibit RESONANCE.

Resonance Example ¡ ¡ CO 32 - 3 resonance structures can be drawn for CO 32 the relationship among them is indicated by the double arrow. the true structure is an average of the 3.

Resonance Example ¡ ¡ CO 32 - 3 resonance structures can be drawn for CO 32 the relationship among them is indicated by the double arrow. the true structure is an average of the 3

Resonance Structures Another way to represent this is by delocalization of bonding electrons: ¡ (the dashed lines indicate the 4 pairs of bonding electrons are equally distributed among 3 C-O bonds; unshared electron pairs are not shown) ¡ l See p. 256

VSEPR valence shell electron pair repulsion

Molecular Shape Lewis structures (electron dot structures) show the structure of molecules…but only in 2 dimensions (flat). ¡ BUT, molecules are 3 dimensional! ¡ l for example, CH 4 is:

Molecular Shape ¡ Lewis structures (electron dot structures) show the structure of molecules…but only in 2 dimensions (flat). ¡ BUT, molecules are 3 dimensional! l but in 3 D it is: a tetrahedron! = coming out of page = going into page = flat on page

Why do molecules take on 3 D shapes instead of being flat? ¡ ¡ Valence Shell Electron Pair Repulsion theory “because electron pairs repel one another, molecules adjust their shapes so that the valence electron pairs are as far apart from another as possible. ”

Why do molecules take on 3 D shapes instead of being flat? ¡ Valence Shell Electron Pair Repulsion theory Remember: both shared and unshared electron pairs will repel one another. Non-Bonding Pairs H—N — H — ¡ H Bonding Pairs

5 Basic Molecule Shapes ¡ Linear ¡ Example: CO 2

5 Basic Molecule Shapes ¡ Bent or angular Example: H 2 O ¡ Notice electron pair repulsion ¡

5 Basic Molecule Shapes ¡ tetrahedral ¡ example: CH 4

5 Basic Molecule Shapes ¡ Pyramidal Example: NH 3 ¡ (note: unshared pair of electron repels, but is not considered part of overall shape; no atom there to contribute to the shape) ¡

5 Basic Molecule Shapes ¡ Trigonal planar or planar triangular ¡ Example: BF 3

Geometry and polarity Three shapes will cancel out polarity. ¡ Shape One: Linear ¡

Geometry and polarity Three shapes will cancel out polarity. ¡ Planar triangles ¡ 120º

Geometry and polarity ¡ Three shapes will cancel out polarity. ¡ Tetrahedral

Geometry and polarity Others don’t cancel ¡ Bent ¡

Geometry and polarity Others don’t cancel ¡ Trigonal Pyramidal ¡