BONDING SOL Review

Introduction to Bonding

Bonding Let’s consider the compound Cesium Fluoride, CsF. The electro-negativity value (EV) for Cs is 0.70; the EV for F is 4.00. The difference between the two is 3.30, which falls within the scale of ionic character. When the electro-negativity difference between two atoms is greater than 1.7 the bond is mostly ionic.

Ionic Bonding Ionic Bonding

Ionic Bonding In an Ionic bond: The electro-negativity difference is extreme, So the atom with the stronger pull doesn’t really share the electron Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction

Ionic Bonding When a metal bonds with a nonmetal an: Ionic bond is formed An ionic bond contains a positive and negative ion. A positive ion is called a cation. A negative ion is called an anion. An Ionic bonding always involves the transfer of an electron from the metal to the nonmetal. The cation and anion are held together by electrostatic attraction.

Characteristics of Ionic Compounds Ionic compounds do not consist of individual molecules. Instead there is a huge network of positive and negative ions that are packed together in a solid brittle crystal lattice. Because their bonds are strong, ionic compounds tend to have very high melting and boiling points -Ionic compounds are electrolytes, which means they can conduct electricity When forming ionic compounds the positive and negative charges must balance Ionic crystals cannot conduct electricity because the ions must be able to move.

Sea of Electrons

nonmetal w/nonmetal = covalent Bonding The take home lesson on electro-negativity and bonding is this: The closer together the atoms are on the P.T., the more evenly their e- interact, and are therefore more likely to form a covalent bond The farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond. metal w/nonmetal = ionic nonmetal w/nonmetal = covalent

Covalent Bonding

Covalent Bonding In a covalent bond: The electro-negativity difference between the atoms involved is not extreme So the interaction between the involved electrons is more like a sharing relationship It may not be an equal sharing relationship, but at least the electrons are being “shared”.

Covalent Bonding Covalent Bonding is between two or more non-metals. Covalent bonds are formed when electrons are shared between two atoms. If they share 2 electrons, the form a single bond; 4 electrons is a double bond; If two atoms share 6 electrons, they form a triple bond.

Covalent Bonding Polar bonds usually involve nitrogen, oxygen or fluorine (NOF) Non-Polar bonds usually involve carbon-hydrogen bonds In polar bonds, the electrons are shared unequally In non-polar bonds, the electrons are shared equally. Covalent compounds can exist in any state (solid, liquid or gas).  They have low melting and boiling points.

Lets look at the molecule Cl2 Covalent Bonds Lets look at the molecule Cl2 Cl Cl Cl + Shared Electrons

Notice 8 e- in each valence shell!!! each atom must have 8 valence e's Cl Cl Notice 8 e- in each valence shell!!! Shared electrons are counted with both atoms Cl 2

How about the molecule HCl? (Polar Covalent) shared, but not evenly Covalent Bonds How about the molecule HCl? H Cl Cl H + 2.1 3.0 (Polar Covalent) shared, but not evenly

So what’s the bottom line? To be stable the two atoms involved in the covalent bond share their electrons in order to achieve the arrangement of a Noble Gas.

Drawing Lewis Structures The Octet Rule Concept of Formal Charge

Lewis Structures Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.

The Octet Rule The most important requirement for the formation of a stable compound is that the atoms achieve a noble gas electron configuration.

Writing Lewis Structures Find the sum of valence electrons of all atoms in the polyatomic ion or molecule. If it is an anion, add one electron for each negative charge. If it is a cation, subtract one electron for each positive charge. PCl3 5 + 3(7) = 26

Writing Lewis Structures The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds. Keep track of the electrons: 26 - 6 = 20

Writing Lewis Structures Fill the octets of the outer atoms. Keep track of the electrons: 26 - 6 = 20; 20 - 18 = 2

Writing Lewis Structures Fill the octet of the central atom. Keep track of the electrons: 26 - 6 = 20; 20 - 18 = 2; 2 - 2 = 0

Writing Lewis Structures If you run out of electrons before the central atom has an octet… …form multiple bonds until it does. Usually occurs with carbon, nitrogen, oxygen

More than One Lewis Structure Resonance Structures More than One Lewis Structure

Resonance This is the Lewis structure we would draw for ozone, O3. -

Resonance But this is at odds with the true, observed structure of ozone, in which… …both O-O bonds are the same length. …both outer oxygens have a charge of -1/2.

Resonance One Lewis structure cannot accurately depict a molecule like ozone. We use multiple structures, resonance structures, to describe the molecule.

Resonance Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.

Resonance In truth, the electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon. They are not localized; they are delocalized.

Exceptions to the Octet Rule

Exceptions to the Octet Rule There are three types of ions or molecules that do not follow the octet rule: Ions or molecules with an odd number of electrons Ions or molecules with less than an octet Ions or molecules with more than eight valence electrons (an expanded octet)

Odd Number of Electrons Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

Fewer Than Eight Electrons Consider BF3: Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine. This would not be an accurate picture of the distribution of electrons in BF3.

Fewer Than Eight Electrons Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.

Fewer Than Eight Electrons The lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.

More Than Eight Electrons The only way PCl5 can exist is if phosphorus has 10 electrons around it. It is allowed to expand the octet of atoms on the 3rd row or below. Presumably d orbitals in these atoms participate in bonding.

More Than Eight Electrons Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.

More Than Eight Electrons This eliminates the charge on the phosphorus and the charge on one of the oxygens. The lesson is: when the central atom in on the 3rd row or below and expanding its octet eliminates some formal charges, do so.

Hydrate Formation In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal. Some ionic compnds have enough space between the ions that water molecules can get trapped in between the ions Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate.

Trapped Water Molecules Hydrated Crystal

Hydrate Formation Hydrates typically have different properties than their dry versions - A.K.A. anhydrides Anhydrous CuSO4 is nearly colorless CuSO4•5 H2O is a bright blue color When Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion. The hydrated name would be Copper (II) Sulfate Pentahydrate

Hydrate Formation Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat? These crystals are there to absorb water that might lead to mildew or mold The formula of a hydrate is XAYB • Z H2O (Z is a coefficient indicating how many waters are present per formula unit)

Molecular Polarity All above are NOT polar Molecules will be polar if a) bonds are polar AND b) the molecule is NOT “symmetric” All above are NOT polar