Chemical Bonding

Chemical Bonds Compounds are made of atoms held together by bonds Chemical bonds are forces of attraction between atoms The bonding attraction comes from attractions between protons and electrons

Types of chemical Bonds Ionic Bonding: is the complete transfer of valence electrons between atoms . Covalent Bonding: is a chemical bond form through the sharing of one or more pairs of electrons.

Lewis Bonding Theory Atoms bond because it results in a more stable electron configuration. more stable = lower potential energy Atoms bond together by either transferring or sharing electrons Usually this results in all atoms obtaining an outer shell with eight electrons octet rule there are some exceptions to this rule—the key to remember is to try to get an electron configuration like a noble gas

Octet Rule When atoms bond, they tend to gain, lose, or share electrons to result in eight valence electrons ns2np6 noble gas configuration Many exceptions H, Li, Be, B attain an electron configuration like He He = two valence electrons, a duet Li loses its one valence electron H shares or gains one electron though it commonly loses its one electron to become H+ Be loses two electrons to become Be2+ though it commonly shares its two electrons in covalent bonds, resulting in four valence electrons B loses three electrons to become B3+ though it commonly shares its three electrons in covalent bonds, resulting in six valence electrons expanded octets for elements in Period 3 or below using empty valence d orbitals

Electron configuration of noble gases Helium 1s2 Neon [He] 2s22p6 Argon [Ne] 3s23p6 Krypton [Ar] 3d104s24p6 Xenon [kr] 4d105s25s6 Radon [Xe] 4f145d106s26p6

Valence Electrons & Bonding Because valence electrons are held most loosely, and Because chemical bonding involves the transfer or sharing of electrons between two or more atoms, Valence electrons are most important in bonding Lewis theory focuses on the behavior of the valence electrons

Determining the Number of Valence Electrons in an Atom The column number on the Periodic Table will tell you how many valence electrons a main group atom has Transition Elements all have two valence electrons. Why?

Lewis Symbol A Lewis symbol is the chemical symbol of an element surrounded by dots equal in number to the number of valence electrons present in atoms of the element. Example:

Practice – Write the Lewis structure for arsenic As is in column 5A, therefore it has five valence electrons.

Lewis symbol of first 20 element

Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond +

Lewis Theory Predictions for Ionic Bonding Lewis theory predicts the number of electrons a metal atom should lose or a nonmetal atom should gain in order to attain a stable electron arrangement the octet rule This allows us to predict the formulas of ionic compounds that result It also allows us to predict the relative strengths of the resulting ionic bonds from Coulomb’s Law

Predicting Ionic Formulas Using Lewis Symbols Electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet Numbers of atoms are adjusted so the electron transfer comes out even Li2O

Example 9.1: Using Lewis theory to predict chemical formulas of ionic compounds Predict the formula of the compound that forms between calcium and chlorine. Cl ∙ Ca ∙ Draw the Lewis dot symbols of the elements. Cl ∙ Cl ∙ Transfer all the valence electrons from the metal to the nonmetal, adding more of each atom as you go, until all electrons are lost from the metal atoms and all nonmetal atoms have eight electrons. Ca ∙ Ca2+ CaCl2

Ionic Bond Model In ionic bonds, the metals loses electrons to become positively charged cations, whereas the nonmetal accepts those electrons to become negatively charged anions. Ionic bonds require an electron donor, metal and an electron acceptor nonmetel. The charged on the cations and anions correspond to the number of electrons denoted or received.

Writing chemical symbols for ions A neutral aluminum atom contains 13 protons and 13 electrons, since the atomic number of aluminum is 13. The aluminum ion formed by the loss of three would still contain 13 protons but the would have only 10 electrons because three electrons were lost. 13 protons = 13 + charge 10 electrons = 10 – charge net charge = 3 + Al3+

Writing chemical symbol for ions Atomic number for sodium is 11 there fore, sodium have to lose one valence electron.

Writing chemical symbol for ions Atomic number for chlorine is 17 therefore, chlorine has to gain one valence electron to satisfy the octet rule.

The sign and magnitude of ionic charge Elements with more than four valence electrons achieve an octet of electrons via electron gain. Conversely, electrons with less than four valence electrons achieve an octet of electrons through electron loss. Nitrogen has five valence electron therefore, Nitrogen need to gain three more electron to achieve and octet of electrons. N3- 1s22s22p6 (Neon configuration)

The sign and Magnitude of ionic charge Electrons with more than four valence electrons achieve an octet of electrons via electron gain. Conversely, electrons with less than four valence electrons achieve an octet of electrons through electron loss. Potassium possesses one valence electron therefore, potassium have to loss one valence electron because it is less than four electrons. K+ 1s22s22p63s23p6 ( argon configuration)

A Lewis Structure A Lewis Structure is a grouping of Lewis symbols the shows either the transfer of electrons or the sharing of electrons in a chemical bond. Lewis symbol involves atoms of individual element. Lewis structure involves compounds.

Covalent Bonding Covalent bond is a chemical bond resulting from two nuclei attracting the same shared electrons. As atoms come together, orbitals overlap to give a common orbital. Electrons like to hang out in the middle.

Covalent Bonding Atoms can have bonding electrons and nonbonding electrons. Bonding electrons are pairs of valence electrons that are shared between atoms. Nonbonding electrons are those that do not participate in bonding.

. . . . . . . Covalent Bonding Using Lewis Structures: Hydrogen H Chlorine Cl Each has one bonding electron. Chlorine has 3 nonbonding electron pairs . . . . . . .

Covalent Bonding Example from your text: Each seeks to have a Noble Gas configuration; to be isoelectronic with the Nobel Gas.

Covalent Bonding Creating Lewis structures with Lewis symbols:

Covalent Bonding Creating Lewis structures with Lewis symbols: Notice that the bonding pair can be represented using dots or a dash. Nonbonding pairs remain dots.

Examples F2 valence electrons of Fluorine 7 H2O valence electrons 1 Oxygen valence 6 Co2 valence electron carbon 4 C2H4 N2 Valence electron of Nitrogen 5 C2H2 Acetylene

Single, Double, and Triple covalent Bond A single Covalent Bond: is a covalent bond in which two atoms share one pair of valence electrons. A Double covalent Bond: is a covalent bond in which two atoms share two pairs of valence electrons. A double covalent Bond is between two atoms is stronger than a single covalent bond between the same atom.

Single, Double, And triple covalent Bonds. A triple covalent bond: is a covalent bond in which two atoms share three pairs of valence electrons. A triple covalent is stronger than a double covalent bond. Example of single , double and triple bond.

Resonance Structures Resonance structures occur when there is more than one way to draw the Lewis structure for a molecule or polyatomic ion that have the same arrangement of atoms. Other resonance structures?

Drawing Lewis Structures for Covalently Bonded Species Determine the total number of valence electrons in the molecule or ion. If the species is charged add or subtract an appropriate number of electrons. Determine the arrangement of atoms. (H is always a terminal atom). Place one pair of electrons between each pair of bonded atoms. Subtract these from the total.

Drawing Lewis Structures for Covalently Bonded Species Place electrons on terminal atoms (except H) until each one has an octet. Subtract what you use from the total. If there are any electrons left, assign them to the central atom. At this point, if the central atom has fewer than 8 electrons, move one or more lone pairs from an terminal atom to form a multiple bond with the central atom.

Covalent Lewis Structure Practice CH4 C2H4 SO32- HCN

Molecular Geometry and VSEPR Now that you can write a Lewis structure, what shape does the molecule have? Molecular Geometry is a description of the three-dimensional arrangement of atoms within a molecule. Molecular Geometry plays an important role in determining the physical and chemical properties of substances.

Molecular Geometry and VSEPR VSEPR – Valence Shell Electron Pair Repulsion theory is a set of procedures for predicting the geometry of a molecule from the information in the molecules Lewis Structure. Nonbonding electron pairs and the number of atoms bonded to an atom determine the geometry of the molecule around that atom. Need to know the number of electron groups.

Molecular Geometry and VSEPR Basically, all electron groups want to be as far away from each other as possible. The electrons all carry a negative charge and like charges repel each other. Think about this in terms of putting two North poles of a magnet next to each other.

Molecular Geometry and VSEPR BeCl2 CCl4 BF3

Molecular Geometry and VSEPR A VSEPR electron group is a group of valence electrons present in a localized region about an atom or molecule. This is slightly different than an electron pair. N N O two VSEPR groups O S O three VSEPR groups Water has 4 VSEPR groups . . . . . . .

Molecular Geometry and VSEPR Atoms arrange themselves to minimize electron pair repulsion (get the pairs as far apart as possible) No distinction is made between bonding electrons (shared) and non-bonding electrons (lone pairs) Single, Double and Triple Bonds are all counted equally as one “pair” – one thing attached to the central atom.

Molecular Geometry and VSEPR Non-bonding electrons contribute to the shape of the molecule, but are invisible. Trigonal Pyramidal Bent (or Angular)

Molecular Geometry Molecules with 2 or 3 VSEPR pairs are planar. Trigonal Planar Bent Linear

VSEPR When the electron pairs are 180° apart, the molecular geometry will be Linear.

VSEPR When the electron pairs are 120° apart, the molecular geometry will be either Trigonal Planar or angular (bent).

VSEPR Tetrahedral Trigonal Pyramidal Bent When the electron pairs are 109.5° apart, the molecular geometry will be one of three possibilities… Tetrahedral Trigonal Pyramidal Bent

Formula VESPR Groups Shape XB2 2 Linear XB3 3 Trigonal planar XB2N1 Bent XB4 4 Tetrahedral XB3N1 Trigonal pyramidal XB2N2 bent X = central atom; B = bonding electron groups N = nonbonding electron groups