Ch 6 Chemical Bonding What you should learn in this section (objectives): Define chemical bond Explain why most atoms form chemical bonds Describe ionic and covalent bonding Explain why most chemical bonding is neither purely ionic or purely covalent Classify bonding type according to electronegativity differences. Introduction to Chemical Bonding There are very few atoms that exist as individual particles in nature. Most atoms are bonded to other atoms to form compounds. A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. One reason atoms bond is to decrease their amount of potential energy. When atoms exist by themselves they have relatively high potential energy. Nature favors arrangements that have minimum potential energy. When atoms bond it decreases the amount of potential energy and creates a more stable arrangement of matter. It takes less energy to bond atoms together than to break the bonds between atoms. Bond energy is the amount of energy it takes to break a chemical bond. Types of Chemical Bonds Ionic bonding- chemical bonding that results from the electrical attraction between cations and anions. In purely ionic bonding atoms completely give up electrons to other atoms. Ionic bonding generally involves metals and nonmetals Covalent bonding- the sharing of electron pairs between two atoms. In a pure covalent bond the electrons are shared by the two bonded atoms. Covalent bonding generally involves two nonmetals. A nonpolar-covalent bond is when the bonding electrons are shared equally by the bonded atoms, resulting in a balanced distribution of electrical charges. When the distribution of charge is uneven we call this polar. Polar covalent bonds occur when the bonded atoms have an unequal attraction for the shared electrons.
Ionic or Covalent? Most chemical bonds are somewhere in between purely ionic and purely covalent. We use the difference in electronegativity values to determine the type of bond that is formed. Remember from Ch 5 that electronegativity is an atom s ability to attract electrons to its self. We use the following image to determine the bond type. Ionic- electronegativity difference is greater than 1.67 Polar covalent- electronegativity difference is less than 1.67 Use electronegativity differences to classify bonding between sulfur and the following elements: hydrogen, cesium, and chlorine. In each pair, which atom will be more negative? Problem A
Covalent Bonding and Molecular Compounds What you should learn in this section (objectives): Define molecule and molecular formula Explain the relationships among potential energy, distance between approaching atoms, bond length and bond energy. State the octet rule List the six basic steps used in writing Lewis structures Explain how to determine Lewis structures for molecules containing single bonds, multiple bonds, or both. Explain why scientists use resonance structures to represent some molecules. A molecule is a neutral group of atoms that are held together by covalent bonds. They can exist as two or more of the same elements bonded together or two or more different elements bonded together. A chemical compound whose simplest units are molecules is called a molecular compound. A chemical formula shows the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts. Remember sub means below. A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound. Formation of a Covalent Bond Nature favors chemical bonding because most atoms have lower potential energy when they are bonded to other atoms than they have as they are independent particles. When 2 hydrogen atoms approach each other 2 bad things happen: electron/electron repulsion and proton/proton repulsion. One good thing that happens: proton/electron attraction. When the attractive forces offset the repulsive forces, the energy of the tow atoms decreases and a bond is formed. Remember, nature is always striving for a lower energy state. too CLOSE too FAR just RIGHT
Bond length is the distance between the two nuclei where the energy is minimal between the two nuclei. In other words, it is the average distance between two bonded atoms. When bonds form individual atoms release energy as they change from isolated individual atoms to molecules. Bond energy is the amount of energy that is required to break the bond. The units for bond energy are usually kj/mol (kilojoule per mole).
Octet Rule Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level (outer most ring of the atom). When two atoms form a covalent bond, their shared electrons form overlapping orbitals. This achieves a noble-gas configuration. The bonding of two hydrogen atoms allows each atom to have the stable electron configuration of helium, 1s 2. Exceptions to the Octet Rule Fewer than 8: H at most only 2 electrons (one bond),beh 2, only 4 valence electrons around Be (only 2 bonds), Boron compounds only 6 valence electrons (three bonds) Expanded valence (more than 8): can only happen if the central element had d-orbitals which means it is from the 3 rd period or greater and can thus be surrounded by more than four valence pairs in certain compounds. The number of bonds depends on the balance between the ability of the nucleus to attract electrons and the repulsion between the pairs. Some of the more elements are fluorine, oxygen, chlorine and noble gases. Electron-Dot Notation To keep track of valence electrons, it is helpful to use electron-dot notation. Electron-dot notation is an electronconfiguration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element s symbol. The inner-shell electrons are not shown.
Lewis Structures H:H An unshared pair, also called a lone pair, is a pair of electrons that is not involved in bonding and that belongs exclusively to one atom. Lewis Structures are formulas in which atomic symbols represent nuclei and inner-shell electrons, dotpairs or dashes between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons. A structural formula indicates the kind, number, arrangement, and bonds but not the unshared pairs of the atoms in a molecule. Example F-F and H-Cl. Single bonds (sigma bonds) are a covalent bond in which one pair of electrons is shared between two atoms. They are represented by two dots (electrons) or one dash. These are the longest bonds, but also the weakest Double bonds (pi bonds) are covalent bonds in which two pairs of electrons are shared between two atoms. They are represented by four dots (electrons) or 2 dashes = Example C=C Triple bonds (pi bonds) are covalent bonds in which three pairs of electrons are shared between two atoms. They are represented by six dots (electrons) or 3 dashes. Example These are shortest, but also the strongest. Carbon, nitrogen, oxygen, phosphorous, and sulfur are the most common elements that form multiple bonds. Drawing Lewis Structures 1. H is always a terminal atom. ALWAYS connected to only one other atom. 2. Lowest electronegativity is the central atom in a molecule. 3. Find the total number of valence electrons by adding up group numbers of the elements. For ions add electrons for negative charges and subtract electrons for positive charges. 4. Place one pair of electrons (sigma bond) between each pair of bonded atoms.
5. Subtract from the total number of bonds you just used. 6. Place lone pairs about each terminal atom (except H) to satisfy the octet rule. Left over pairs are assigned to the central atom. 7. If the central atom is not yet surrounded by four electron pairs, convert on or more terminal atom lone pairs to a double or triple bond ( pi bonds). Only C, N, O, P, and S can form multiple bonds (pi bonds) Resonance Structures Resonance refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure. Ozone (O 3 ) exists as an average of these two images, so it must be shown both ways.
Ionic Bonding and Ionic Compounds What you should learn in this section (objectives): Compare and contrast a chemical formula for a molecular compound with one for an ionic compound. Discuss the arrangements of ions in crystals Define lattice energy and explain its significance List and compare the distinctive properties of ionic and molecular compounds Write the Lewis structure for a polyatomic ion given the identity of the atoms combined and other appropriate information. Formation of an Ionic Compound Ionic compounds are composed of positive (cation) and negative (anion) ions that are combined so that the numbers of positive and negative charges are equal Ionic Bond - Completely transfer electrons. Positive charge cation lost electrons to the anion. Negative charge anion gained electrons from the cation. Positive charge must equal and, therefore, cancel the negative charge. Example: Sodium Chloride sodium wants to lose one electron to become stable and chlorine wants to gain one electron to become stable. Formula unit a chemical formula of the smallest sample of an ionic compound. Ionic Character Ionic compounds have the greatest ionic character with full on charged ions. The further the ions are apart in electronegativity, the more the ionic character. Molecular compounds have very low electronegativity. The closer the ions are in electronegativity, the less the ionic character. Characteristics of Ionic Bonding Ionic compounds are crystalline solids at room temperature. They are arranged in repeating three-dimensional pattern called a crystal lattice. These structures are very orderly and stable. Example: In solid NaCl, each Na is surrounded by six Cl and each Cl is surrounded by six Na. These crystalline solids also have very high melting points. It is extremely hard to break the attraction between the cations and anions because of their stability. Example: NaCl melts at 800 Celsius. The energy released when one mole of an ionic crystalline compound is formed from gaseous ions is called lattice energy. Ionic compounds conduct electric currents when molten (liquid) or dissolved in water (aqueous). The cations and anions are then able to migrate freely.
Ionic compounds are electrically neutral salts (solids). Many of these compounds appear as minerals in the Earth s crust. Comparing Ionic and Molecular Compounds The force that holds ions together in an ionic compound is a very strong electrostatic attraction. In contrast, the forces of attraction between molecules of a covalent compound are much weaker. This difference in the strength of attraction between the basic units of molecular and ionic compounds gives rise to different properties between the two types of compounds. Molecular compounds have relatively weak forces between individual molecules. They melt at low temperatures. The strong attraction between ions in an ionic compound gives ionic compounds some characteristic properties, listed below. o very high melting points o hard but brittle o not electrical conductors in the solid state, because the ions cannot move Polyatomic Ions Polyatomic Ions a group of atoms that acts as a unit with a single charge Begin memorizing polyatomic ions get the list from the website and make flashcards. Know the formula, the charge, and the correct spelling of the name of the polyatomic ions listed on the website. We will have a quiz over these.
Metallic Bonding What you should learn in this section (objectives): Describe the electron-sea model of metallic bonding, and explain why metals are good electrical conductors Explain why metal surfaces are shiny Explain why metals are malleable and ductile but ionic crystalline compounds are not. The Metallic-Bond Model Metallic bonding is the chemical bonding that is a result from the attraction between metal atoms and the surrounding sea of electrons. A sea of electrons refers to the free moving valence electrons in an atom. These electrons are delocalized which means that they can freely move to any other atom Properties of Metals Good conductors of electricity electrons enter one end of the metal bar and leave the other. Ductile can be stretched into wires. Malleable can be pounded into shapes. Metals ions slide passed one another in a sea of drifting
Molecular Geometry What you should learn in this section (objectives): Explain VSEPR theory Predict the shapes of molecules or polyatomic ions using VSEPR theory Explain how the shapes of molecules are accounted for by hybridization theory Describe dipole-dipole forces, hydrogen bonding, induced dipoles, and London dispersion forces and their effects on properties such as boiling and melting points VSEPR Theory - Valence Shell Electron Pair Repulsion The VSEPR theory states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible. The electron dot structures are not flat 2D structures, but are 3D in real life. Molecules adjust their shapes so that the valence electron pairs are as far apart as possible. **See the chart on pg. 200 of your textbook Linear angles are 180 degrees definitely will be linear if only have two atoms in the molecule. No lone pairs and two covalent bonds or three lone pairs and one covalent bond around central atom. Example: CO 2 Bent again, unshared pair(s) strongly repels the covalent bonding pairs. Two lone pairs and two shared pairs around central atom. All angles are 105 degrees. Example: H 2 O Trigonal-Planar three shared pairs (covalent bonds ) separate as much as possible, but are unaffected by a lone pair (no lone pairs) of electrons like the pyramidal structure. Example: BF 3 Trigonal-Pyramidal one unshared pair strongly repels the three shared pairs (covalent bonding), pushing them closer together. All angles are 107 degrees. Example: NH 3 Tetrahedral four faced four shared pairs and no lone pairs, all angles are 109.5 degrees. Example: CH 4 Trigonal Bipyramidal five shared pairs separate as much as possible, but are unaffected by a lone pair of electrons (no lone pairs). Example: PCl 5 Octahedral six shared pairs separate as much as possible, but are unaffected by a lone pair of electrons (no lone pairs). Example: SF 6 Hybridization Hybridization - two atoms combine, their atomic orbitals overlap to produce molecular orbitals. One electron from each atomic orbital combines to create a shared pair in a molecular orbital.
sp hybridization has electrons in 2 orbitals sp 2 hybridization has electrons in 3 orbitals sp 3 hybridization has electrons in 4 orbitals
Intermolecular Forces (IMF) The forces of attraction between molecules are known as intermolecular forces. These forces vary in strength and are generally weaker than bonds The strongest IMF exists between polar molecules. A dipole is created by equal but opposite charges that are separated by a short distance. The direction of a dipole is from the dipole s positive pole to its negative pole. The negative region in one polar molecule attracts the positive region in adjacent molecules. So the molecules all attract each other from opposite sides. Such forces of attraction between polar molecules are known as dipole-dipole forces. Dipole-dipole forces act at short range, only between nearby molecules. Dipole Interactions - when polar molecules are attracted to one another: opposite charged regions of polar molecules are attracted. Hydrogen Bonds a particularly strong dipole interaction specifically involving hydrogen at the partially positive pole. Hydrogen is covalently bonded to a very electronegative atom AND to an unshared pair of another atom.
Hydrogen is able to bond with the unshared pair of electrons from another molecule because its valence electrons are not shielded from the nucleus by another layer of electrons (hydrogen s valence electrons are directly up against the nucleus). Example: H 2 O The more electronegative the element that hydrogen is bonded to the stronger the intermolecular attractions. Dispersion Forces - weakest of all molecular interactions caused by the motion of electrons. Vibrating electrons may end up moving randomly closer to one atom or another creating a momentary dipole. The more electrons there are the greater the interaction between nonpolar molecules.