CHEMICAL BONDS SC2. Obtain, evaluate, and communicate information about the chemical and physical properties of matter resulting from the ability of atoms to form bonds.  a. Plan and carry out an investigation to gather evidence to compare the physical and chemical properties at the macroscopic scale to infer the strength of intermolecular and intramolecular forces.  b. Construct an argument by applying principles of inter- and intramolecular forces to identify substances based on chemical and physical properties. c. Construct an explanation about the importance of molecular level structure in the functioning of designed materials. d. Develop and use models to evaluate bonding configurations from nonpolar covalent to ionic bonding. 

Intermolecular vs. Intramolecular forces Intramolecular forces are the forces that hold atoms together within a molecule. These are the strong forces of chemical bonding (ionic/covalent/metallic) Intermolecular forces are forces that exist between molecules. These are much weaker interactions (dipole-dipole/hydrogen bonding/van der Waals), but they influence physical properties

What is a chemical bond? Remember that although atoms are neutral (# of protons = # of electrons), they are not all stable (unreactive). The most stable atoms are those that have a full outer energy level of valence electrons. Which elements have the most stable atoms?

What is a chemical bond? So, for an atom to achieve that “noble gas-like” stability, it must gain or lose valence electrons. Atoms must, therefore, interact with other atoms to exchange or share electrons. This interaction of atoms is called bonding. Lewis structures are commonly used to “track” electron exchange/sharing.

What is a chemical bond? Bond formation releases energy, while bond cleavage (breaking) absorbs energy. The amount of energy required to break a bond is called bond energy, and it is a measure of the strength of a bond. Higher bond energy = stronger bond Bond length is inversely related to bond energy Higher bond energy = shorter bond length

Types of Bonding If atoms exchange electrons, it results in the formation of ions – either positive (cations) or negative (anions). This interaction is called ionic bonding. If atoms share electrons, it is called covalent bonding. If metal atoms share their delocalized "sea" of electrons, it is called metallic bonding.

Ionic bonding The ion formed by an atom depends on its current number of valence electrons and how close the atom is to noble gas configuration. Remember that valence electron count is predicted by the group number. Generally, atoms will only exchange up to 3 (rarely 4) electrons to gain the “octet.”

IONIC BONDING Ionic bonds are essentially electrostatic attractions between atoms. Which types of elements will tend to form ionic bonds between their atoms (ions)? Why? Ions will bond so that the resulting compound is neutral.

bond

IONIC BONDING Properties resulting from ionic bonding include: Crystalline solid Very high melting point Soluble in H2O Nonconductor of heat and electricity Conducts electricity in aqueous solutions Examples: NaCl, CaCO3

COVALENT bonding Atoms that participate in covalent bonding exchange two electrons per bond. The electrons come from overlapping orbitals – typically one from each atom Which kind of elements tend to form covalent bonds? Why? A covalently bonded compound is called a molecule.

COVALENT bonding Multiple covalent bonds may form if more than one pair of electrons is shared. This is common with C, N, O, P, S

COVALENT bonding An alternate form (coordinate covalent bond) occurs when one atom contributes both electrons to the bond

COVALENT bonding Atoms that covalently bond can either result in a charged ion or a neutral compound.

COVALENT bonding For several compounds, multiple valid Lewis structures may be drawn. These are called resonance structures.

COVALENT bonding Some elements that participate in covalent bonding also undergo a process called hybridization Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridized orbitals are very useful in the explanation of the shape of molecules.

COVALENT bonding There are 3 types of hybrid orbitals: sp            s orbital + 1 p orbital Ex: C2H2

COVALENT bonding There are 3 types of hybrid orbitals: sp2           s orbital + 2 p orbitals Ex. C2H4

COVALENT bonding sp3           s orbital + 3 p orbitals Ex. CH4

COVALENT bonding When orbitals hybridize, additional orbital bonds form: Sigma bonds (σ) form when orbitals overlap end-to-end Pi bonds (π) form when orbitals overlap side-by-side Sigma bonds are stronger than pi bonds

COVALENT bonding

COVALENT BONDING How to describe the shape of a molecule: VSEPR (valence shell electron pair repulsion) theory states that the valence electron pairs surrounding an atom tend to repel each other and will, therefore, adopt an arrangement that minimizes this repulsion, thus determining the molecule's geometry.

COVALENT BONDING Three basic electron geometries: Linear Trigonal planar Tetrahedral There are others, but they involve greater degrees of hybridization

COVALENT BONDING Not all atoms share equally: Polar covalent bonds occur among atoms that share unequally, producing a dipole Often occurs in compounds containing halogens, N and O

COVALENT BONDING Not all atoms share equally: Nonpolar covalent bonds occur among atoms that share nearly equally Generally among same kinds of atoms Polar covalent bond strength > nonpolar c.b.s.

COVALENT BONDING Properties resulting from covalent bonding include: Gas, liquid, or a soft solid Low melting point and low boiling point Insoluble in H2O Soluble in nonpolar solvents Nonconductor of heat and electricity Nonlustrous Examples: CO2, CH3COOH

Predicting Bonds Calculating the difference in electronegativity can predict ionic vs. polar covalent vs. nonpolar covalent bonds Difference  1.9  ionic Difference 0.5 – 1.9  polar covalent Difference < 0.5 nonpolar covalent

metallic BONDING Metallic bonding occurs when transition metals bond to either themselves or mixed with other metals in alloys Having generally low electronegativity they tend to lose their valence electrons easily  The valence electrons detach from the atoms but are not held by any one of the other atoms – they are only held collectively by the entire assemblage of atoms

metallic BONDING In a metal the ion cores are held in an ordered, or crystal, lattice. The valence electrons are free to move about under applied stimulation, such as electric fields or heat.

metallic BONDING Properties resulting from metallic bonding include: Malleable solid High melting point and boiling point Insoluble in H2O Insoluble in nonpolar solvents Conducts heat and electricity Lustrous Examples: gold, copper

Intermolecular forces Intermolecular forces are generally referred to as van der Waals forces. These are distance-dependent interactions between atoms or molecules. These forces play a fundamental role in polymer science, nanotechnology, and surface science, and define many properties of compounds, including solubility in polar and nonpolar solvents.

Intermolecular forces There are 2 types: Dipole-dipole Electrostatic interactions between molecules which have permanent dipole(s)

Intermolecular forces There are 2 main types of intermolecular forces: Dipole-dipole Hydrogen bonding is a form of dipole-dipole interaction

Intermolecular forces London Dispersion Forces Arise from interaction between uncharged atoms or molecules