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• Forces that hold atoms together
• Ionic bonds: the forces of attraction between ions; involves a transfer of electrons.
• Covalent bonds: the forces of attraction
between two atoms that are sharing electrons
• Result from reaction between metal and nonmetal to form a cation and an anion
• An ionic bond is the attraction between a positive ion and negative ion.
• The ions are arranged in a pattern called a crystal lattice.
• Sharing pairs of electrons • Molecules attracted to each other weakly • Often found between nonmetal atoms N2 Cl2 HF
• Covalent bonding between unlike atoms results in unequal sharing of the electrons.
• The result is bond polarity; HF
H F • • δ+ δ−
Electron Sharing in HF
If electrons were equally shared Actual molecule
• Measure of the inherent ability of an atom to attract shared electrons
• A larger electronegativity means that an atom attracts shared electrons more strongly; fluorine in HF
• The difference in electronegativity between two atom in a bond is a measure of bond polarity. A larger difference in electro- negativity means a more polar bond.
If the difference is ≥2, the bond is ionic If the difference is >0 and <2, the bond is polar covalent If the difference is 0, the bond is covalent
Choose the bond in each pair that will be more polar.
• H-P or H-C: • O-F or O-I: • N-O or S-O: • N-H or Si-H:
Classify the following bonds as ionic, covalent, or polar covalent
• HCl • NaF • Cl2 • Cl-F • NH3
• Any molecule that has a center of positive charge and a center of negative charge at different points in space is said to have a dipole moment.
Dipole Moment (cont.) • In polyatomic molecules the dipole
moment depends upon the atoms involved and the three-dimensional structure.
Dipole Moment (cont.) The dipole moment affects the attractive forces between
molecules, and therefore the physical properties of the substance. For example, water readily dissolves NaCl.
Electron Arrangements and Ionic Bonding
• Metals lose their valence electrons to form cations. • Nonmetals gain electrons to form anions. • Both try to acquire the electron configuration of a
noble gas; for example, NaCl
Na: [Ne]3s1 → [Ne] + 1e- Cl: [Ne]3s23p5 + 1e- → [Ar]
What are the electron configurations of the ions in the following compounds?
• MgO • SrF2
Electron Arrangements and Ionic Bonding (cont.)
• Formulas are predicted by achieving electrical neutrality using the component cations and anions.
What is the formula of the ionic compound formed from Al and S?
• In polyatomic ions, the atoms in the ion are connected with covalent bonds. The ions are attracted to oppositely charged ions to form an ionic compound.
Properties of Ionic Compounds
• All are solids at room temperature. – Melting points are greater than 300°C.
• Many are soluble in water. When dissolved
the solution becomes an electrical conductor.
Bonding & Structure of Ionic Compounds
• Crystal lattice: geometric pattern determined by the size and charge of the ions.
• Anions are almost always larger than cations. • Anions are generally considered “hard” spheres
packed as efficiently as possible, with the cations occupying the “holes” in the packing.
• Compounds with polyatomic anions contain covalent bonds within the anion structure, which are ionically bonded to the cation; CuSO4
• Use the symbol of the element to represent the nucleus and inner electrons.
• Use dots around the symbol to represent valence electrons.
• Elements try to achieve a noble gas configuration. sodium chloride
Lewis Structures (molecules with covalent bonds)
• Hydrogen shares two electrons (duet rule).
• Helium already has two, so it does not form bonds. • The second row nonmetals require eight electrons to fill the 2s and 2p orbitals (octet rule). example: Cl2 • Neon has eight electrons, so it does not form bonds.
Writing Lewis Structures of Molecules
• Count the total number of valence electrons from all the atoms.
• Attach the atoms together with one pair of electrons. • Arrange the remaining electrons in pairs so that all
hydrogen atoms have two electrons (one bond) and other atoms have eight electrons (combination of bonding and nonbonding). Nonbonding pairs of electrons are also know as lone pairs.
Draw Lewis structures for the following molecules:
Multiple Covalent Bonds
• Single covalent bond: atoms share two electrons (one pair); HBr
• Double covalent bond: atoms share four electrons (two pairs); C2H4
• Triple covalent bond: atoms share six electrons (three pairs); HCN