121lec8 - <?xml version="1.0"...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Chemistry 121 Chapter 8 BASIC CONCEPTS OF CHEMICAL BONDING 8.1 Chemical Bonds, Lewis Symbols, and the Octet Rule The properties of many materials can be understood in terms of their microscopic properties. Microscopic properties of molecules include: o The connectivity between molecules and o The three-dimensional shape of the molecule. When atoms are strongly attracted to one another, we say that there is a chemical bond between them. o In chemical bonds, electrons are shared or transferred between atoms. Types of chemical bonds include the following: o Ionic bonds (electrostatic forces that hold atoms together, e.g. NaCl) o Covalent bonds (result from the sharing of electrons between atoms, e.g. Cl 2 ) o Metallic bonds (refer to metal nuclei floating in a sea of electrons, e.g. Na) Lewis Symbols The electrons involved in bonding are the valence electrons . o Valence electrons are in the outermost shell of an atom. Pictorially, we represent the electrons of an atom as dots around the symbol for the element. o These symbols are called Lewis symbols or Lewis electron-dot symbols. o The valence electrons available for bonding are indicated by unpaired dots. o We generally place the dots on four sides of a square around the element’s symbol. Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons: this is known as the octet rule . o An octet consists of full s and p subshells. o We know that s 2 p 6 is a noble gas configuration. o We assume that an atom is stable when surrounded by eight electrons (four electron pairs). 1 Study material for this chapter: Textbook sections 8.1 through 8.8
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
8.2 Ionic Bonding Consider the reaction between sodium and chlorine: Na(s) + ½ Cl 2 (g) → NaCl(s) ∆H f 0 = -410.9 kJ/mol o As indicated by the large negative change in enthalpy, this reactrion is violently exothermic. We infer that NaCl is more stable than its constituent atoms. Why? o Sodium has lost an electron to form Na + , and chlorine has gained an electron to form Cl - o Note that Na + has a Ne electron configuration and Cl - has an Ar configuration. o In other wors, both of these ions are surrounded by an octet of electrons. NaCl forms a very regular structure in which eac Na + ions is surrounded by six Cl - ions. o Similarly, each Cl - ion is surrounded by six Na + ions. o There is a regular arrangement of Na + and Cl - in three dimensions. o The ions are packed as closely as possible. o Also note that it is not easy to find a molecular formula to describe the ionic lattice. Energetics of Ionic Bond Formation The heat of formation of NaCl is exothermic: Na(s) + ½ Cl 2 (g) → NaCl(s) ∆H f 0 = -410.9 kJ/mol Separation of NaCl into sodium and chlorine ions is endothermic: NaCl(s) → Na + (g) + Cl - (g) ∆H f 0 = +788 kJ/mol The energy required to separate 1 mol of ions in an ionic lattice into gaseous ions is called the lattice energy , ∆H lattice
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/14/2009 for the course CHEM 121 taught by Professor Wyzlouzil during the Fall '07 term at Ohio State.

Page1 / 10

121lec8 - <?xml version="1.0"...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online