Unit 1 – The Diversity of Matter and Chemical Bonding Chemistry 20 Unit 1 – The Diversity of Matter and Chemical Bonding

ammonia water Structural formulas show the way in which atoms are bonded to each other. methane

+ + - - - Attraction Attraction A chemical bond results from the simultaneous attraction of electrons by two atomic nuclei.

The electrons that occupy the outermost energy level are called valence electrons. The max. number of electrons in each energy level equals the number of elements in each period. Electrons are located at certain specific energy levels around the nucleus. The number of energy levels occupied by electrons equals the period number.

Bonding Theory: Valence Electrons and Orbitals Electrons exist in orbitals. An orbital is a region of space where there is a high probability of finding electrons of a particular energy. An orbital can be occupied by either one or two electrons.

Bonding electron: a single electron occupying an orbital The maximum number of bonding electrons determines an atom’s bonding capacity, the maximum number of single covalent bonds formed by an atom. Lone pair: two electrons occupying a filled orbital (non-bonding)

Electron Energy Level Diagrams 1 p+ 3 p+ 8 p+ 10 p+ 11 p+ 12 p+ H Li O Ne Na Mg Level Max. # of Electrons 1 2 8 3 4 18 valence electrons

Electrons “spread out” to occupy any empty valence orbitals before forming electron pairs.

Atomic Models: Lewis Symbols two lone pairs and two bonding electrons Gilbert Lewis (1875 – 1946) In a Lewis symbol (or Lewis dot diagram), valence electrons are represented as dots.

e- - Cl Li 3.0 1.0 Electronegativity + Electronegativity is the tendency for an atom to attract electrons to itself when it is chemically combined with another element. e- - + Cl Li 3.0 1.0 high electronegativity low electronegativity Lithium loses the e- tug-of-war with chloride.

Metals tend to have low electronegativities and non-metals tend to have high electronegativities.

Trends in Electronegativity Electronegativity increases from left to right in a period. e.g. period 2 Li Be B C N O F Li Electronegativity decreases as we move down a group. Na K Rb e.g. group 1 (alkali metals) Cs Fr

Homework: Read pgs. 78 – 81 pg. 82 Practice #’s 1 – 5

Covalent Bonding Covalent bonds form between non-metal atoms (all with high electronegativities). Covalent bonds involve sharing a pair of electrons, each atom approximating a complete outer shell because of the shared electrons.

Ionic Bonding Electrons are transferred from metal (low electronegativity) to non-metal (high electronegativity) atoms. The result is the formation of positive cations and negative anions.

All ionic compounds are arranged in a 3-dimensional crystal lattice structure. Each positive ion is surrounded by negative ions and vice versa.

Metallic Bonding Metal atoms generally have lower electronegativity. Pure metals do not consist of metal atoms, but of closely packed cations suspended in a “sea” of free electrons which can move between the cations (+ ions). The electrons act like a “glue” that holds the positive nuclei together.

Free e- transmit kinetic energy rapidly, thus metals are excellent conductors of heat.

The free electrons move rapidly in response to an electric field, making metals excellent conductors of electricity.

Individual atoms are held loosely to other atoms, so atoms slip easily past one another, so metals are ductile.

Homework: Read pgs. 82 – 84 pg. 84 Section 3.1 Questions #’s 1 – 10, 12