Chemistry lab Bonding

Chem 1A – Expt-13 Molecular Models and Lewis Structures – C, H, N, O

Dr. Alex Madonik Page 1 College of Alameda

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Part A: Covalent Bonding – How Atoms Share Electrons (13 points) Two atoms form a covalent bond when they share a PAIR of electrons, represented by two dots (:) or a dash (-). In general, each atom contributes one electron to the bond. Hydrogen (H) has just one electron to share, so it can form only one bond. By sharing an electron with another atom, it fills its 1s orbital, giving it a full shell, similar to the stable full shell in helium. H2 is the simplest possible molecule: H:H or H-H. The next eight elements fill orbitals in the second shell – the 2s orbital can hold two electrons, and the three 2p orbitals can hold six electrons. The number of outer shell (“valence”) electrons determines what ions or bonds an element can form. Lithium easily loses its 2s electron, forming a stable cation (Li+) with a full first shell and empty second shell, like helium. Most compounds of lithium are ionic. Similarly, beryllium can lose two electrons to form a stable Be2+ ion. Or, it can share those two electrons to form two covalent bonds. Cl – Be – Cl is linear (180°). Boron can make three bonds by sharing its three valence electrons. In these compounds, the boron outer shell contains three pairs of electrons. The electron pairs repel each other and spread out as far apart as possible; the resulting molecular shape is flat (“Trigonal Planar”) with 120° angles between the bonds.

CH4

(methane)

Carbon has four valence electrons; it can make four bonds to four different atoms. Draw Lewis structures for methane and ethane.

H3C-CH3 (ethane) The four bonds spread out like a triangular pyramid; the shape is “tetrahedral” with 109° angles. Four bonds = eight shared electrons = a stable “octet.” The second shell is filled, as it is for neon.

H2C=CH2 (ethylene)

Carbon can also form double bonds, where it shares two pairs of electrons with another atom. Compounds with double bonds are flat because there are just three atoms attached to carbon.

H2C=O

(formaldehyde)

This geometry is trigonal planar” (120° angles). Draw Lewis structures – oxygen has lone pairs!

Chem 1A – Expt-13 Molecular Models and Lewis Structures – C, H, N, O

Dr. Alex Madonik Page 2 College of Alameda

NH3 (ammonia) Nitrogen has five outer shell electrons, so it can reach a stable, “octet” configuration by adding three shared electrons. So, nitrogen usually forms three covalent bonds. The other two electrons in the outer shell aren’t needed for bonding, but they occupy an orbital as a “lone pair.”

NH4+

(ammonium ion)

When nitrogen forms three bonds to three different atoms, it shares one electron with each of them, and the resulting compounds are pyramidal (109° angles). The lone pair of electrons occupies the fourth corner of the pyramid. If this pair bonds with H+, it forms the ammonium ion. Draw the Lewis structures (include lone pairs and charges).

Carbon and Nitrogen can form triple bonds, in which two atoms share a total of six electrons. Compounds with triple bonds are linear. Draw these Lewis structures.

N2 (nitrogen) HCCH (acetylene) HCN (hydrogen cyanide)

Oxygen has six outer shell electrons, so it can reach a stable “octet” configuration by adding two shared electrons. Oxygen usually forms two covalent bonds with a 109° angle (BENT). What about the other four electrons in the outer shell? They aren’t needed for bonding, but they occupy two orbitals as “lone pairs.” Draw these Lewis structures – don’t forget, carbon and oxygen must have octets!

H2O (water) O2 (oxygen) formic acid HCO2H

carbon dioxide CO2

Note: the halogens (F, Cl, Br, I) have seven valence electrons and complete their outer shell by forming one bond. That leaves three lone pairs of electrons. Part B: Build models for at least 7 of your Lewis structures Assign ALL MODELS to one of these four categories (7 points, more possible):

Linear Molecules Bent Molecules Trigonal planar Pyramidal or Tetrahedral