Dry Lab 3 Atomic and Molecular Structure Objectives

1
Dry Lab 3 Atomic and Molecular Structure -
Objectives

• To view and calibrate the visible line spectra
  for a number of elements.
• To identify an element from its visible line
  spectrum.
• To identify a compound from its infrared
  spectrum.
• To predict the 3-D structure of molecules and
  molecular ions.

2
Dry Lab 3 Introduction (EM Spectra)

• The sun emits energy in the form of
  electromagnetic radiation.
• This energy is either absorbed or transmitted.
• When the energy is absorbed by a molecule,
  something in that molecule must be capable of
  absorbing the energy. Within compounds, those
  somethings are electrons, vibrating bonds and
  rotating atoms.

3
Dry Lab 3 - Introduction

• When elemental atoms absorb energy, only
  electronic energy states are available for energy
  absorption the electrons absorb the energy to
  reach excited states. As an atom returns to its
  ground state, energy is given off as photons.
• Since every element is different, every atom of
  an element possesses a unique set of electronic
  energy states that can be used to identify the
  element.

4
Dry Lab 3 - Introduction

• When the electron returns to its ground state,
  the energy is emitted as a photon. When the
  photon(s) pass through a prism, an emission line
  spectra is produced. Each line corresponds to a
  photon of fixed energy and wavelength.
• Visible light is responsible for all the colors
  of nature. The colors our eyes detect are the
  wavelengths of visible light not absorbed, i.e.,
  the transmitted light.

5
Dry Lab 3 Introduction (IR Spectra)

• The infrared (IR) portion of the spectrum can be
  used to identify atom arrangements in molecules.
• Ex The O-H bond absorbs IR between 3700 and 3500
  cm-1.

6
Dry Lab 3 Introduction

• Fig. D3.5D has a peak at 3700-3500cm-1 which
  correlates to the O-H atom arrangement as per
  Table D3.2. Another peak at 3000cm-1 correlates
  to C-H, one at 1200cm-1 correlates to C-O. Look
  at the structures below for one that has a C-H
  bond, an O-H bond and a C-O bond.
• Ethanol, methyl formate, and acetic acid all do,
  but methyl formate and acetic acid both have a
  CO bond which absorbs at 1800cm-1. There is no
  absorption peak at 1800cm-1, so the compound must
  be ethanol.

7
Dry Lab 3 - Introduction

• Lewis Theory
• Atoms form bonds by losing, gaining, or sharing
  electrons until each atom of the molecule has the
  same number of valence electrons (8) as the
  nearest noble gas in the periodic table.
• The resulting arrangement of atoms forms the
  Lewis structure of the compound.
• Atoms of period 3 or greater often extend valence
  electrons to accommodate additional bonds the
  central atom has more than 8 valence electrons.
• Ex SF4 6 valence electrons on S and 4 on F.

8
Dry Lab 3 - Introduction

• A Lewis structure accounts for bonding, but does
  not predict the 3-D structure of the molecule.
• Valence Shell Electron Pair Repulsion Theory
  proposes that the structure of a molecule is
  determined by the repulsive interaction of the
  electron pairs of its central atom. In other
  words, the orientation maximizes the distance
  between electron pairs, or the electron pair
  interactions are minimized.

9
Dry Lab 3 Procedure/Questions

• p. 157 A, B.1, B.2.a, C (pick one not 3.5D)
• p. 158 D.1.a, c, D.2.b, d, D.3.a, c, D.4.b, d
• p. 159-160 2, 6, 10