An Introduction to the Basics of Chemistry Forensic Investigation Ch 300.

An Introduction to the Basics of Chemistry

Forensic InvestigationCh 300

protons (nucleus) – positively charged particles

neutrons (nucleus) – particles with no charge

electrons ( in orbit) – negatively charged particles

All matter is composed of atoms

The Atom

The Periodic Table

element – a substance that cannot be chemically broken down

atomic number – number of protons and electrons an element possesses

mass number – sum of protons and neutrons in a particular element

Compounds- a substance made up of two or more elements

H2O

CO2

O2

NH3

O

O O

O O

C

N

O

O O

CO O

N

N

18

44

32

17

Compounds

O O

N

Two types:

Organic Compounds – chemical compounds composed mostly of carbon and hydrogen (may contain N, O, S, P and other atoms as well)

All of the chemical compounds we will talk about in forensics this term will be organic compounds.

Inorganic Compounds – chemical compounds resulting from reactions of any and all the other elements on the periodic table

Of the 22 million compounds known to man, 20 million of them are organic.

Chemical Bonding

Two types:

Ionic Bonding – attraction between a positive atom (ion) and a negative atom (ion); many inorganic substances interact via ionic bonds

Covalent Bonding – where two atoms share electrons; organic compounds always share electrons to make bonds

Most of the chemicals we will talk about in forensics will involve covalent bonds!

Na + Cl_

O

Important Chemical Properties

Electronegativity – the degree to which atoms attract electrons (increases up and to the right on the periodic table)

fluorine, oxygen, nitrogen, chlorine, bromine - some of the most electronegative

Important Chemical Properties

Electronegativity within compounds leads to a concept called polarity.

Polarity is an imbalance in the position of electrons in a compound due to differing electronegativities of the individual atoms; the electrons are attracted to one side (top vs bottom, left vs right) of a compound more than the other.

The way they are drawn, which of the following compounds are polar?

O

CO O

N

Cwater

carbon dioxide

methane

ammonia

Important Chemical Properties

Polarity leads to a phenomenon known as intermolecular forces. Intermolecular (IM) forces are the forces that attract one molecule to another – an attraction between molecules.

There are three types of IM forces that we will consider in forensics:

a. Dipole-dipole

b. Hydrogen bond ( a type of dipole-dipole)

c. London forces

Polarity influences the way chemicals interact (their behavior) through IM forces.

Chemical Reactivity

There are two factors that affect the reactivity of a chemical compound (two, at least, that we will consider in this class):

a.shape of the molecule

b. the energy of the reactants vs the energy of the products in

a reaction

Reactants Products

Oklahoma City 1995