The Chemical Context Of Life CHAPTER 2

CHAPTER 2 PAGE 1 : Elements n Life depends on chemical reactions n Matter takes up space and occupies mass n There are 92 naturally occurring elements n 25 of the 92 elements are essential to life

CHAPTER 2 PAGE 1 : Elements n Four elements, Hydrogen, Oxygen,Carbon, Nitrogen make up 96% of living matter n Most of the remaining 4% is accounted for by P, Ca, S, K n Trace elements are required in small amounts, usually as catalysts n Defn: An element can’t be broken down to any other substance by a chemical reaction

Chapter 2 Page 2 : Atoms n Atoms composed of protons, neutrons and electrons n Atomic number = # protons n Atomic mass /wt = protons + neutrons, we will not differentiate the two terms n Electrons have orbitals and corresponding energy levels n The outermost orbital is called the valence shell. This is the part of the atom that takes part in chemical reactions. n “Valence” denotes the bonding capacity of an atom. Number of electrons needed n Valence Electrons = the number of unpaired electrons in the outermost orbital { Valence Shell }

Chapter 2 Page 3:Bonding n Octet Rule = when atoms react they do so to obtain stability in the valence shell. In most of the atoms we will deal with in this class, 8 electrons will confer the stability. The exception is hydrogen, it has only the first level orbital. The first orbital level can only ever hold two electrons. Hydrogen will be stable with two electrons in its valence shell.

Chapter2 Page 4 : Bonding cont. n When the valence electrons of atoms interact chemical bonds are formed. Several types are important in biological systems. n There are three major types of bonds we will discuss: covalent, ionic, and hydrogen. Please read about Van der Waals interactions in your book. n It is extremely important that you understand these concepts. Some confusion about them at first is usual. If after studying you do not understand please come and see me or a teaching assistant.

Chapter 2 Page 5: Covalent Bonds n Electrons are shared in covalent bonds n Polar = charged n Non - Polar = not charged n In non - polar covalent bonds the electrons are shared equally ; Ex. H2 n In polar covalent bonds one atom is more electronegative and the electrons are not equally shared; Ex H2O n Water also contains special bond s that we will discuss in a moment

Chapter 2 Page 6 : Ionic Bonds n Ionic bonds involve transfer of electrons. n Normally occurs between atoms with charges n charged atoms are called ions n NaCl is one example of an ionic compound. Please know the definitions in the chapters in your book. n Lets do a problem:What is the valence of Chlorine? Will it form a covalent or ionic bond? Why? How may bonds will it form?

Chapter 2 Page 7 : Hydrogen Bonds n Hydrogen bonds are very important in biological systems, and humans. n They occur when a hydrogen atom is bonded covalently to an electronegative atom and is also attracted to another electronegative atom. n H2O is a good example ; the most electronegative atom is “stronger” and pulls the electrons into orbiting around it for a higher percentage of the time. Subsequently partial charges form on the bonded elements, and attractions from to the partially charged areas

Isotopes n Differ in the number of neutrons n Used in many medical tests – PET Scans

Molecular Shape Is Critical To Life

Van der Waals Interactions