The Chemical Context of Life and Properties of Water Chapter 2 The Chemical Context of Life and Properties of Water

Why are we studying chemistry? Chemistry is the foundation of Biology

Basics You Should Know Matter Elements Compounds Atoms Subatomic Particles Atomic Number Atomic Mass Isotopes Electron Shells/Energy Levels Electron Orbitals

Everything is made of matter Matter is made of atoms Hydrogen 1 proton 1 electron Oxygen 8 protons 8 neutrons 8 electrons Proton + Neutron Electron –

Different kinds of atoms = elements The World of Elements H C N O Na Mg P S K Ca Different kinds of atoms = elements

Life requires ~25 chemical elements About 25 elements are essential for life Four elements make up 96% of living matter: • carbon (C) • hydrogen (H) • oxygen (O) • nitrogen (N) Four elements make up most of remaining 4%: • phosphorus (P) • calcium (Ca) • sulfur (S) • potassium (K)

Deficiencies If there is a deficiency of an essential element, disease results (a) Nitrogen deficiency (b) Iodine deficiency (Goiter)

Trace Elements Trace elements Are required by an organism in only minute quantities Minerals such as Fe and Zn are trace elements

How does this atom behave? Bonding properties Effect of electrons electrons determine chemical behavior of atom depends on number of electrons in atom’s outermost shell valence shell How does this atom behave?

Bonding properties How does this atom behave? What’s the magic number? Bonding properties Sulfur on the LEFT Magnesium on the RIGHT How does this atom behave? How does this atom behave?

Elements & their valence shells Elements in the same row have the same number of shells Moving from left to right, each element has a sequential addition of electrons (& protons)

Elements & their valence shells Elements in the same column have the same valence & similar chemical properties Oxygen has medium electronegativity so doesn’t pull electrons all the way off hydrogen whereas chlorine would. So oxygen forms a polar covalent bond. Carbon has only a weak electronegativity so forms a nonpolar covalent bond

This tendency drives chemical reactions… Chemical reactivity Atoms tend to complete a partially filled valence shell or empty a partially filled valence shell This tendency drives chemical reactions… and creates bonds – –

Ionic bonding High electronegativity difference strips valence electrons away from another atom Electron transfer creates ions (charged atoms) Cation (positive ion); anion (negative ion) Ex: Salts (sodium chloride)

Covalent Bonding Sharing pair of valence electrons Number of electrons required to complete an atom’s valence shell determines how many bonds will form Ex: Hydrogen & oxygen bonding in water; methane

Nonpolar covalent bond Pair of electrons shared equally by 2 atoms example: hydrocarbons = CxHx methane (CH4 ) balanced, stable, good building block

Polar covalent bonds Pair of electrons shared unequally by 2 atoms example: water = H2O oxygen has higher electronegativity oxygen has stronger “attraction” for the electrons than hydrogen water is a polar molecule + vs – poles leads to many interesting properties of water… H Oxygen + – – – – +

Hydrogen bonds Hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom (oxygen or nitrogen)

On Earth, it exists in three phyiscal states Properties of Water Capillary Action High Specific Heat Surface Tension Supports Life Adhesion Good Solvent On Earth, it exists in three phyiscal states Cohesion Solid Gas liquid

Properties of Water Earth is ¾ Water Supports All of Life Most animals and plants contain more than 60% water by volume. All living organisms require water more than any other substance

Properties of Water Cohesion/Adhesion Surface tension Temperature Moderation High specific heat Evaporative cooling Universal solvent

Effects of Changes in pH Water can dissociate Into hydronium ions (H+ or H3O+) and hydroxide (OH-) ions Changes in the concentration of these ions Can have a great affect on pH in living organisms H Hydronium ion (H3O+) Hydroxide ion (OH–) + – Figure on p. 53 of water dissociating

Acids and Bases An acid Is any substance that increases the hydrogen ion concentration of a solution A base Is any substance that reduces the hydrogen ion concentration of a solution (more OH- ions)

Buffers The internal pH of most living cells Buffers Must remain close to pH 7 Buffers Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution