Chapter 2: Nature of Molecules and Properties of Water

Matter consists of chemical elements in pure form and in combinations called compounds

  • Organisms are composed of matter

  • Matter is anything that takes up space and has mass

  • Mass exists in different forms

Law of conservation of matter

  • Matter can neither be created or destroyed during a chemical reaction

Mass of reactants = Mass of products

Dalton’s atomic theory

  • Matter is composed of indestructible particles called atoms

  • All atoms of a given element have the same properties that differ from the properties of all other elements

  • Elements and commands are composed of definite arrangement of atoms and chemical change occurs when atomic arrays are rearranged

What is an element

  • A substance that cannot be broken down to other substances by chemical reactions 

Elements and Compounds 

  • A compound is a substance consisting of two or more elements in a fixed ratio 

  • An elements properties depend on the structure of its atom

  • Each element consists of unique atoms 

  • An atom is the small unit of matter that still retains the properties of an element 

  • Atomic number = # of protons 

Why are atoms neutral?

  • Atomic mass = P+N (protons + neutrons)

Math breakdown

Atomic Number = Protons 

Electrons = Positive, Negative, or Neutral

Mass = Protons + Neutrons 

Atomic Number = Protons = Electrons 

  • Protons - Positively Charged, Located in the nucleus 

  • Electrons - negatively charged, located in the e- shell

  • Neutrons - No Charge (Neutral) - Located in the nucleus 

Isotopes 

  • All atoms of a given element have the same number of protons, but some atoms have more neutrons than other elements (different atomic mass) 

  • formed either naturally through radioactive decay of elements 

Some applications of radioactive isotopes in biological research 

  • dating fossils 

  • tracing atoms through metabolic processes 

  • diagnosing medical disorders

Energy levels of electrons 

  • Any electrons are involved in chemical reactions between atoms 

  • An atom’s electrons vary in the amount of energy they possess

  • energy = capacity to cause change 

  • potential energy = energy possessed because of location or structure 

Energy level of electrons 

  • Third shell = Highest energy 

  • Second Shell = Higher energy

  • First shell = Lowest energy 

Electron distribution and chemicals properties 

  • # of electrons in an atom’s electrons shell determines its chemical behavior 

  • Depends mainly on the # of electrons in the outermost shell

  • Atoms with the same number of these electrons show similar chemical behavior 

  • Inert = unreactive atoms = full or completed valence e- shell 

  • Reactive atoms = incomplete valence e- shell

Electron Orbitals 

  • An orbital is the three - dimensional space where an electron is found 90% of the time

  • Each electron shell consists of a specific number of orbitals 

Ring 1 = 2, Ring 2 = 8, Ring 3 = 18, Ring 4 = 32 (fill rings based off of electrons / protons, ring numbers show how many it can hold)

  • Spread within ring, start from innermost (2 → 32)

What is a molecule?

  • Two or more atoms held together by covalent bonds 

  • Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms 

  • These interactions usually result in atoms staying close together held by attractions carried other atoms

Covalent Bonds

  • Atoms share a pair of valence electrons

  • the orbitals overlap, which allows atoms to share electrons

Ionic Bonds

  • atoms sometimes strip electrons from their bonding partners

  • transfer of an electron from sodium to chlorine

  • after the transfer, both atoms have charges

  • charged atom/molecule is an ion

  • Cations = Lose, Anions = Gain

Electronegativity

  • a measure of the attraction of an atom for the electrons in a covalent bond

  • the more electronegative an atom is, the harder it pulls shared e- to itself

  • Fluorine is the most reactive non-metal; it has the highest value since it has the greater attraction for the e- being shared by the other leements

  • Oxygen is highly electronegative and has a strong attraction for electrons

  • Metals have low electronegativities since they have weak attraction for any shared e-

  • For ionic bonds: higher than 1.7

Hydrogen bonds

  • form when a hydrogen atom covalently bonded to one electronegative atom is also attracted to an electronegative atom of a different molecule

  • Chemical reactions are the making and breaking of chemical bonds

  • The starting of molecules = Reactants, Final Molecules = Product

Molecular shape and function

  • Structure determines functions

  • A molecules shape is very important to the function

  • All chemical reactions are reversible

Keep in mind: Polar = Unequal, Non Polar = Equal

Group # = Group of electrons

Water Properties

Polarity of Water

  • Because oxygen is more electronegative than hydrogen, shared electrons are pulled more toward oxygen

  • This results in a partial negative charge on the oxygen and a partial positive charge on the hydrogens

Hydrogen vs Polar Covalent Bonds (test question!!)****

Properties: Cohesion, Adhesion, Surface tension

  • Cohesion: Water to Water Bonding

  • Adhesion: Water to other surfaces bonding

  • Surface Tension: Difficulty of stretching or breaking surface

Water has a very high surface tension

  • Surface is a measure of how hard it is to break the surface of a liquid

  • Water has an unusually high surface tension due to hydrogen bonding between the molecules at the air-water interface and to the water below

Moderation of temperature by water

  • water absorbs heat from warmer air and releases stores heat to cooler air 

  • Water can absorb or release a large amount of heat with only a slight change in its own temperature

Temperature and heat

  • Kinetic energy is the energy of motion

  • The kinetic energy associated with random motion of atoms or molecules is called thermal energy

  • Temperature represents the average kinetic energy of the molecules in a body of matter

  • • Thermal energy is transfer from one body of matter to another is defined as heat

A calorie (cal) is the amount of heat required to raise the temperature of 1g of water by 1°C

  • • It is also the amount of heat released when 1g of water cools by 1

Water’s specific heat

  • The specific heat of a substance is the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1C

  • The specific heat of water is 1 cal/g*C)

  • Water resists changing its temperature because of its high specific heat

  • Water’s high specific heat can be traced to hydrogen bonding between the

  • Heat is absorbed when hydrogen bonds break

  • Heat is released when hydrogen bonds form

  • The high specific heat of water minimizes temperature fluctuations to within limits that permit life

  • A large body of water can absorb and store a huge amount of heat from the sun in daytime and during summer while warming up only a few degrees

  • At night and during the winter the gradually cooling water can warm the air

  • This serves to moderate air temperature in coastal areas

Evaporative cooling

  • evaporation or vaporization is transformation of a substance from liquid to gas

  • Heat of vaporization is the heat a liquid must absorb for 1g to be converted to gas

  • AS a liquid evaporates, its remaining surface cools, through a process called evaporative cooling

  • Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water

Floating of ice on liquid water

  • water is less dense as a solid than as a liquid

  • At 0 Celsius, water molecules are locked into a crystalline article

  • The hydrogen bonds keep the molecules far enough apart to make the ice ~10% less dense than liquid water

  • Water reaches its greatest density at 4 Celsius

  • If ice sank, all bodies water would eventually freeze solid, making life impossible on earth

Water: The solvent of life

  • a solution is a liquid that is a completely homogeneous mixture of substances

  • The solvent is the dissolving agent of a solution

  • The solute is the substance that is dissolved

  • The aqueous solutions is one in which the water is the solvent

Acids and bases

Acid

  • A substance which, when dissolved in water, releases protons

  • Acid gives more hydrogen, making it more acidic

  • Acid strength correlates with the number of protons released

Base

  • a substance which, when dissolved in waters decreases proteins and increases the hydroxide concentration

  • base strength correlates with hour many protons are removed from solution

Buffer

  • A buffer is a solution of a weak acid and its conjugate base that resists changes in pH in both directions-either up or down