Taking too long? Close loading screen.
Just a sec. Gotta wake up the lions.

Learning Outline

Introductory Chemistry

heading icon

Pre-A&P (BIO 095)

Chemistry is the basis for biology

Basic structural concepts

  • Atom: smallest part of a pure substance, or element, that still has the properties of that substance
  • Molecule: a particle made up of one or more atoms bound together
  • Element: a pure substance, made entirely of one kind of atom (all known elements are listed in the Periodic Table)
  • Compound: substance made of molecules that contain more than one kind of atom

Atomic structure and function video

  • Many different models exist; none are perfect slide
    • We’ll look at the Bohr model and electron cloud model first —then later structural and space-filling models
    • Bohr model (named for Niels Bohr) shows electrons orbiting the nucleus like planets orbit the sun
    • Electron cloud model (ECM) shows electrons as a cloud surrounding the nucleus
  • Nucleus
    • Protons have positive (+) charge; number of protons is “atomic number” and signifies kind of atom (which element)
    • Neutrons have no charge; optional and variable in number (isotopes)
  • Electrons
    • Electrons have a negative (-) charge and do not affect the type of element
    • Electrons may be present in regions called energy levels
      • The further from the nucleus, the more energy is needed by the electron to stay there –so the higher energy levels are farther from the nucleus
      • The lowest (first) energy level can contain up to two electrons
        • The next few can hold up to eight electrons each (the “octet rule”)
      • Energy levels fill from the inside out
Bohr model

Bohr model (hydrogen atom)

Bohr oxygen

Bohr model of oxygen atom (s and p are suborbitals) Credit

Energy levels

Electrons move to new energy levels as they gain or lose energy (squiggly arrow)

Electron cloud model (helium)

Electron cloud model (helium)

Behavior of atoms: chemical bonds slide

  • Atoms want to be “happy”—meaning having a full outer energy level
  • One path to happiness: ionic bonds
    • Occurs when an atom gives one or two electrons to another atom, giving both “full” outer energy levels
    • Ion: charged particle (atom or group of atoms)
    • Ions “stick” together because opposite charges attract —forming a bond
    • Electrolyte: molecule that dissolves in water to form ions
      • Sometimes the individual, dissolved ions are also called electrolytes
  • Another path to happiness: covalent bonds video
    • Outer level is “valence” level (from Latin word for outer garment)
    • Occurs when atoms share valence electrons (co-valent, get it?)
    • Sharing may not be equal slide
      • Polar molecules occur when sharing is NOT equal
        • Polar molecules are hydrophilic, or “get along with water”
        • Ions also “get along with water”
      • Nonpolar molecules occur when sharing IS equal
        • Nonpolar molecules are often hydrophobic, “don’t get along with water” slide

Other attractions

  • Sometimes also referred to as “bonds” or “weak attractions”
  • Hydrogen bonds (formed when polar molecules stick together) slide icon
  • Other weak attractions from temporary shifts in charge within molecules


  • Water is cohesive—hydrogen bonds make it “sticky” slide
  • May be represented H2O or H-O-H or HOH
  • Solvent: something in which other molecules (solutes) are dissolved
  • Solution: mixture of solutes dissolved in a fluid solventaintaining a relatively stable pH
water with hydrogen bonds

Hydrogen bonds (dotted lines) form between polar water molecules, making water cohesive

water molecule

Water molecule (space-filling model)

water molecule

Water molecule (structural model)

glucose dissolved in water

Glucose (center molecule) is polar, forming attractions with surrounding water molecules (also polar) to form a solution.

salt dissolving

When table salt (sodium chloride crystals) is put into water, the sodium and chloride ions dissociate, and become surrounded by water molecules, forming an aqueous (watery) solution (see images to right)

sodium ion in water

Sodium ion (center) is positive and forms attractions with the negative regions of surrounding water molecules (polar), forming a solution.

chloride in water

Central chloride ion (negative) forms attractions with surrounding polar water molecules, forming a solution

pH: acids, bases, and buffers slide

  • Water molecules may dissociate into H+ and OH
    • H+ is the hydrogen ion
    • OH is the hydroxide ion
  • Solutions with equal proportion of H+ and OH are “neutral”
  • Acids: solutions with higher proportion of H+
  • Bases: solutions with a lower proportion of H+ (alkaline solutions)
  • Can be expressed as “power of Hydrogen” or pH
    • pH scale is an inverse (base 10) logarithm of relative H+ concentration
    • 7 = neutral
    • acid = anything lower than 7
    • base = anything higher than 7
    • human blood plasma = pH 7.35-7.45
  • Buffer: system of molecules that absorb or release H+ —maintaining a relatively stable pH

This is a Learning Outline page.
Did you notice the EXTRA menu at the top right of each Learning Outline page with extra helps?

Last updated: June 27, 2018 at 20:02 pm