Chapter 5a Chemical Principles

 

Basics of Chemistry

 

    element       electron

    atom            ion

    proton          cation / anion

    neutron        molecule

 

 

 

Parts of an Atom

 

Atom the smallest unit of matter that displays all of the properties of the element

 

Nucleus - the center of the atom, with most of the mass

 

 composed of:

 

    protons positively charged particles   (+)

 

    neutrons neutrally charged particles  (0)

 

     

correct pronunciation:   “nuclear”  NOT   “nucular”

 

Electrons - negatively (-) charged particles that orbit the nucleus in discrete shells

   

 

An electrically stable atom has the same number of protons as electrons

 

  the charges balance  (+) = (-)

 

nuclei are small:                 10–15 meters,  1.67 x 10–27 kilograms

electrons are smaller:        10–19 meters,  9.1 x 10–31 kilograms

 

Elements

An element is defined by the number of protons in the nucleus

 

 

Examples:

 

  1 proton     hydrogen

  6 protons    carbon

  8 protons    oxygen

26 protons    iron

92 protons    uranium

 

The Periodic Table of the Elements

A way of organizing the elements to show their physical properties

      elements identified by atomic number

 

 

Electron Shells

 

Determine chemical reactions

Electrons orbit the nucleus in specific shells

Each shell has a maximum number of electrons, which is its stable configuration

 

An atom is unstable if it has only one or two electrons in the outer shell,

      or if it is missing one or two electrons in the outer shell

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For example, hydrogen and oxygen combine to form water (H2O):

 

 

 

 

 

 

 

 

 

      The first two shells are filled by 2 and 8 electrons, respectively.

 

Hydrogen has one electron in the inner shell, and needs to add a second electron to be stable

 

Oxygen has 2 electrons in the inner shell (which is stable), but only 6 in the second shell, and needs to add 2 electrons to be stable

 

If oxygen “shares” electrons with 2 hydrogen atoms, the outer shell of the hydrogen has 2 electrons and is stable, and the outer shell of oxygen has 8 electrons and is stable

 

This sharing of electrons forms a covalent bond between the oxygen and hydrogen atoms

 

Ions

if the charges on an atom don’t balance:

 

 

  cation – a positive ion, “missing” one or

              more electrons  { cat ion }

 

              most metals produce cations

              example:  Na+

 

 

  anion – a negative ion, with one or more

              “extra” electrons  { an ion }

 

              example:  Cl–  

 

 

Elemental cations and elemental anions

 

the configuration of the electron shells determines the preferred ionic charge for an element

 

Elemental cations

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Elemental anions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Groups of elements with similar properties

      also determined by the electron shells

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Isotopes of an Element

An isotope has the same number of protons

  but a different number of neutrons

 

For example:  Isotopes of oxygen

      ”regular” oxygen:  8 protons (p), 8 neutrons (n)

      this is 16O, total mass of 16 amu (atomic mass units)

 

      also:  17O (8 p, 9 n), and 18O (8 p, 10 n)

 

Important use for isotopes: 

tracing processes and the fate of different elements or compounds,

for example, tracing water vapor through the hydrologic cycle

 

Stable Isotopes and Radio-Isotopes

 

      Stable isotopes do NOT decay radioactively

 

 

Isotopes of Hydrogen:

      1H (hydrogen, 99%),  2H (deuterium, 1%)

      3H (tritium, radioactive)

 

All of the isotopes of oxygen (16O, 17O, and 18O) are stable

 

Isotopes of Carbon

      12C  carbon-12   6p 6n   98.89%

            13C carbon-13   7n    1.11%

 

 

 

 

 

 

     

14C carbon-14  8n  radioactive

 

Molecules – two or more atoms bonded together

 

 

 

 

 

 

 

Example: water molecule

one  oxygen atom shares electrons with two hydrogen atoms

      to form a

covalent  bond

 

Very few elements occur naturally as single atoms

 

Primarily just the noble gases (He, Ne, Ar, Kr, Xe, Rn)

   and some metals (Au, Ag, Pt)

 

Common Polyatomic Ions

      (ions that are molecules)

 

      most of these are used by organisms

 

 

 

 

 

 

 

 

 

 

 

 

 

States (or Phases) of Matter

GAS                 LIQUID                        SOLID

disorder           short-range order        long-range order

 

What Is Heat?

 

      Heat results from the vibrations of atoms – this is kinetic energy

 

     

Heat is transferred along a gradient

  conductive – particle to particle

  radiative – by electromagnetic radiation

      (infra-red radiation)

 

 

Heat is measured with a thermometer

  BUT, how does a thermometer work?

 

The effect of heat on density of matter

 

Phase transitions are controlled by:

  heat (energy available – outward force)

  pressure (constraining force)

 

 

At 5,000 meters altitude, water boils at a:

   higher or lower temperature?

 

 

At 5,000 meters in the ocean, water boils

   at a: higher or lower temperature?

 

 

Think about it this way:

      Is the constraining force (atmospheric pressure) higher or lower at high altitude?

 

So…as temperature increases:

 

 

  atoms (or molecules) vibrate faster

    and with greater amplitude

 

 

  these vibrations “push” the atoms farther apart

 

    which lowers the density of the material

 

Phase Transitions

Melting is the transition from solid to liquid

  freezing is the reverse

 

 

Evaporation (vaporization) is the transition

  from liquid to gas

  condensation is the reverse

 

Measuring Heat

How does a mercury thermometer work?

 

 

How does a bi-metallic thermometer work?

 

 

How does a thermistor work? 

 

  Density is not the only property of matter

    that is affected by temperature.

 

 

And, by the way, how does a thermostat work?

 

 

Hydrogen Bonding of Water

Because a water molecule

  is bipolar, it has

  hydrogen bonding

  between the

  individual molecules

 

 

 

 

 

This is only 4% of the force of the covalent bond,

  BUT it gives water unique properties

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Unique Properties of Water

 

Higher melting and boiling point than other hydrogen compounds

 

 

Higher heat capacity

       [amount of heat  needed to raise the temperature of 1 gram of water 1oC]

     

than any substance other than ammonia

 

 

Greater solvent power than any other substance

 

      Highest surface tension of any other liquid

 

Only substance that naturally occurs as a solid, liquid, and a gas

      at Earth-surface temperatures and pressures

 

Heat Capacity of Water

Heat capacity

  1o C per calorie per gram between 0 and 100o C

 

 

  latent heat of vaporization

      540 calories / gram

 

 

  latent heat of fusion (freezing)

      80 calories / gram

 

   

As Water Freezes

The angle between the hydrogen atoms increases from 105o to 109.5o

 

The molecules form hexagons and “lock in place”

 

Energy (heat) is released

 

Maximum Density of (Fresh) Water

      occurs at 3.98º C

 

Water Clusters

      groups of water molecules bound together by hydrogen bonds

      water clusters control density before freezing 

 

Ice Floats!  {duh}

  But why?

 

      The center of the hexagons in ice are empty space, so…

           

ice is 8% less dense than liquid water

 

Hydration of Ions

      Ionic bonds are only 1/10 as strong as covalent bonds

     

Water molecules form a hydration sheath around single ions

      Example of dissolving NaCl (salt)