Chemistry Lecture

 

     Chemistry is the study of the nature of matter. Matter is anything that occupies space and has mass. Matter can exist as a solid, liquid or gas.
     Matter can undergo:
  Physical change including dividing a mass or changing the state such as what occurs when ice is melting (solid to liquid) or water is boiled (liquid to water; or,
  Chemical change that involves altering the composition of a substance such as what happens during digestion (proteins and other molecules are broken down) or growth (amino acids are linked to form proteins).
 

Composition of Matter
     All the matter in the universe can be reduced to a limited number of elements that are shown in the periodic table. An element is a substance that cannot be broken down into simpler substances by normal chemical means. There are 92 elements that occur naturally. All elements are made up of units called atoms.
  Atom
  An atom is the smallest unit of an element that retains the properties of the element. The atom of each element is represented by its atomic symbol that uses the first one or two letters of the element name. Some straightforward examples are:
Carbon - C
Nitrogen - N
Oxygen - O
Helium - He
  Some other examples are tricky because the Latin term for the element is used instead:
Sodium - Na (natrium)
Potassium - K (kalium)
  Atomic Structure
  Atoms are composed of the following subatomic particles:
  Protons – have a positive charge (+) and a mass of 1.0 atomic mass units (amu).
  Neutrons – have no charge (are neutral) and a mass of 1.0 amu
  Electrons – have a negative charge (-) and a mass of 0.0 amu (electrons have a mass but it is practically negligible)
  Positive charges repel other positive charges and negative charges repel other negative charges but positive and negative charges attract one another. In other words, like charges repel one another and unlike charges attract one another.
  Each element can be defined by the number of protons, neutrons and electrons they contain. Elemental atoms are electrically neutral so that the positive charges equal the negative charges, in other words, the number of protons equal the number of electrons. However, it is possible for an atom to gain or lose an electron and then they are called ions.
  Periodic Table
   The following information about atomic structure can be found in the periodic table:
Atomic number
     The atomic number is equal to the number of protons the atom possesses. Each element has its own unique atomic number. The number of electrons equals the number of protons in an electrically neutral atom.
Atomic mass units (amu)
     The atomic mass number is the sum of the mass of protons and neutrons within each nucleus. An element can have individual atoms that vary in their atomic mass number because the number of neutrons can vary, particularly with the heavier elements. These variations are called isotopes. The atomic mass units is equal to a weighted average of the naturally occurring isotopes of an element. YouTube: Weighted Average
 
Atomic Interactions
     The properties of elements are understood by visualizing atomic structure using models. YouTube: Basic Atomic Structure
  Planetary model
  The planetary model visualizes an atom as consisting of a central nucleus that contains protons and neutrons and electrons that orbit around the nucleus.
  Orbital model
  The orbital model keeps the extremely dense nucleus but the location of the electrons can only be approximated as being in “orbitals”. The orbital can be visualized as an “electron cloud”. Most of the volume of the atom is taken by the electrons which are practically massless.
     The behavior of the atom is primarily determined by the electrons.

  

Molecules and Compounds
     Molecules are formed when two or more atoms bind together. The atoms can be of the same element such as when two atoms of hydrogen bind to from molecular hydrogen (H2). Compounds form when two or more atoms of different elements such as when one atom of carbon binds with four atoms of hydrogen to form methane (CH4).

  

Chemical Bonds and Reactions University of Oxford Web Page
     Chemical bonds form between atoms as a result of the interaction of their electrons. The formation of these bonds is best understood by considering the behavior of the electrons in their orbitals
  Energy shells and valence
      In an atom, electrons occupy different energy levels or energy shells. The energy levels closest to the nucleus are the most stable while those further away are less stable. Only the electrons in the outer most shell determine the behavior of the atom. This is called the valence shell. This is the only shell that participates in the bonding.
   Rule of 8s
     Except for the smallest atoms such as H and He, the most stable number of electrons to have in the outer shell is 8. Atoms will interact with one another to achieve the stability of having 2 electrons in the first shell (for small atoms like H or He) or 8 electrons in the outer shell.
     The interactions between electrons of the valence shells results in the following types of chemical bonds: YouTube: Ionic and Covalent Bonds
  Ionic bonds
     Ionic bonds result when the electrons of the valence shell are completely transferred from one atom to another. The atom that loses an electron becomes positively charged and is called a cation. The atom that gains electrons becomes negatively charged and is called an anion. The cations and anions attract one another and form ionic bonds.
  Covalent bonds
     Stable bonds can form between atoms that share the electrons of their outer shells. Covalent molecules result when atoms share electrons by forming these covalent bonds. Examples of covalent molecules include, H2, O2, and CH4.
  Non-polar vs. polar bonds
  When two atoms share electrons equally the result is a non-polar bond. If one atom, such as oxygen (O), attracts electrons more strongly than an atom to which it is covalently bonded the result is a polar bond because the electrons are “pulled” by the O atom more strongly and the region around the O atom becomes more negative.
  Water as a polar molecule Animation
  Water (H2O) is an important example of a polar molecule because its polarity gives rise to the properties that make it essential for life.
  In H2O, two H atoms form a single covalent bond with an O atom. However, because of the interactions with the other electrons in the outer shell of O, the bonds form at an angle less than 180o (about 105o). The electrons spend more time around the O atom because of its greater attraction for electrons and this gives the region around the O atom a partial negative charge, expressed as d-. The H atoms have a partial positive charge, expressed as d+ and the water molecule is a polar molecule.
  Hydrogen bonds Animation
      Hydrogen bonds are relatively weak bonds. Hydrogen bonds result when an H atom covalently bonds with either O or N. The strong attraction that O and N has for the electrons causes the H to have a partial positive charge. The H atoms are then attracted to any nearby O or N atoms that have a partial negative charge. For example, the H atoms on water molecules form hydrogen bonds with the O of neighboring water molecules.

  

Patterns of Chemical Reactions 
     Chemical reactions involve the breaking or making of covalent bonds.
     There are three kinds:
   (Dehydration) Synthesis reaction
  Synthesis reactions occur when atoms or molecules combine to form larger molecules. Synthesis reactions involve bond formation and require energy.
  Decomposition (Hydrolysis) reaction
  Decomposition reactions occur when a molecule is broken down into smaller molecules. Decomposition reactions involve the breaking of bonds and release energy.
  Exchange reaction
  Exchange reactions occur when bonds are both made and broken and involve the exchange of parts between two molecules.