Electron Dot and Lewis Structures

Drawing Electron Dot and Lewis Structures

• the nucleus is represented by the Atomic symbol
• for individual elements you have to determine number of valence electrons
• electrons are represented by dots :      .   
• 4 orbitals (max. of 2 per each orbital)
• each orbital has 1 electron before they pair up
• the dots are placed into 4 groups of one/ 2 electrons, with 8 electrons representing a closed shell (noble gas configuration)
• the dots are placed on the 4 sides in pairs

Ex:       Li .

Each bond in the structural formula represents 2 electrons. All valence electrons must be used. Each element must have a full valence shell (8 electrons), except hydrogen which only needs 2 electrons.

Ex:      C2H6       Carbon- 4e  Hydrogen- 1e
A structural diagram is one that shows each bond as a line. Each line represents 2 dots. Lone pairs aren't shown in structural diagrams:        H       H     
                                                                            H-C    -  C -H
                                                                                H        H    <----(structural lines connecting bottom and top H's with the C's.)

Lewis Structures

• structural diagrams show the number of bonds in a compound
• electron dot diagrams show the electrons involved in the bond
• if you know the structure of a molecule it means you know which atoms are bonded to which

Octet Rules

• H only needs 2 electrons
• CNOF (carbon, nitrogen, etc) always follow the octet rule

Rules for Writing Lewis Structures

For ex: CCl4

1. The central atom (H and F are never in the center)

• if a metal is present, place it in the center
• if a molecule has only 1 atom of a particular element and several atoms of another, the single one is the one in the center
• place the atom that needs the most electrons in the center

    2.   Place one pair of the electrons into each bond

    3.   Can't up the total number of valence electrons in a molecule. Adjust the number by subtracting one
          electron, for every positive charge. Adding one for negative charges.

    4.   Find the number of valence electrons remaining.

    5.   Use the number of valence electrons remaining to complete the octets of the atoms bonded to the central atoms.

   6.   Then place any remaining electrons on the central atoms in pairs.

   7.   If the central has less than an octet, make multiple bonds.

   8.   Replace each pair of electrons engaged in a bond with a dash "  - "


Check out this link for extra help!
http://www.youtube.com/watch?v=K7EAteDWxPQ

-Lauren

The History of the Periodic Table and the Periodic Table Families

          The very first scientist arrange elements in order is an English chemist, John Newlands. He is the one who pioneered the discover of the periodic table. Newlands arranged the elements by their relative atomic weights in 1863.
         The next year in 1864, Dmitri Mendeleev a Russian chemist, came up with the very first version of the peridic table. Unlike John Newlands, Mendeleev put the elements in order by similar properties in stead of their relative atomic mass. Mendeleev left blanks in the periodic table for the elements yet to be discovered.
        Glen Seaborg is the last to make changes in the modern periodic table. Seaborg dicovered elements 94 to 102. Element  106 has been named after him.
      
        For the organization of the periodic table, the horizontal rows are called a period. Vertical columns are called families or groups.
        Alkali metals: highly reactive metals that are located in group 1 in the periodic table.
        Alkaline earth metals: reactive metals that are located in group 2 in the peridoic table
        Halogens : Non metals that are highly reactive and react with water, located in group 17.
        Noble Gases: odourless, colourless, non metals that have low reactivity because their valence sheels are full. They are in group 18.
       Lanthanide Series : elements 57-71
      Actinide Series : elements 89-103, all are radioactive.

Atomic History

Aristotle (384 BC – 322 BC)

-         Believed you could divide matter an infinite number of times

-         Believed world was made of earth, fire, water and air.

-         These elements were acted upon by gravity and levity

Democritus (460 BC)

-         Believed matter was made up of tiny particles called ATOMS

-         Matter could not be divided indefinitely

-         Could not prove his hypothesis

Lavoisier (1777)

-         founded the law of definite proportions

-         discovered oxygen and hydrogen

-         reasoned that the ratio between elements would always be the same in a compound no matter how much of that compound was available.

Dalton (1800s)

            Five Point Atomic Theory

-         All matter is made up of atoms

-         Atoms of a given element differ from the atoms of another element

-         All atoms of a given element are identical

-         Chemical reactions do not change the elements but change the way the elements are grouped together

-         Atoms cannot be created, divided or destroyed through chemical process. 

Henri Bequerel(1851)

-         discovered radioactivity

-         studied the effect of xrays on photographic film

-         some chemicals decomposed and gave off penetrating rays

Thomson (1897)

-         Discovered the electron

-         Used a cathode ray tube to experiment with the magnetic deflection

-         Created the plum pudding model

-         This model states that negatively charged electrons exist randomly throughout an atom. No mention of protons or a nucleus.

Bohr

-         Created the bohr model of the atom

-         Electrons do not spiral around the nucleus – they orbit at certain allotted distances from nucleus

-         Atoms radiate energy- jump from higher to lower orbits

-         His model had protons, electrons and neutrons

Rutherford

-         Changed the arrangement of particles in an atom

-         Gold foil experiment- shot alpha particles through gold foil – some of them bounced back

-         He concluded that atoms have a positive dense centre and electrons orbit outside it

-         This was the first planetary model – electrons spin around the nucleus.

Millikan (1921)

-         Discovered the negative charge on the electron

-         Used the oil drop experiment and observed the speed of the particles

James Chadwick (1932)

-         discovered the neutron

-         Observed that previously thought gamma radiation was radiation from neutral particles

- Candace

Trends on the Periodic Table

Last class we learned some terms in chemistry. Let's review them!
Atomic Radius: The atomic radius of a chemical element is a measure of the size of its atoms
Melting Point: The temperature a chemical reaches when it melts
Boling Point: The temperature a chemical reaches when it boils
Ionization Energy: The energy required to move an electron from an atom
Electronegativity: The tendency of an atom or radical to attract electrons in the formation of an ionic bond

The Trend of Periodic Table
- Elements close to each other have similar characteristics
Atomic Radius  - radius decreases moving → on the table
 - increases moving ↓ (a family)
Melting Point and Boiling Point - elements in the center of the table of highest MP and BP
- Noble gas has the lowest
- Increases moving → until the middle of the table
Ionization Energy - energy increases going ↑ and →
- Noble gases have the highest ionization energy because they have a full valence shell
Electronegativity -  energy increases going ↑ and →
Metallic Properties - elements become more metallic going ↓ a family

Visit  http://www.ptable.com for a dynamic and fun periodic table to visualize the trends!

- Catherine