May 10-- Electron Dot and Lewis Diagram

Electron Dot diagram is the other name of Lewis Diagram

 - the electrons are represented by dots

Around the nucleus, there are 4 orbitals and each orbital can hold a maximum 2e.We are using dots to represent electrons. The dots are placed on the 4 sides in pairs.

Here is an example.

exercise 1

Draw the electron dot diagram ---HCl

First of all, list up all the valence electrons

Hydrogen 1e

Chlorine   7e

For this question, there is no central atom.The one furthest from a full shell is the central atom.

* we also can use "-" instead or " ."

May.18--Chemical Bonding

First of all, there are three types of chemical bonds: polar covalent bond, non polar covalent bond, and ionic bond.

What are the differences among those three bonds?

A Polar Covalent Bond is formed when the electrons are shared unequally.

A Non Polar Covalent Bond, on the other hand, is formed when the electrons are shared equally.

An Ionic Bond is formed when the electrons are transferred between two atoms.

All chemical bonding is based on the electrostatic relationships.

1. Opposite charges attract each other. 

2. Like charges repel each other.

3. The greater the distance between two charged particles, the smaller the attractive force existing between them. 

4. The greater the charge on the particles, the greater the force of attraction between them.

Note: This force of attraction is called electrostatic force.

But what exactly is the definition of electrostatic force???

- It is a force that exists between charged particles as a result of attraction or repulsion. It operates equally in all directions. (Positivelly-charged particles attract negatively-charged ones from all around it.)

Now, it's time to introduce electronegativity.

Electronegativity is the measure of the tendency of an atom to attract electrons from a neighbouring atom.

Depending on the difference in electronegativity between two atoms, the degree of electron sharing between those two atoms can be different.
So, what does that mean???
Let's look at this periodic table below, which shows a scale called the Pauling Scale.
The scale is used to measure the electronegativity value of each element. It ranges from 0.7 to 4.0.

Atoms with higher electronegativity values are more likely to attract its electrons and electrons from neighbouring atom.
In general, metals have low electronegativity values, and non-metals have relatively higher electronegativity values.


The difference between the electronegativity values of the two bonded atoms determines whether electrons will be shared or transferred.
How to determine the type of bond?

Energy Difference = | ENeg1 - ENeg2 |

- If ENeg Diff. < 0.5, it is a Non-Polar Covalent Bond.
- If ENeg Diff. is between 0.5 and 1.8, it is a Polar Covalent Bond.
- If ENeg Diff. > 1.8, it is an Ionic Bond.

 Non-Polar Covalent Bonds:
- formed when two atoms having less than full shells are able to share their electrons with each other to attain full shell (a very stable arrangement).
- two atoms share electrons EQUALLY

Characteristics of covalent bonds
- also very strong
- have lower melting points than expected 
This is weird. WHY? 

It has something to do with intramolecular and intermolecular forces. 

Intramolecular Forces: are found within a molecule, and are responsible for holding the atoms of a molecule together.

Intermolecular Forces: are found between the molecules, and are responsible for the bonding between molecules.


Now we can explain why covalent bonds have low melting points.
During the melting process, the covalent bonds within each molecule are not affected. The weak bonds (intramolecular forces) are broken instead. Since they are easily broken, the melting points of covalent bonds are relatively low.





Polar Covalent Bonds:

Polarity-a molecule's electrical balance. 
Polar: when there is an imbalance with electrical charge
Non Polar: when the electrical charge is the same strength on all sides of the molecule.


Polar covalent bonds are bonds that contains unequal sharing of electrons.


In a polar covalent bond, the atom with the greater electronegativity value will pull the electrons in the bond more towards itself.


Why is the bond called polar covalent?
- The atom with higher electronegativity will form a Partial Negative charge (between 0 and -1)
- The atom with lower electronegativity will form a Partial Positive charge (between 0 and 1)


We usually use an arrow sign to indicate the migration of electrons (towards the more electronegative atom).

Ionic Bonds:

- metals lose electrons and become positively-charged ions (cations) --> lower electronegativity

- non-metals gain electrons and become negatively-charged ions (anions) --> higher electronegativity

After bonding, the electron arrangement in the ions become very stable (the same as a noble gas).

Characteristics of ionic bonds:
- very strong
- high melting points 
 

Dipole: when one end of a molecule has a slight positive charge and the other end has a slight negative charge, the charge separates partially.

History of Periodic Table

History of Periodic Table

-In the beginning: discover elements 52 by 1817, 62 by 1863

 Some kinds of organizations were needed:

·         First attempt done in 1820’s

·         1857 William Ordling separated the known elements into 13 groups based on their physical and chemical properties. (It was a start)

·         Between 1863 and 1866 John Newlands assigning Hydrogen -mass1 and ordering the known elements by their masses, every eighth element shared a common set of properties

(“Law of octaves”). Weaknesses: cannot predict elements, and he kept changing the orders

·         1869- Dimitri Mendeleev published a method of organizing the elements according to both their masses and their properties. The elements are listed according to masses, certain properties recur PERIODICALLY. (Rows (PERIOD) and columns (GROUP).)

-can accurately predict elements

-chemists can understand and organize data and predict new properties

Modern periodic table

·         Is organized according to atomic number rather than atomic mass.

·         The periodic law summarizes the periodic table. The Periodic Law: Properties of the chemical elements recur periodically when the elements are arranged from lowest to highest atomic numbers

 Period: The set of all elements in a given row going across the table.

Group or Family: The set of all elements in a given column going down the table.

Alkali Metal = elements in the first column (Except H)

Alkaline Earth Metals = The elements in the second column

Halogens = second column from the end on the right hand side. Starting with Fluorine.

Noble Gases = Far right side of the table. Starting with helium.

Lanthanides = elements in the first row shown underneath the table. Starting with lanthanum.

Actinides = underneath the Lanthanides. Starting with actinium.

·         Metals, non-metals, semiconductor