Periodic Trends

As we examine the periodic table each elements have certain characteristics and looking at the periodic table as a whole, some trends are noticed.

Recall that Dmitri Mendeleev created a periodic table of the 63 known elements which he arranged by atomic mass.

First we must know some important vocab.

Density: of an object is its mass per unit volume. It increases as the atomic numbers increase.
Ionization energy: is the energy needed to remove electrons from an atom. It increases as you go across a period(left----> right) and decreases as you go down in families.

Melting and Boiling Points: This is the temperature at which an object liquefies and  becomes a gas respectively.
These properties increase as you go from bottom to top in a group.

Electronegativity: This is the tendency of an atom or functional group to attract electrons towards itself(form negative ions) In increases left------------->right

Atomic Radius: Is the measure of the size of an atom. It decreases left to right and increases going down.

Valence Electrons

Valence electrons are the electrons that exist on the outermost shell of an atom.
Are the ones involved in chemical reactions

An open shell is a shell that has less than the maximum amount of electrons in it.
A closed shell is a shell that is completely full with the max. amount of electrons that can occupy it.

How to determine the number of valence electrons from electronic configurations:

Sodium: has 11 electrons.

1s^2 2s^2 2p^6 3s^1

Now we must put it into core notation:
[Ne] 3s^1

When counting valence electrons, only the s and p subshells are considered, unless the d and f subshells are full.

Therefore, Na has one valence electron.

http://www.youtube.com/watch?v=1TZA171yxY4&feature=fvwrel

Cheers

Electrons in an Atom

Niels Bohr proposed that atoms exist in energy levels surrounding the nucleus.

Electrons exist in two states:
1. Ground state- when electrons are in their lowest possible energy level
2-Excited State-when one or more electrons of an atom are in energy levels above their lowest possible level.

An energy level is the amount of energy an electron can possess.
The quantum of energy is the difference in energy between two levels.

Recall that electrons exist in shells,which are sets of orbitals having the same 'n' value
and an orbital is  the space an electron takes up in a particular energy level.
A subshell is a set of orbitals of the same type.

There are 4 different types of orbitals
S, D, P and F.
Each subshell has

1 S Orbital
3 P orbitals
5 D orbitals
7 F orbitals

This means that the most electrons you can put in

S subshell- 2
P subshell- 6
D subshell-10
F subshell-14

This is governed by Pauli's Exclusion Principle which states that each orbital contains 2 electrons max.

This is a diagram which will help when filling orbitals.

To write the electronic configuration of neutral atoms:

1) The Aufbau Principle states that you must start with the lowest energy level.
2) Figure out how many electrons you have.
3)According to the diagram, that is the order you follow, so 1s,2s,2p etc..
4. Remember how many electrons in each subshell
5) Keep filling until you run out of electrons.

Example 1:

Oxygen- has 8 electrons.

So 1s^2 2s^2 2p^4

You write the name of the subshell and then the number of electrons in it as an exponent.

Example 2:

Krypton has 36 electrons.

1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6

Note that 4p^6 is filled up. This is because krypton is a noble gas.

This is the visual way to display electron configuration. Note how each shell is filled with up and down arrows because they have opposite spins. Remember to fill each subshell with all up arrows and then go back and fill the down arrows.

Another way to display electron configuration is to use core notation.

The core-the set of electrons with the configuration of the nearest noble gas

The outer part of an atom- the electrons other than the core.


How:

1) Locate the closest noble gas to that element on the periodic table.
2) Cite the noble gas in [ ] and then write the rest of the configuration.

Example 3:

Nitrogen:
Has 7 electrons= 1s^2 2s^2 2p^3

What you do is backtrack and find the nearest noble gas. Go backward across the periodic table(left) until you run off the table and come out one level up on the right side.

In this case the nearest NG is helium.

Its atomic number is 2 so, [He].
Then eliminate the first 2 electrons from the configuration and add the rest.

2s^2 2p^3

[He] 2s^2 2p^3.


Exceptions
Instead of: Cr --> [Ar] 4s^23d^4
Cu--> [Ar] 4s^23d^9
In actuality:
Cr--> [Ar] 4s^13d^5
Cu--> [Ar] 4s^13d^10 

http://www.youtube.com/watch?v=YURReI6OJsg&feature=relmfu 

That should help!

Atomic Number, Mass and Mass Number- Subatomic Particles

Today we learned about the different characteristics of modern atoms.

Recall that an atom is made up of 3 types  of subatomic particles.

The Proton
Has a charge of +1.
Has a mass of 1.
Location: Nucleus

The Electron
Has a charge of -1
Has a mass of (not equal to) zero
Location: Energy shells surrounding nucleus

The Neutron
Has a charge of 0
Mass of >1.
Location: Nucleus

The atomic number(Z) of an element is the number of protons in its nucleus.

Atoms can form ions, which are charged atoms.
To form a negative ion(anion) electrons must be added to a neutral atom.
To form a positive ion(cation) electrons must be taken away from a neutral atom.

To find the charge of an atom: # Protons- #Electrons.

The mass number of an element is the sum of its protons and neutrons
The atomic mass is the average of masses of existing isotopes of an element.

An isotope of an element always has the same number of protons but a different number of neutrons.

How to calculate atomic mass.

Three naturally occurring isotopes with their percent abundance.

12(60%), 13(36%), 14(4%)

Calculate atomic mass.

12 x 60%= 7.2
13 x 36% = 4.68
14 x 4% = 0.56

Now add: 7.2 + 4.68 + 0.56 = 12.4.

Atomic Theory

The history of atomic theory has evolved over thousands of years. Several new ideas, concepts and models have been put forth and have contributed to our understanding of the structure of an atom.

Democritus- believed that matter was made up of atoms. Had a theory that atoms remain unchanged but move about in space to form macroscopic objects.

Aristotle- His theory was that atoms were made up of 4 elements; earth, wind, fire and water. When these elements combine they created dryness, wetness, coldness and hotness.

Antoine Lavoisier- accidentally discovered oxygen and created an incomplete table of the elements. Created early version of Law of 1) Definite Proportions 2) Conservation of Mass

Joseph Proust- created the Law of Definite Proportions-a given compound always has the same elements in the same proportions by mass.

John Dalton- he proposed Dalton's Atomic Theory
1. elements are made up of atoms
2.atoms of same elements are identical
3.atoms of different elements can be told apart by atomic weight
4.atoms of different elements can combine in a reaction to form chemical compounds in fixed ratios.

JJ Thomson-proved the existence of the electron, proposed the raisin bun model of the atom.

 Ernest Rutherford- discovered the positively charged, central, dense nucleus.
Did this using his gold foil experiment in which he fired radioactive particles through thin foil; most passed through but some particles were reflected.

Niels Bohr- proposed the planetary model of the atom
-Electrons exist in energy levels surrounding the nucleus.
-studied gaseous samples of atoms

The Atom today:
It is the smallest particle of an element and cannot be broken down.
It contains 3 subatomic particles- the proton(+), the electron(-) and the neutron (0).
The protons and the neutrons occupy the nucleus and electrons exist in levels around the nucleus.

Percent Yield and Percent Purity

Today we learned the last things for the test.

1) Percent Yield: it is the amount of actual product obtained as a percentage of the theoretical amount of product.

It is found using the formula:

Actual mass produced(g) / theoretical mass produced(g)

Lets try some problems!!!!

Example :

The reaction SiO2(s) + 4 HF(g) -----> SiF4(g) + 2H2O(l) produces 3.0 grams of H2O when 13.30 g of SiO2 is treated with small excess of HF.

What mass of SiF4 is formed?

Looking back at previous lessons this is fairly straightforward.


Mass of SiF4 formed:

3.0 g H20 x 1mole/ 18.0 x 1SiF4/2H20 x 104.1/1 mole=8.7 g of SiF4


What mass of SiO2 is left unreacted?

Mass of SiO2 used - 3.0 g H2O x 1mole/18.0 x 1 SiO2/2H2O x 60.1/1mole=5.0 g SiO2

13.30-5.0 g = 8.3 g SiO2 in excess

What is percent yield of reaction?

amount of SiF4 expected:  13.30 g SiO2 x 1 mole/ 60.1 x 1 SiF4/1SiO2 x 104.1/1mole = 23.0 g SiF4

Now remember Percent Yield is amount obtained/amount expected x 100

8.7/23 x 100 = 37 .8% yield.


Percent Purity:

(Mass of pure substance/mass of impure substance) x100

If the mass of silver ore is 66.4 grams and contains 12.0 g of pure silver what is percent purity?

12.0/66.4

x 100 = 18.1 % purity





http://www.youtube.com/watch?v=LicEaaXhlEY