The integer that you find in each box of the Periodic Chart is the atomic number. The atomic number is the number of protons in the nucleus of each atom. Another number that you can often find in the box with the symbol of the element is not an integer. It is oversimplifying only a little to say that this number is the number of protons plus the average number of neutrons in that element. The number is called the atomic weight or atomic mass.

How can it be that an element must have an averaged atomic weight? The number of protons defines the type of element. If an atom has six protons, it is carbon. If it has 92 protons, it is uranium. The number of neutrons in the nucleus of an element can be different, though. Carbon 12 is the commonest type of carbon. Carbon 12 has six protons (naturally, otherwise it wouldn’t be carbon) and six neutrons. The mass of the electrons is negligible. Carbon 12 has a mass of twelve. Carbon 13 has six protons and seven neutrons. Carbon 14 has six protons and eight neutrons. Carbon 14 is radioactive because, as other atoms with the wrong percentage of neutrons to protons, it is unstable. The nucleus tends to pop apart. The proper ratio of protons to neutrons is about one to one for small elements and about one proton to one and a half neutrons for the larger elements. Types of an element in which every atom has the same number of protons and the same number of neutrons are called isotopes. Carbon 14 is a radioactive isotope of carbon. Any carbon 14 that was made at the time the earth was formed is now almost all gone. Carbon 14 is continuously made from high energy electromagnetic radiation hitting nitrogen atoms in the ozone layer of the earth. This carbon 14 when taken into plants as CO2 will also be taken into animals. We can find out how much carbon 14 that normally is in a living plant or animal and from there we can find the actual amount of carbon 14 left in a plant or animal long dead. We can get a very good idea of how long ago that plant or animal was living from the amount of carbon 14 remaining in the dead body. This process is called ‘carbon dating.’ The stable, non-radioactive isotopes of carbon play no part in this. As a whole element, carbon has a more or less fixed proportion of the various carbon isotopes. For this reason, we can determine a weighted average of the isotopes for all elements. On a periodic chart you may see some atomic weights that are integers or in parentheses. These are usually on the very large or very rare or very radioactive elements. That is not really an integer atomic weight, but the atomic weight has been estimated to the nearest integer.

FORMULA WEIGHT OR MOLECULAR WEIGHT OR FORMULA MASS OR MOLAR MASS

Now with the atomic weight information we can consider matching up atoms on a mass-to-mass basis. Let’s take hydrogen chloride, HCl. One hydrogen atom is attached to one chlorine atom, but they have different masses. A hydrogen atom has a mass of 1.008 AMU and a chlorine atom has a mass of 35.453 AMU. Practically speaking, one AMU is far too small a mass for us to weigh in the lab. We could weigh 1.008 grams of hydrogen and 35.453 grams of chlorine, and they would match up exactly right. There would be the same number of hydrogen atoms as chlorine atoms. They could join together to make HCl with no hydrogen or chlorine left over. If we take one gram of a material for every AMU of mass in the atoms of just one of them, we will have a mol (or mole) of that material. One mol of any material, therefore, has the same number of particles of the material named, this number being Avogadro’s number, 6.022 E 23.

The formula weight is the most general term that includes atomic weight and molecular weight. In the case of the HCl, we can add the atomic weights of the elements in the compound and get a molecular weight. The molecular weight of HCl is 36.461 g/mol, the sum of the atomic weights of hydrogen and chlorine. The unit of molecular weight is grams per mol. The way to calculate the molecular weight of any formula is to add up the atomic weights of all the atoms in the formula. CuSO4·5H2O is copper II sulfate pentahydrate. The formula has one copper atom, one sulfur atom, nine oxygen atoms, and ten hydrogen atoms. To get the formula weight of this compound we would add up the atomic weights. Copper II sulfate pentahydrate is not a molecule, strictly speaking, but you will hear the term ‘molecular weight’ used for it rather than the more proper ‘formula weight.' Since the unit of formula weight is grams per mol, it makes good sense to use the formula weight of a material as a conversion factor between the mass of a material and the number of mols of the material.

ELECTRON CONFIGURATION

Protons have a positive charge and electrons have a negative charge. Free (unattached) uncharged atoms have the same number of electrons as protons to be electrically neutral. The protons are in the nucleus and do not change or vary except in some nuclear reactions. The electrons are in discrete pathways or shells around the nucleus. There is a ranking or heirarchy of the shells, usually with the shells further from the nucleus having a higher energy. As we consider the electron configuration of atoms, we will be describing the ground state position of the electrons. When electrons have higher energy, they may move up away from the nucleus into higher energy shells. As we consider the electron configuration, we will be describing the ground state positions of the electrons.

A hydrogen atom has only one proton and one electron. The electron of a hydrogen atom travels around the proton nucleus in a shell of a spherical shape. The two electrons of helium, element number two, are in the same spherical shape around the nucleus. The first shell only has one subshell, and that subshell has only one orbital, or pathway for electrons. Each orbital has a place for two electrons. The spherical shape of the lone orbital in the first energy level has given it the name ‘s’ orbital. Helium is the last element in the first period. Being an inert element, it indicates that that shell is full. Shell number one has only one s subshell and all s subshells have only one orbital. Each orbital only has room for two electrons. So the first shell, called the K shell, has only two electrons.

Beginning with lithium, the electrons do not have room in the first shell or energy level. Lithium has two electrons in the first shell and one electron in the next shell. The first shell fills first and the others more or less in order as the element size increases up the Periodic Chart, but the sequence is not immediately obvious. The second energy level has room for eight electrons. The second energy level has not only an s orbital, but also a p subshell with three orbitals. The p subshell can contain six electrons. The p subshell has a shape of three dumbbells at ninety degrees to each other, each dumbbell shape being one orbital. With the s and p subshells the second shell, the L shell, can hold a total of eight electrons. You can see this on the periodic chart. Lithium has one electron in the outside shell, the L shell. Beryllium has two electrons in the outside shell. The s subshell fills first, so all other electrons adding to this shell go into the p subshell. Boron has three outside electrons, carbon has four, nitrogen has five, oxygen has six, and fluorine has seven. Neon has a full shell of eight electrons in the outside shell, the L shell, meaning the neon is an inert element, the end of the period.

Beginning again at sodium with one electron in the outside shell, the M shell fills its s and p subshells with eight electrons. Argon, element eighteen, has two electrons in the K shell, eight in the L shell, and eight in the M shell. The fourth period begins again with potassium and calcium, but there is a difference here. After the addition of the 4s electrons and before the addition of the 4p electrons, the sequence goes back to the third energy level to insert electrons in a d shell.

The shells or energy levels are numbered or lettered, beginning with K. So K is one, L is two, M is three, N is four, O is five, P is six, and Q is seven. As the s shells can only have two electrons and the p shells can only have six electrons, the d shells can have only ten electrons and the f shells can have only fourteen electrons. The sequence of addition of the electrons as the atomic number increases is as follows with the first number being the shell number, the s, p, d, or f being the type of subshell, and the last number being the number of electrons in the subshell.

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6

It is tempting to put an 8s2 at the end of the sequence, but we have no evidence of an R shell. One way to know this sequence is to memorize it. There is a bit of a pattern in it. The next way to know this sequence is to SEE IT ON THE PERIODIC CHART. As you go from hydrogen down the chart, the Groups 1 and 2 represent the filling of an s subshell. The filling of a p subshell is shown in Groups 3 through 8. The filling of a d subshell is represented by the transition elements (ten elements), and the filling of an fsubshell is shown in the lanthanide and actinide series (fourteen elements).

Here is a copy of the periodic chart as you have usually seen it.

And here is the same chart re-arranged with the Lanthanides and Actinides in their right place and Group I and II afterward. Both of these charts are color coded so that the elements with the 2s subshell on the outside (H and He) are turquoise. All other elements with an s subshell on the outside (Groups I and II) are outlined in blue. Lanthanides and actinides are in grey. Other transition elements are in yellow, and all of the elements that have a p subshell as the last one on the outside are in salmon color.

You may be able to see it better with the subshell areas labeled.

There are several other schemes to help you remember the sequence.

The shape of the s subshells is spherical. The shape of the p subshells is the shape of three barbells at ninety degrees to each other. The shape of the d and f subshells is very complex.

Electron configuration is the "shape" of the electrons around an atom, that is, which energy level (shell) and what kind of orbital it is in. The shells were historically named for the chemists who found and calculated the existence of the first (inner) shells. Their names began with "K" for the first shell, then "L," then "M," so subsequent energy levels were continued up the alphabet. The numbers one through seven have since been substituted for the letters. Notice that I have included an "R" shell (#8) that is purely fantasy but makes the chart symmetrical.

The electron configuration is written out with the first (large) number as the shell number. The letter is the orbital type (either s, p, d, or f). The smaller superscript number is the number of electrons in that orbital.

Use this scheme as follows. You first must know the orbitals. An s orbital only has 2 electrons. A p orbital has six electrons. A d orbital has 10 electrons. An f orbital has 14 electrons. You can tell what type of orbital it is by the number on the chart. The only exception to that is that "8" on the chart is "2" plus "6," that is, an s and a p orbital. The chart reads from left-to-right and then down to the next line, just as English writing. Any element with over 20 electrons in the electrically neutral unattached atom will have all the electrons in the first row on the chart. For instance, scandium, element #21, will have all the electrons in the first row and one from the second. The electron configuration of scandium is: 1s2 2s2 2p6 3s2 3p6 4s2 3d1 Notice that the 2s2 2p6 and 3s2 3p6 came from the eights on the chart (2+6). Notice that the other electron must be taken from the next spot on the chart and that the next spot is the first spot on the left in the next row. It is a 3d spot due to the "10" there and only one more electron is needed, hence 3d1.

The totals on the right indicate using whole rows. If an element has an atomic number over thirty-eight, take all the first two rows and whatever more from the third row. Iodine is number fifty-three. For its electron configuration you would use all the electrons in the first two rows and fifteen more electrons. 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 from the first two rows and 4d10 5p5 from the third row. You can add up the totals for each shell at the bottom. Full shells would give you the totals on the bottom.

We have included an R shell (#8) even though there is no such thing yet proven to exist. The chart appears more symmetrical with that shell included. The two electrons from the R shell are in parentheses. We have not yet even made elements that have electrons in the p subshell of the Q shell.

ELECTRON CONFIGURATION CHART

Here is another way to consider the same scheme. The inert elements appear at the end of either the first two, an eight, a six. Wherever there is the six of a p subshell there is the two of an s subshell above it to make eight electrons in the outer full shell of a noble gas. The electron configuration for xenon is:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6

ELECTRON CONFIGURATION CHART

“Und” is the undiscovered inert element that would be below radon on the periodic chart.

Another type of electron configuration chart is below. These are more commonly known schemes. All you have to do is follow the arrows through the points to find the sequence. Add up the number of electrons as you go, and stop when you have equaled or almost exceeded the number. There have been a large number of variations on this idea, but they all work the same. Arrange the subshells in a slanted order and go through the array in straight lines, as in the first scheme, or arrange the subshells in a straight line and go through the array in slanted lines, as in the second scheme. In these schemes the inert elements appear after the first s subshell and after every p subshell. As the other type, this scheme type has its advantages and disadvantages, but they all lead to the same sequence.

COMMON ELECTRON CONFIGURATION SCHEME A

COMMON ELECTRON CONFIGURATION SCHEME B

Any of these schemes, if used correctly, will give you the same thing, the sequence of the addition of the electrons to the shells. This pattern is correct for all of the elements that are not Transitional Elements or Lanthanides or Actinides. Of the Transitional Elements and Lanthanides and Actinides about one third of the elements do not follow the pattern. The Periodic Chart below is arranged sideways to show the electron configuration by shell. As you work with the schemes for finding the electron configuration of elements, you can check to see if your answer is correct by adding the electrons in each shell (downwards in the first scheme) and comparing with the Sideways Periodic Chart. The elements that do not fit the pattern have an asterisk by them. In the Transition Elements that do not follow the scheme, only the s subshell of the outer shell and the d subshell of the next to last shell have some trading between them. In the Lanthanide and Actinide series any trading of electrons are between the d subshell of the next to last shell and the f subshell of the second to last shell, the one filling as the elements progress up that series.

Useful Weak Acids and Bases

What is an acid-base indicator?
An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the iogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range.

How is an indicator used?
Weak acids are titrated in the presence of indicators which change under slightly alkaline conditions. Weak bases should be titrated in the presence of indicators which change under slightly acidic conditions.

What are some common acid-base indicators?
Several acid-base indicators are listed below, some more than once if they can be used over multiple pH ranges. Quantity of indicator in aqueous (aq.) or alcohol (alc.) solution is specified. Tried-and-true indicators include: thymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, neutral red, phenolphthalein, thymolphthalein, alizarin yellow, tropeolin O, nitramine, and trinitrobenzoic acid. Data in this table are for sodium salts of thymol blue, bromphenol blue, tetrabromphenol blue, bromcresol green, methyl red, bromthymol blue, phenol red, and cresol red.

pH of salts formed from reactions of acids & bases	Strong Base	Weak Base
Strong Acid	pH = 7	pH <>
Weak Acid	pH > 7	pH = 7

http://www.ausetute.com.au/indicata.html

Examples of Indicators:

	Acid							neutral	Base
[H+] (mol/L)	100	10-1	10-2	10-3	10-4	10-5	10-6	10-7	10-8	10-9	10-10	10-11	10-12	10-13	10-14
pH	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	pH range
Universal indicator	red	red	orange- red	orange	pale orange	orange- yellow	pale yellow	green- yellow	green	dark- green	blue	blue	blue	blue	blue
cyanidin (red cabbage water)	red	red	red	cerise	purple	blue	blue	blue	aqua- marine	emerald- green	lime	lime	yellow	yellow	yellow
blue litmus indicator	red	red	red	red	red	red	red	blue	blue	blue	blue	blue	blue	blue	blue	5.0 - 8.0
red litmus indicator	red	red	red	red	red	red	red	red	blue	blue	blue	blue	blue	blue	blue	5.0 - 8.0
phenol- phthalein indicator	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	colour -less	pink	pink	pink	pink	pink	pink	pink	8.3 - 10.0
thymol blue indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	8.0 - 9.6
phenol red indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	red	red	red	red	red	red	red	6.8 - 8.4
bromo- thymol blue indicator	yellow	yellow	yellow	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	blue	blue	6.2 - 7.6
methyl red indicator	pink	pink	pink	pink	pink	pink	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	4.4 - 6.0
bromo- cresol green indicator	yellow	yellow	yellow	yellow	yellow	pale blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	blue- green	3.8 - 5.4
methyl orange indicator	red	red	red	red	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	3.1 - 4.4
bromo- phenol blue	yellow	yellow	yellow	yellow	blue	blue	blue	blue	blue	blue	blue	blue	blue	blue	blue	3.0 - 4.6
cresol red	red	red	red	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	yellow	0.2 - 1.8
pH	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	pH range

hello goodbye; 9:55 PM

what we had learnt

Atoms, Molecules and Stoichiometry


1. Relative masses of atoms and molecules
2. The mole ,Avogadro Constant
3. Calculation of Empirical and Molecular Formulae
4. Reacting masses and volumes (of solution and gases)



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Atomic Abundance Example Chemistry Problem:

The element boron consists of two isotopes, ¹⁰₅B and ¹¹₅B. Their masses, based on the carbon scale, are 10.01 and 11.01, respectively. The abundance of ¹⁰₅B is 20.0%.
What is the atomic abundance of and the abundance of ¹¹₅B?

Solution

The percentages of multiple isotopes must add up to 100%.
Since boron only has two isotopes, the abundance of one must be 100.0 - the abundance of the other.

abundance of ¹¹₅B = 100.0 - abundance of ¹⁰₅B

abundance of ¹¹₅B = 100.0 - 20.0
abundance of ¹¹₅B = 80.0

Answer

The atomic abundance of ¹¹₅B is 80%


http://chemistry.about.com/od/workedchemistryproblems/a/abundanceexampl.htm

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No. of mole= Mass divided by Mr

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To get the percentage composition from formulas, we calculate the percent of each element present,

as follows:

% Element X = atomic weight of the element X number of atoms of that element _X 100%

Formula weight of the compound

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The theoretical yield is the maximum amount of product possible from the available reactants. We can calculate

the theoretical yield, and we are usually given the actual yield. The definitions and procedure for calculating the % yield are shown below.

1. Theoretical Yield - the amount of product predicted by the balanced equation based on the amount of the limiting reactant.

2. Actual Yield - the amount of product obtained when the reaction is actually carried out in the laboratory.

3. Per Cent Yield:

% Yield = Actual yield X 100%

Theoretical yield

hello goodbye; 9:20 PM

http://09s4yan.blogspot.com/

Tuesday, February 24, 2009

Feed back on the race movie

It was a movie about a little trying all his best to face his failure with his father continuous encouragement.
A movie which start with a man who look into a photo frame and flashback of his started. He recalled back about a race that he fall down so many times that he knew he could not catch up with the rest of his friends. Although, he is the last but yet due to his persistence he manage to finish the race.
After watching this movie, my chemistry tutor said she wanted us to be like his boy. Face your failure and start again. Well, i do believe that everyone in my class had a common goal, all of us wanted to go to UNI. All i wish is that all of us will hold on tight in this 3 years.

hello goodbye; 9:20 PM

titration experiment ! :D

hello goodbye; 7:25 PM