Notes for Chem Academic

8/3/2019 Notes for Chem Academic

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NOTES! HIP HIP HOORAY!

All the elements of the periodic table are neutral as is; that is, every element has a set number

ofprotons (positive charge) and a set number ofelectrons (negative charge). The number

of protons and electrons in each specific element are equal, and the intensity of the negative

charge from the electrons is equal to the intensity of the positive charge from the protons. Thismeans that the charges cancel out and that the atom has an overall neutral charge.

Since there is an equal number of electrons and protons in each atom, the specific number of

electrons and protons for a particular atom can be represented by one number. We call that

number the Atomic number(represented by the symbol: Z).

In addition to protons and electrons, elements also have neutrons. Neutrons have no charge at

all. They exist along side the protons in the center of the atom, while the electrons orbit them in

specific energy levels we sometimes call shells. Without the neutrons, the positive charges of

the protons would repel each other.

Protons, neutrons and electrons are extremely small, but of the three electrons are the smallest.

Electrons are so small that it would take about 2000 of them to make the size of a proton. and

just to give you an idea of how small a proton is, it would take 6.02 x 10^23

(thats 602 000 000 000 000 000 000 000) protons to have a mass of 1 gram (about the weight

of a paperclip). Since they are so light, and becasue no element comes anywhere near having

2000 electrons, when determining the mass of a particular element, we ignore the electrons.

That means that the weight of the atom comes entirely from the weight of the neutrons

and protons (represented by the symbol: A).

lets look at one of the elements off of the periodic table.

In general, the short hand notation for any element can be seen in the figure below, where X


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is the elements atomic symbol; A is the atomic massfor that element; and Z is the atomic

number for that element.

Sometimes to visually represent atoms we use a Bohr model. A Bohr model shows the element

name in the center (or sometimes the number of protons and neutrons for that element), and the

electrons orbiting around it in their respective energy levels (shells). As far as were concerned

for now, elements fill up their energy levels, or shells, using the octet rule which states that eachenergy level except the first can hold 8 electrons (the first one only being able to hold 2). Using

a Bohr model, lets take a look at one of the elements for an example.

What do we have here?Nitrogen! how do we know? looking at the periodic table we notice that the atomic number for


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nitrogen is 7, this means that nitrogen should have 7 electrons and 7 protons... just count!

This is all fine and dandy, but we all know that elements do not exist only by themselves,elements interact with other elements and can form compounds.

All the elements of the periodic table are trying to attain noble gas configuration. What thatmeans is that all the other elements think the noble gases are awesome and want to be justlike them. The noble gases all have complete orbitals; in other words, each 'shell' they have isholding the maximum number of electrons possible for that shell.Soooo, for the Bohr model above, we have an element whose first shell is completely filled,and whose next shell needs 3 more electrons to be completely filled. That means if, somehow,nitrogen picks up 3 more electrons, it will have filled its octet and have achieved noble gaselectron configuration, (nitrogen will have the same number of electrons as Neon).So lets pretend it gets 3 more electrons (we'll look at how it does that soon... just keep reading).

Now both shells are filled and nitrogen is happy. But wait!... if nitrogen has the same number ofelectrons as neon now, why doesn't nitrogen actually BECOME neon?Count the protons... there are still only 7 protons, and neon has 10 protons and 10 electrons. So

nitrogen is still nitrogen, it just has a few extra electrons that he stole from some other element

(or elements... maybe he got all 3 from one element, or maybe he just stole one electron from

3 different elements). Since each electron has a negative charge, and nitrogen now has 3 more

electrons than it SHOULD have, the overall charge on nitrogen is now -3. This is now a nitrogen

ion

An ion is an atom or molecule with a net electric charge due to the loss or gain of one or more

electrons.

Lets look at an example of who nitrogen might have stole these extra electrons from... who everhe stole it from, they are now an ion as well.


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Here is lithium:

Lithium has a total of 3 electrons, and just like nitrogen, lithium would like to be like the noblegases too. In order to do this, lithium would have to gain 7 electrons to have a full outer shelland be like neon (too hard to do and doesnt happen)... OR lithium could just loose one electronand be like helium!

So lithium gives away the outer most electron to nitrogen and now lithium is happy! Twoadditional lithium's would have to do this to give nitrogen the 3 electrons that it needs to fill itsouter shell.

When lithium gives away an electron lithium will have 3 protons and 2 electrons, and sinceprotons have a positive charge and electrons a negative charge, 3 protons and 2 electrons givelithium a net charge of +1.

It is important to note that the only electrons involved in bonding is the electrons in the highestenergy level; in other words it is the electrons in the outer most shell or the valence shell thatare involved in transfer, or as well will see, sharing of electrons.HANG ON ONE SECOND!!!!!!!!!We just talked about 3 atoms of lithium giving up one electron each and one atom of nitrogenaccepting 3 electrons. When this happens these elements cant just part ways and go offhappily with a noble gas configuration. By donating/accepting electrons, these elements havewritten their vows and said I do and are now bonded together by the strongest bond out


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there... an ionic bond. Lets view the equation for this reaction:

From this equation we see that Lithium metal reacts with nitrogen gas to from a compoundcalled lithium nitride.The top equation tells you what is involved in the reaction; in other words what things arereacting and what happened (what is formed) after the reaction is over.The bottom equation is the same equation balanced. But what does that mean? Whats the

point, and how is it useful? Welllll...., If you were to want to create lithium nitride in this way inthe lab, this equation would be useful. The coefficients (big numbers on the left) tell you howmany thingys are required for this reaction to happen. To avoid potential confusion, for now,were going to call the quantity of substance thingys. So in this case, it would take 6 thingys oflithium metal and 1 thingy of nitrogen gas to make 2 thingys of lithium nitride (solid).Its the same as baking a cake. This is the ingredients for a cake:

250g butter, chopped

3 teaspoons finely grated lime rind

180g white chocolate, chopped

1 1/3 cups caster sugar

3/4 cup milk

1 1/2 cups plain flour, sifted

2 eggs

280g jar lemon butter

For simplicity reasons, lets focus on just the eggs and flour. It says we need 2 eggs and 1.5

cups of flour to make 1 cake... well in chemistry we try to avoid using anything but whole

numbers for balancing our equations. So another way to say this is that it takes 4 eggs and 3

cups of flour to make 2 cakes.


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Weve learned thus far that an ion is an element (or molecule, eg. polyatomic ion) that has lost

or gained electrons when taking part in an bond (ionic bond). Once you have an ionic compound

that is made up of a cation (positively charged ion; eg. element lost one or more electrons)

and an anion (negatively charged ion; eg. element gained one or more electrons), even

though these are tightly bonded together with an ionic bond, it is possible for the compound todissociate in a solvent. An example of this would be dissolving table salt (NaCl) in water. Na has

an atomic number of 11. Following the octet rule there would be 2 electrons in the first energy

level (shell) 8 electrons in the next, and with one electron left over, there would be one electron

in sodiums valence (outermost) shell.

Side note 1! in fact, if we ignore the transition metal, following the octet rule well

notice that all the elements in the first group (up and down on periodic table)

end up having 1 valence electron. All the elements in group 2 have 2 valence

electrons and so on.

Side note 2. Using the the fact from side note 1 we can also determine the

charges each element would have (excluding the transition metals) if they were

to become ions. Eg. everything in group 1 has 1 valence electron, which means

if it were to ionize it would only have one electron to loose. If it looses that one

electron, it has one less negative charge making it an ion with a charge of +1.

All the elements in group 2 have 2 valence electrons, which means if they were

to become ions they could only loose 2 electrons. This would mean that if an

element in group two becomes an ion it would have a charge of +2. When we

reach the fourth group (remember skipping the transition metals), the group

starting with carbon, these elements are right in the middle. They can either

loose or gain four electrons in order to attain noble gas configuration, so theircharge is +/- 4.

Elements in groups 5,6 and 7 all receive electrons when ionizing. (eg. its easier

for chlorine, with a valence number of 7, to gain one electron that to loose 7).

Receiving an electron or electrons give your a more negative charge. Therefore

elements in these groups end up having negative charges when becoming ions

(they become anions).

Covalent bondsWeve talked about ionic bonds, the strongest of them all, now lets talking about the nextstrongest bond type; covalent bonds.

In a covalent bond, instead of atom donating or accepting electrons, they pass them back andforth between each other; they share non-lone pairvalence electrons. Before we look at anexample of this, it is useful to learn about Lewis dot diagrams.Lewis dot diagramsSince it is only the valence shell that takes part in chemical bonding, it become tedious drawing

Bohr models showing every shell. What the Lewis dot diagrams do is ignore the lower energy


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levels and only draw valence electrons. Lets look at an example:

Nitrogen has 7 electrons total, and weve already looked at the Bohr model of this. The Lewis

dot diagram only shows the valence electrons, so in the case of nitrogen there are 5 valence

electrons.

First you draw the symbol for the element, then you put dots clockwise around the element at12:00, 3:00, 6:00, and 9:00 and repeat again until youve used all the valence electrons.


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To show ionic bonding in Lewis dot diagrams, see

below:

Naming

When given the molecular formula to name a compound, first identify if the cation is a metal ora non-metal. Non-metals are on the right side of the staircase on the periodic table starting bycarbon and aluminum.As a rule, if the cation IS a metal then it is an ionic molecule with an ionic bond. If the cation is

a non-metal then the molecule is a molecular compound with a covalent bond.

For ionic compounds you simply name the first element and then change the ending to ide

Eg. NaCl is Sodium Chloride

For molecular compounds you need to know your prefixes heres a list:

Prefix Number Prefix Number

mono- 1 hexa- 6

di- 2 hepta- 7

tri- 3 octa- 8

tetra- 4 nona- 9

penta- 5 deca- 10

For molecular compounds you simply name the element and state how many atoms of each

their are using prefixes.

Eg. CO2 = Carbon dioxide N205 dinitrogen pentaoxide.

Note 1: In both cases, if a polyatomic ion is used (from your polyatomic ion list)

then instead of changing the name to -ide, just write the name of the polyatomic


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ion

Eg. Ca3PO4 = Calcium Phosphate

Note 2: with IONIC COMPOUNDS sometimes the metal is a transition metal

(or tin or lead) if so, you must be sure to write THE CHARGE of the transition

metal in roman numerals next to the name. To figure out what the charge is youmust look at what the transition metal is bonded with. Use the charge of what it is

bonded with to determine what the transition metals charge would have to be to

cancel out the charge.

Eg. SnO

right away you can see that the cation is a metal, but is not an alkali

or alkaline earth metal, so you know youre going to need roman

numerals to tell the reader what the CHARGE is.

Step 1: Write the name like you would for any other ionic compound

and leave room for the roman numerals

Tin ( ) Oxide

Step 2: Determine the charge on the anion

Oxygen has a charge of -2

Step 3: Determine the positive number required to cancel out the

negative charge

+2

Step 4: Finish the name by writing in the number (in roman

numerals) you determined in step 3

Tin (II) Oxide

Eg.2 SnO2Step 1: Tin ( ) Oxide

Step 2: Oxygen has a charge of -2, and there are 2 of them: total = -4Step 3: Need +4 to balance charges

Step 4: Tin (IV) Oxide

Eg.3. FeSO4Step 1: Iron ( ) Sulfate

Step 2: Sulfate, a polyatomic ion, has a charge of -2, and there is only

1

of them

Step 3: Need +2 to balance charges

Step 4: Iron (II) Sulfate


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Writing Molecular Formulas:

If you are given the name of an ionic compound, to write the molecular formula you need to

determine the individual charges of the cation and the anion in the molecule and then balance

those charges using appropriate subscripts.

Eg. Calcium Carbonate

Step 1: Write the symbols for each element (or polyatomic ion, as in thisexample) you may wish to leave a tiny space in case you do end up putting a

subscript in the last step.

Ca CO3Step 2: Determine the charges on the cation and the anion

Calcium, group 2, has a charge of +2; the carbonate ion has a

charge of -2

Ca+2 CO3-2

Step 3: If the charges arent already equal, determine how many of each one you

need to make them equal, then write that quantity as a subscript. If the charges

are already equal, than youre done. Just write it all together.

CaCO3Eg.2. Iron (II) Phosphate

Step 1: Fe PO4Step 2: The roman numeral tells us that the charge on iron is +2

Fe+2 PO4-3

Step 3: The charges are +2 and -3; 6 is the first number that these both go into.

So we need three +2 charges, and two -3 charges. In other words, three irons and

2 phosphate

Fe3(PO4)2

Molecular compounds are even easier:For molecular compound, use the prefixes to tell you what number your subscripts

should be for each atom in the compound.

Eg1. Carbon dioxide

Carbon has no prefix, so there is an assumed 1

di tells us that there are 2 atoms of the element to follow, and in this case, the element

that follows di is oxygen. Remember that compound names change the ending to ide, so oxide

is oxygen.

Carbon dioxide = CO2Eg2. Carbon tetrachloride

Carbon has no prefix, so there is only 1 of them.

Write: C

tetra means 4, and chloride is chlorine

Write: Cl4

Put it altogether

Write CCl4Types of reactions:

Synthesis: The definition of synthesis means to combine 2 or more entities together to


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make something new.

The general equation for a synthesis reaction is:

A + B --> AB

Decomposition: Decomposition is the opposite of synthesis and means to take a

compound and break it into its individual parts

AB --> A + BCombustion: Combustion is the process of burning something. To burn something you

need a fuel source, heat, and oxygen, and complete combustion always

yields carbon dioxide and water.

CxHy + O2 --> CO2 + H2O

Single Displacement: The reaction of an element (or diatomic molecule; HOFBRINCL)

with a compound. If the lone element is a cation it will replace the cation in

the compound, if the lone element is an anion, it will replace the anion in the

compound. For this one you have to keep an eye on the part of the reactants that

is by itself. It is IMPORTANT to remember that we always write the cation in a

compound first followed by the anion.

If A is a cation: A + BC --> AC + B

If A is an anion: A + BC --> BA + C

Double Displacement: Similar to single displacement, but in this type of reaction the

cation of one compound replaces the cation of the other compound.

AB + CD --> AD +BC

Again, it is important to remember that the cation always goes first and the anion

always goes last.

Steps to complete a reaction.Step 1: Determine what type of reaction it is and write JUST THE ELEMENT SYMBOL on theproducts side (ignore all subscripts for this step.)Step 2: Just like when you are given the name of a compound and have to come up with

the formula, in this step you look at the elements you have written on the products side and

determine the charge for all species (cations and anion). Then balance the charges by putting

the appropriate subscripts.

Step 3: Balance the whole equation by putting coefficients in front of lone elements, diatomic

molecules, or compounds.

Eg.

Na2CO3 + HCl -->


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Step 1: This is a double displacement. Na will go with Cl and H will go with CO3.

Hint - its a good idea to leave a tiny space for potential subscripts that you may add in step 2.

Na2CO3 + HCl --> Na Cl + H CO3

Step 2: Now add the subscripts to balance the compound you just made on the right.

Na2CO3 + HCl --> Na+1 Cl -1 + H+1 CO3-2The Na and Cl can be written as is because the charges cancel.

We need 2 Hydrogen ions to combined with one carbonate ion.

Na2CO3 + HCl --> NaCl + H2CO3

Step 3: Now just balance the whole equation.Na2CO3 + 2HCl --> 2NaCl + H2CO3

Notes for Chem Academic

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