Understanding Chemical Formula

H2O Most of us are familiar with the formula for water being H2O but it is vital for understanding chemistry to understand what this means. From the diagram we can see h2o means two H’s and 1 O. so it is important to record that the subscript that follows an element is equal to the number of atoms of that element in the compound. Notice also that the O for oxygen is not followed by a number 1 this is because we would not mention the element unless their was at least one atom of it in the molecule

Copper sulfate forms some of the most beautiful crystals the formula for copper sulfate is written above but what does this mean in terms of elemental ratio. (Click) well from the formula we can see one copper atom combined with one sulfur atom and 4 oxygen atoms notice again that the subscript that follows the element tells us how many of that element are bonded in the compound.

Compound Two or more different types of atom chemically combined in fixed proportion There a three important features of this definition. (Click) firstly the atoms are of different elements (click) then they are chemically combined ( bonded) and finally the part of the definition we will focus on today (click) the atoms are in fixed proportion. This is the atomic ratio shown by the subscripts.

.H20 Many crystals come combine with water of crystallization. This is water that is chemically bonded to the molecule and is in fixed proportion. How ever when the compound dissolves these water particle dissolve separately into the solution

A dot is used to represent water of crystallization Eg MgSO4.7H2O means MgSO4 with 7molecules of water added

A dot is used to represent water of crystallization Cu(OH)2.5H2O means 1 molecule of Cu(OH)2 with 5 waters attached. The period means that the following number of molecules are attached

Here there is a total of 14 atoms 2 x Al 3 x C 3 x 3 O Use of parenthesis Example: Mg(OH)2 1 x Mg 2 x O 2 x H Al2(CO3)3 Here there is a total of 14 atoms 2 x Al 3 x C 3 x 3 O Use parenthesis to show how many polyatomic ions are required Al2(SO4)3 This formula means 2 Aluminum atoms and 3 sulfate polyatomic ions The use of parenthesis mean that the subscript number applies to all atoms within the parenthesis In the above example there are two OH’s so the total number of atoms is 5

Why fixed proportion Elements have no choice they can only bond in specific ratios. Let me explain further

Today we will focus on ionic compounds and their formula Ionic compounds are those which are formed between a metal and a non metal The large electronegativity difference mean that instead of electrons being shared they are completely exchanged On the board is an atomic shell diagram of sodium on the left and chlorine on the right. Sodium Chlorine

P P N N Sodium Chlorine Electron configuration Sodium has 11 protons locked away in its nucleus and chlorine has 17 Sodium Chlorine Electron configuration

P P Sodium Chlorine Electrons Protons

P P Chlorine Sodium Electrons Electrons Protons Protons Charge Charge A sodium atom has 11 negative electrons encircling the nucleus in orbitals A chorine atom has 17 negative electrons encircling its nucleus This means that both Sodium and Chlorine Atoms have the same number of electrons as they have protons This is true for all atoms Sodium Chlorine Electrons Protons Electrons Protons Charge Charge

P 11 P 17 Sodium Ion Atom Chloride Ion Chlorine Atom I wish my outside shell was full I wish my outside shell was empty P 11 P 17 However Sodium atoms and chlorine atoms are not Happy and now we will see why It is important to point out that atoms are inanimate so they don’t have feelings in fact they are more stable when they have a full outside shell meaning they have lower chemical potential energies. Sodium Ion Atom Chloride Ion Chlorine Atom Electrons 11 Protons 11 Electrons 17 Protons 17 10 18 Charge Charge -1 +1

So one sodium atom can make one chlorine atom happy So one magnesium atom can make two chlorine atoms happy Gives 2 e- Accepts 1 e- So we saw that one sodium could supply one electron to chlorine this is the lowest possible ratio for reacting atoms, It is a one to one ration one sodium reacts loses one electron and one chlorine atom receives it. This is because a sodium atom must lose one electron as the shortest possible route to becoming isoelectric with Ne Equally a chlorine atom must gain one electron to become isoelectric with Argon. In the case of magnesium, which has 2 valence electrons it must lose 2 electrons to become isoelectric with its nearest noble gas Neon.

-1 -1 2+ Chlorine Chlorine Magnesium So Magnesium needs to lose two electrons but a chlorine only needs one. This means that now one chlorine does not have the capability to accept enough electrons to make a magnesium atom become stable. A magnesium atom requires two chlorine atoms so that they can both take one of the two valence electrons that magnesium must lose to become isoelectric with neon. Magnesium

We can represent the need to donate electrons or acquire that as jigsaw pieces A tab that sticks out represents a valence electron that must be donated but a blank that sticks in represents a space in the outside shell that must be filled in order to complete the out side shell and become more stable

Group 1 reacting with Group 7 Now we can look at how the elements from different groups will interact Here we see group one reacting with group 7A referred to on your periodic table as group 17 This is a rematch of sodium with chlorine that we saw earlier sodium on the left has one electron to give away and chlorine on the right a space for one more electron. We can see then that one atom from group one reacts with one atom from the halogens to make a compound.

Group 1 reacting with Group 6 Now is we look group one the alkali metals reacting with any member of group 6A or group 16 Here we see that the chalcogen’s (group 16) require two electrons to gain a full outer shell Alkali metals have only one to donate, so two group 1 atoms are required to satisfy the needs of a group 16 atom

Group 1 reacting with Group 5 Elements in group 5A require three electrons before their valence shell is filled so would require 3 alkali metal to do this job Group 1

Group 2 reacting with Group 7 Now we will look at Group 2 metals the Alkaline Earth Metals, such as magnesium When they react is the halogens it takes two halogens to satisfy the AEM This will be a 1:2 ratio such as MgCl2 that we saw earlier

Group 2 reacting with Group 6 But when group 2 react with group 6 it is a perfect match for example One magnesium requires on oxygen since magnesium needs to lose two electrons and oxygen needs to gain two One magnesium can supply all the electrons that one oxygen needs and visa versa

Group 2 reacting with group 5 Now when group 2 react with group 5 things become a little more complicated As you see when one group 2 atom reacts with one group 5A atom there is still a requirement for another electron in the outer shell of the Group 5 non metal But If two group atoms react with one group 5a atom then one valence electron on the group 2 element is left undonated However if we continue we see that a combination of three group 2 atoms with 2 group 5A atoms works perfectly meaning that the formula for group2 reacting with group 5A would be 3:2 Group 5 Group 2

Group 4 Can Give four or accept four On the periodic table that I have supplied for you will see I didn’t assign a jigsaw piece for carbon this is because carbon can donate four or collect four electrons. However because of this carbon mostly shares its electron and makes covalent compounds

Try these questions How many Chlorines could an Aluminium make happy? How many Chlorines could a Beryllium make happy? How many Lithiums could an Oxygen help out? How many Oxygens could an Aluminium make happy? What is the formula of Sodium fluoride? What is the formula of magnesium fluoride? What is the formula of Aluminium fluoride? What is the formula of Aluminium oxide?