The Prefix Table on the Board

The prefix table on the board In naming a binary(binary compound is a substance composed of exactly two different elements, which are substances that cannot be simplified further by chemical means) molecular compound, the number of atoms of each element present in the molecule is noted by using prefixes. When Naming them Prefix Number of atoms Mono 1 atom of that type of element in the molecule Di 2 atom of that type of element in the molecule Tri 3 atom of that type of element in the molecule 4 atom of that type of element in the molecule Tetr or tetra Pent 5 atom of that type of element in the molecule Hexa 6 atom of that type of element in the molecule hepta- 7 Octa 8 nona 9 Deca 10 Example Carbon Monoxide (CO) Carbon dioxide(CO2) Diphosphorous mono chloride ( P2Cl) Carbon trioxide(CO3) Carbon tetrachloride( CCl4 ) Vanadium pentoxide( V2O5) Use of suffix -ide Tells us that There are only 2 types of atoms present in the molecule carbon dioxide (CO2) • The previous table will be used to explain the formation of the following compounds • CO is the formula for carbon monoxide • SO2 is the formula for sulphur dioxide • SO3 is the formula for sulphur trioxide Steps in writing formula and names for covalent molecules 1. Asses the structure and determine how many atoms of each element exists in the structure. 2. Write the Formula by writing the symbol of the central atom/ atoms first and the other atoms next. Use subscripts to identify the number of each. Except water 3. Write the name of the formula by using the Prefixes that you have learnt. The prefix comes before the element name when writing the name of chemical formulas (See example 7). Please note there is no need to write mono if the first element is only one but you must use mono if the second element is only one( see example 5 and 6 ) . 4. Change the last part of the name of the second element in the formula to “ide” 5. E.g CO2 is carbon dioxide 6. Eg. CO is Carbon monoxide 7. Eg. Se2Br2 8. More examples on slide 21 is diselenium dibromide • The prefix is placed BEFORE THE ELEMENT TO WHICH IT IS REFERRING afterwards you must end word in the prefix -ide. If you don’t see a subscript for the first element in the formula write the name of the first element as seen in periodic table. • Special cases/ Special names--It is a good idea to learn the molecular formula of covalent compounds as you come across them as there are exceptions to the above rules as some names cannot be derived from their compounds/molecules eg. H2S hydrogen sulphide, Ammonia(NH3), methane for CH4, water H2O some of these were named long before it was discovered what atoms they consist of. Practice Name the following covalent molecules • N2O3 – • N2O4• SF6• XeF6• CCl4 – • CF4SN- Convert the following from word formulas to chemical formulas • Write the formula from the names of the covalent substances below • 1. Selenium trioxide – • 2. Carbon disulphide• 3. Carbon tetrabromide• 4. silicon dioxide• 5.silicon trioxide• 6. nitrogen dioxide- Special Names and exceptions • H O- water 2 • All acids eg HCl (Hydroclhoric acid), H2SO4(Sulphuric acid). • Ammonia NH3 • Ammonium NH4+ • And all hyrdocarbons eg CH4 is methane The diatomic moleculesAll of these compounds can be found in nature as diatomic molecules. Meaning there is two atoms of the same kind combined together in a covalent bond like a set of twin holding hands. If any of these elements is being used or produced in a chemical reaction ensure the subscript two is beside them Eg. Structure of oxygen gas as found in nature Name ( in free state) Formula Hydrogen Iodine Bromine Chlorine Oxygen Nitrogen Fluorine H2 I2 Br2 Cl2 O2 N2 F2 Popular acids – Memorize them- You can consider their names as special cases of names given for covalent substances which they are Acid name Formula Hydrochloric acid Hydrobromic Acid Hydroiodic Acid hydrofluoric acid Hydrogen Sulphide/hydrosulfuric acid Acetic acid/ Ethanoic acid phosphoric acid carbonic acid Sulfurous Acid Sulfuric acid Nitric Acid Perchloric Acid Chloric Acid HCl HBr HI HF H2S CH3COOH H3PO4 H2CO3 H2SO3 H2SO4 HNO3 HClO4 HClO3 Drawing Lewis structures for ions 1.Find the total number of valence electrons for all atoms in the molecule. 2.Draw the Lewis structure of the molecule with single bonds first . 3.Determine how many electrons are needed to satisfy the octet rule of the outside atoms and stabilize them first by distributing the correct amount of valence electrons to them. 4.Check the stability of the central atom. If it was not stabilized by the single bonds then Stabilize the central atom one double bonds or triple bonds. To do this, use a lone pair of electrons from an electronegative non-central atom and make it into a bonding pair shared with the central element that lacks electrons. 5.Check to make sure you have the lowest formal charge for each • Lets draw the lewis structure for the nitrate anion and atom. Also ensure the octet rule is not violated. carbonate anion – 6.Put the molecule in a parenthesis with the charge to the top right hand corner. • NO 3 • CO32- Coordinate( Dative bond) covalent bonding • A coordinate or dative covalent bond is formed when both electrons in the shared pair come from the same atom. • In ordinary covalent bonding each atom contributes one electron to the shared pair of electrons. This is described as simple/ ordinary covalent bonding. • The sharing of electrons can occur in another way where all of the electrons in the bond is contributed by one of the combining atoms. • The first atom uses a pair of non- bonded electrons( lone pair) to bond with the second atom. The second atom contributes nothing to the bond.