Periodic Table, Valency and Formula

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1 Periodic Table, Valency and Formula Origins of the Periodic Table Mendelѐѐv in 1869 proposed that a relationship existed between the chemical properties of elements and their atomic masses. He noticed a pattern when elements were placed in horizontal rows and that chemically similar elements occurred in vertical columns. Knowing that at that time his knowledge of chemistry and the elements were limited, he decided to leave gaps in his table were elements should exist even though they were not known at time, he also correctly predicted the properties of those missing elements. H. G. Mosely ( ) Contributed to the periodic table of elements by arranging the elements according to their atomic number instead of their mass number. Mosely worked with x- rays to do this. The Periodic Table The periodic table is a classification of all the elements based on atomic numbers. The modern table is composed of horizontal periods and vertical groups which contain elements arranged in: Order of increasing atomic number In relation to the electron structure of their atoms In relation to their chemical properties Periods: - A new period starts when a new electron enters a new shell. There are seven periods numbered from 1-7 with the following properties: 1. Elements of one period all have the same number of shells. 2. Going across a period each element has one more proton than the previous element. 3. Going across a period the chemical properties of elements become less metallic and more non- metallic.

2 Groups: - These are vertical columns of elements they are 8 groups numbered I- VIII. 1. Each element in a group has the same number of electrons in its outer shell. For element in group I- VIII, this number is called the group number. 2. Going down a group each element has one more shell than the previous element. 3. Elements in the same group have similar chemical properties. 4. The activity of metals going down a group increases. 5. The activity of non- metals increases going up a group. The periodic table divides the elements into metals, non- metals and semi- metals (metalloids). There are two groups that include metals only. These are group I and II. There are two groups which contain metals with the exception of Boron. The metalloids are: B group III Si, Ge group IV As group V Te, Po group VI Al group III

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4 Valency of Elements Valency is the number of electrons an atom has to lose, gain or share to attain a stable electronic structure. Formula E.g. 1 The formula of water is (H 2 O) this tells us we have 2 Hydrogen atoms to 1 Oxygen atom. E.g. 2 The formula of carbon dioxide is (CO 2 ) this tells us we have 2 Oxygen atoms and 1 Carbon atom. The chemical formula of a compound shows a ratio of the atoms of each element by mass. E.g Ammonium Sulphate (NH 4 ) 2 SO 4 : Nitrogen 2 Hydrogen 8 Sulphur 1 Oxygen 4 The formula for copper nitrate is written as: Cu(NO 3 ) 2 and not as CuN 2 O 6 This is because the compound is a nitrate and has properties of a nitrate. The nitrate group is recognized as NO 3. Here is a list of some common group names: 1. SO 4 Sulphate e.g. K 2 SO 4 2. CO 3 Carbonate e.g. Na 2 CO 3 3. Cl Chloride e.g. NaCl 4. OH Hydroxide e.g. KOH 5. O Oxide e.g. Al 2 O 3 6. S Sulphide e.g. ZnS 7. NO 3 Nitrate e.g. Cu(NO 3 ) 2 8. HCO 3 Hydrogen Carbonate e.g. NaHCO 3

5 Writing the Formula of Compounds To write a simple chemical formula you need to know: 1. The symbol or formula of the elements and groups. 2. The valencies of the elements or groups. (The valency is a measure of combining power) Element (Metals) Symbol Valency Sodium Na 1 Potassium K 1 Silver Ag 1 Gold Au 1 and 3 Barium Ba 2 Calcium Ca 2 Zinc Zn 2 Copper Cu 2 and 1 Iron Fe 2 and 3 Lead Pb 2 Magnesium Mg 2 Aluminum Al 3 Element (Non- Metals) Symbol Valency Hydrogen H 1 Chlorine Cl 1 Bromide Br 1 Iodide I 1 Oxygen O 2 Sulphur S 2, 4 and 6 Nitrogen N 3 and 5 Phosphorous P 3 and 5 Carbon C 4 Group Symbol Valency Ammonium NH 4 1 Hydroxide OH 1 Nitrate NO 3 1 Sulphate SO 4 2 Carbonate CO 3 2 Hydrogen Carbonate HCO 3 1 Phosphate PO 4 3

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