 How are the elements on the periodic table organized?

Chapter 3.4

 Dmitri Mendeleev first began to organize the elements by similarities in mass. ◦ He was the first one to create a periodic table, though we no longer organize it by atomic mass

 Arrange by atomic number (number of protons)  Organizes elements into a grid of horizontal rows called periods and vertical columns called groups or families  Elements in the same group have similar chemical and physical properties

 Elements- pure substances that cannot be separated into simpler substances by physical or chemical means ◦ 92 naturally occurring ◦ Others have been developed by scientists (synthetic) ◦ Not equally abundant  Examples: 75% of universe is hydrogen and oxygen, carbon, and hydrogen make up 90% of the human body and francium is the least abundant element with only 20g total in the Earth’s crust ◦ Each has a unique chemical name and symbol

 Symbols are 1, 2, or 3 letters  First letter is ALWAYS capitalized  Remaining letters are ALWAYS lowercase  Universally accepted system so that scientists can communicate

 4-5 quizzes ◦ 3 announced ◦ 1-2 unannounced  Quiz Monday, 24 th (Actinium –Hydrogen)  Quiz Wednesday, 26 th (Iodine-potassium)  Quiz Friday, 28 th (Radium-Zirconium)

 Made up of 2 or more different elements that are chemically combined ◦ Most matter exists in this form ◦ 10 million known compounds ◦ New ones are created at a rate of 100,000/year ◦ Can be broken down into simpler substances by chemical means ◦ More stable than individual elements

 Coefficient: A number that comes in front of a chemical formula, indicates how many molecules of that compound you have. ◦ Examples: 4 CO 2 3 H 2 O  Subscripts: a number that is below the elements listed in the compound, indicates how many of each element you have. ◦ These numbers only apply to the number in FRONT of them. ◦ Examples: 4CO 2 3H 2 O

 Figure out how many atoms are in each compound!

 Determine how many atoms are present in the following compounds: ◦ 1. H 3 PO 4 ◦ 2. NH 4 H 2 PO 4 ◦ 3. 3CaCrO 4 ◦ 4. 4K 2 Cr 2 O 7

 A compound is always composed of the same elements in the same proportion by mass ◦ The same amounts of the elements in a compound can be expressed by percent mass x 100

 A 200.0g sample of sugarcane contains 32.5g of hydrogen. What is the percent by mass of hydrogen in the compound?

 The same elements can form more than one compound ◦ Happens if different compounds have different mass compositions  Example: Water and Hydrogen Peroxide Mass ratio of compound I Mass ratio of compound II *Look at the example on page 89*

 Chapter 3 pg 977: 1-4  Practice Problems: pg 88  Practice Problems: pg 90