Key Concept: Elements make up the periodic table Ch. 16.2 Key Concept: Elements make up the periodic table

I. Elements can be organized by similarities Atomic Mass Periodic Table Mendeleev- Organize elements by chemical/physical properties Rows- similar chemical properties Period Columns- similar physical properties Family Group

C. Groups & Periods Group- elements listed in a vertical column Labeled by a # at the top of the column Sometimes called a family Period- elements listed in a horizontal row Properties change in a predictable way from one end of a period to another

II. The periodic table organizes the atoms of the elements by properties and atomic number Reading the Periodic Table # at the top= atomic number Chemical Symbol- abbreviation for an element Name Atomic Mass- indicates AVERAGE mass of all isotopes of the element Color of Symbol- (if present) indicates the physical state of the element at room temperature

C. Trends Atoms on the left side form positive ions easily (Group 1 form +1 ions Group 18 forms no + ions). Atoms on the right side form negative ions easily Atom size decreases as you move to the right of the table. Atom size increases as you move down the table. Density generally increases from the top of a group to the bottom. Atoms to the right and left are the least dense and the atoms in the middle are the most dense.