Elements and Compounds Section 3.4 Elements and Compounds

Composition of Matter

Pure Substances

Elements Building blocks of matter A pure substance that cannot be separated into simpler substances 91 elements occur naturally on Earth 116 total known elements

The Periodic Table The periodic table is the most useful tool to a chemist. You get to use it on every test. It organizes lots of information about all the known elements.

Pre-Periodic Table Chemistry was a mess!!! No organization of elements. Imagine going to a grocery store with no organization!! Difficult to find information. Chemistry didn’t make sense.

Dmitri Mendeleev HOW HIS WORKED… Put elements in rows by increasing atomic weight. Put elements in columns by the way they reacted. SOME PROBLEMS… He left blank spaces for what he said were undiscovered elements. (Turned out he was right!) He broke the pattern of increasing atomic weight to keep similar reacting elements together.

Rows or Periods Now the elements are put in rows by increasing ATOMIC NUMBER!! The horizontal rows are called periods and are labeled from 1 to 7.

Columns/Groups/Families The vertical columns are called groups are labeled from 1 to 18. Elements in the same group have similar chemical and physical properties!! (Mendeleev did that on purpose.) Because they share common traits, they are also referred to by family names

More on the Periodic Table in Chapter 6!!

Compounds Two or more elements chemically combined 10 million known compounds Table salt, aspirin, water

Law of Definite Proportions Characteristic of compounds A compound is always composed of the same elements in the same proportion by mass

Percent by Mass Percent by mass (%) = mass of element x 100 mass of compound

Law of Multiple Proportions Characteristic of compounds When different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the other element in a ratio of small whole numbers