Elements and Compounds Chapter 3-4

Substances are either: a) elements, or b) compounds

Substances: element or compound Elements- simplest kind of matter cannot be broken down any simpler and still have properties of that element! all one kind of atom. Compounds are substances that can be broken down only by chemical methods when broken down, the pieces have completely different properties than the original compound. made of two or more atoms, chemically combined (not just a physical blend!)

Compound vs. Mixture Compound Mixture Made of one kind of material Made of more than one kind of material Made by a chemical change physical change Definite composition Variable

Elements vs. Compounds Compounds can be broken down into simpler substances by chemical means, but elements cannot. A “chemical change” is a change that produces matter with a different composition than the original matter.

Classification of Matter

Law of Definite Proportions Regardless of the amount, a compound is always composed of the same elements in the same proportions by mass. percent by mass = mass of element x 100 mass of compound

Problem 1 A 78.0 g sample of an unknown compound contains 12.4 g of hydrogen. What is the percent by mass of hydrogen in the compound?

Problem 2 If 3.5 g of X reacts with 10.5 g of Y to form the compound XY, what is the percent by mass of X in the compound? The percent mass of Y?

Law of Multiple Proportions 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.