Describing Matter 13-1 Notes Part 2

Describing Matter: part 2 Elements ● An element is a pure substance that cannot be broken down into any other substances by chemical or physical means. (refer to periodic table) ● Elements are the simplest substances. ● Each element can be identified by its specific physical and chemical properties. ● Elements cannot be broken down into simpler substances. ● Scientists have discovered about 114 different elements. Example: Aluminum (Al) is used for foil. ● The first 92 elements occur naturally on Earth. ● Elements after 92, are synthetic – man made! Particle of Elements – Atoms: ● An atom is the basic particle from which all elements are made. ● Atoms are the building blocks of matter. ● They are made up of three types of things (subatomic particles): protons, neutrons, and electrons. ● Protons = positive charge, + ● Neutrons = neutral (no) charge ● Electrons = negative charge, ● Different elements have different properties because their atoms are different. When Atoms Combine: ● When atoms combine, they form a chemical bond, which is a force of attraction between two atoms. ● In many cases, atoms combine to form larger particles called molecules -- groups of two or more atoms held together by chemical bonds. ● A molecule of water, for example, consists of an oxygen atom chemically bonded to two hydrogen atoms. Compounds ● A compound is a pure substance made of two or more elements chemically combined in a set ratio. ● A compound may be represented by a chemical formula, which shows the elements in the compound and the ratio of atoms, CO2. ● For example, part of the gas you exhale is carbon dioxide. Its chemical formula is CO2. The number 2 below the symbol for oxygen (subscript) tells you that the ratio of carbon to oxygen is 1 to 2. (If there is no number after the element’s symbol, the number 1 is understood.) ● If a different ratio of carbon atoms and oxygen atoms are seen in a formula, you have a different compound. ● For example, carbon monoxide—a gas produced in car engines—has the formula CO. ● When elements are chemically combined, they form compounds having properties that are different from those of the uncombined elements. ● *COMPOUNDS CANNOT BE SEPARATED! This snail’s shell is made mostly of calcium carbonate – a compound made from calcium, carbon, and oxygen. Mixtures ● A mixture is made of two or more substances—elements, compounds, or both—that are together in the same place but are not chemically combined. Example: vegetable soup! ● Mixtures differ from compounds in two ways: Each substance in a mixture keeps its individual properties. Also, the parts of a mixture are not combined in a set ratio (no chemical formula). ● *MIXTURES CAN BE SEPARATED!! *Review: -Elements are pure substances in their simplest form! -A molecule is two or more atoms, chemically combined! - Compounds are two or more elements chemically combined! Cannot separate! - Mixtures are two or more elements, compounds, or both that are not chemically combined. Can separate! Review Questions: 1. How are elements and compounds similar? How are they different? - Both elements and compounds are pure substances. While elements are made of one simple substance, compounds are made of two or more elements. 2. Plants make a sugar compound with the formula C6H12O6. What elements make up this compound and how many of each element? - C (Carbon)- 6 - H (Hydrogen)- 12 - O (Oxygen)- 6 3. Why is it correct to say that seawater is a mixture? - Seawater is mixture of salt, water, and many other substances. 4. List three examples of elements from the periodic table with their symbol. (You will be responsible for knowing elements 1-18!) - Calcium, Ca - Sodium, Na - Hydrogen, H 5. What is an atom? - The basic particle from which all elements are made – building blocks of matter! 6. When atoms combine, what holds them together? - Chemical bonds.