Unit 7: Compounds and Chemical Reactions Section 7.1 Compounds and Chemical Bonds March 9, 2011 Page 175/177

Elements, Compounds, and Mixtures An element is a substance made up of one kind of atom Gold (Au) and Silver (Ag) are elements Some elements are made up of molecules with more than one atom – for example, Hydrogen Gas (H2) and Oxygen Gas (O2) are both elements.

8.1 Molecules and covalent bonds A chemical bond forms when atoms transfer or share electrons. There are two kinds – ionic (giving/taking electrons) and covalent (sharing electrons) Covalent Bond Na+ Cl- Ionic Bond Extra electron from Na

Elements, Compounds, and Mixtures A compound is formed when two or more elements form a chemical bond. Water (H2O) is a compound, and so is salt (NaCl). Compounds have different properties than the elements that make them!!!!!!!!!!

Elements, Compounds, and Mixtures If two or more elements or compounds are put together but don’t bond (exchange or share electrons), they make a mixture Mixtures keep the properties of the substances that make them up Salt water is a mixture – it’s still got the properties of both salt and water!

8.1 Chemical formulas and diagrams Molecules are represented by a chemical formula. The chemical formula tells you the number of each kind of atom in the molecule. The number next to each element’s symbol is the subscript.

8.1 Structure and function The properties of a compound depend MUCH more on the shape of its molecule than on the elements of which it is made.

8.2 Electrons and chemical bonds Chemical bonds are formed only between the electrons in the highest unfilled energy level. These electrons are called valence electrons. To figure out how many valence electrons an atom has, count right (but skip the transitional metals)

What Atoms Want Atoms want to have full electron shells (8, or 2 in the case of atoms from H to C) Noble gases have full electron shells, so they don’t form molecules Other non-metals can get full shells easier by getting electrons. Metals can get full shells easier by giving away electrons Metalloids can give or get depending on what they bond with