Combinations of Atoms

1. Combinations of Atoms Chapter 8.2

2. Compounds • Compounds • Elements are rarely found pure in the earth, they are generally found as compounds. • A compound is a substance that results when the atoms of two or more elements are chemically combined.

3. Compounds • The result is a new substance with properties different from those of elements that compose it. • Example: Water is a compound formed from the atoms of hydrogen and oxygen.

4. Compounds • A molecule is the smallest complete unit of a compound. Water is formed from two hydrogen molecules and one oxygen molecule. H-O-H • Diatomic molecules are molecules that exist naturally as two atoms.

5. Compounds • Hydrogen atoms always exist naturally as a diatomic molecule. • The oxygen atoms you breath are O2 atoms.

6. Energy Levels • Energy Levels • Energy levels are the arrangement of electrons within the electron cloud of an atom; they are also the specific energies an atom can have. • Atoms have a specific number of energy levels to contain the amount of electrons they have.

8. Energy Levels • The electrons in an atom travel on the energy levels in a set of “paths” called orbitals. • An orbital is the region of space in the energy level where an electron is likely to be found.

10. Energy Levels The following table describes the energy levels, orbitals, and # of electrons.

11. Energy Levels • When electrons are in their highest energy levels the atom is said to be in an excited state. • When electrons are in their lowest energy levels the atom is said to be in a ground state.

12. Energy Levels • Atoms with filled energy levels are stable and do not react. This group of elements is the “Noble Gases”. • Elements that are near the edges of the period table are the most reactive, because they are closest to having a complete set of electrons. • Elements nearest the bottom are also more reactive because they have more energy levels.

13. Chemical Bonds • Chemical Bonds • A chemical bond is produced by the interaction of valence electrons (electrons in the outermost energy) and is the force that holds atoms together. • Atoms can form chemical bonds by either sharing electrons or transferring electrons from one atom to another.

14. Ionic Bonds • An ionic bond is a bond formed through the transfer of electrons from one atom to another. • When an ionic bond is formed, both atoms become either positively or negatively charged and are called ions. • An ion is an atom or group of atoms that carry a charge.

15. Ionic Bonds • When an atom gives away electrons the atom becomes positively charged and is called a cation. • When an atom gains electrons the atom becomes negatively charged and is called an anion.

17. Covalent Bonds • Covalent Bonds • A covalent bond is a bond between atoms that share electrons. • When electrons are shared in a diatomic molecule the nuclei of each atom pull on the electrons with the same force and create a neutral molecule.

18. Covalent Bonds • When electrons are shared in a molecule of different elements the electrons are shared unequally. • For example, in a water molecule the two oxygen molecules pull on the electrons greater than the one hydrogen molecule which gives the molecule a slightly positive charge at its hydrogen end and a slightly negative charge at its oxygen end.

20. Chemical Formulas • Chemical Formulas • A chemical formula is symbols indicating the elements a compound contains and the relative number of each element. • In a compound the number of elements in the same compounds are always found in the same proportion. • For example, the chemical formula for water is H2O, which represents two hydrogen atoms and one oxygen atom for every H2O molecule.

21. Mixtures • Mixtures • A mixture is material that contains two or more substances that are not chemically combined. • The substances in a mixture keep their individual properties, and can be separated by a physical means.

22. Solutions • A solution is a mixture in which one substance is uniformly dispersed in another substance. • An example of a solution is Kool-Aide. The sugar is completely dissolved in the water. • Not all solutions are liquids. Gases and solids can also form solutions. • An alloy is a solution of two or more metals, such as brass (Cu & Zn) and bronze (Cu & Sn).