Combinations of Atoms

Chapter 8.2

Compounds

1.Compoundsa.Elements are rarely found pure in the

earth, they are generally found as compounds.

b.A compound is a substance that results when the atoms of two or more elements are chemically combined.

Compounds

i.The result is a new substance with properties different from those of elements that compose it.

ii.Example: Water is a compound formed from the atoms of hydrogen and oxygen.

Compounds

a.A molecule is the smallest complete unit of a compound. Water is formed from two hydrogen molecules and one oxygen molecule. H-O-H

b.Diatomic molecules are molecules that exist naturally as two atoms.

Compounds

i. Hydrogen atoms always exist naturally as a diatomic molecule.

ii.The oxygen atoms you breath are O2 atoms.

Energy Levels

1.Energy Levelsa.Energy levels are the arrangement of

electrons within the electron cloud of an atom; they are also the specific energies an atom can have.i.Atoms have a specific number of energy

levels to contain the amount of electrons they have.

Energy Levels

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

Energy Levels

The following table describes the energy levels, orbitals, and # of electrons.Energy Level Operation Orbital Operation # of Electrons

1 X Self 1 X 2 2

2 X Self 4 X 2 8

3 X Self 9 X 2 18

4 X Self 16 X 2 32

Energy Levels

a.When electrons are in their highest energy levels the atom is said to be in an excited state.

b.When electrons are in their lowest energy levels the atom is said to be in a ground state.

Energy Levels

a.Atoms with filled energy levels are stable and do not react. This group of elements is the “Noble Gases”.i.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.

ii.Elements nearest the bottom are also more reactive because they have more energy levels.

Chemical Bonds

1.Chemical Bondsa.A chemical bond is produced by the

interaction of valence electrons (electrons in the outermost energy) and is the force that holds atoms together.

b.Atoms can form chemical bonds by either sharing electrons or transferring electrons from one atom to another.

Ionic Bonds

a.An ionic bond is a bond formed through the transfer of electrons from one atom to another. i.When an ionic bond is formed, both atoms

become either positively or negatively charged and are called ions.

ii.An ion is an atom or group of atoms that carry a charge.

Ionic Bonds

i.When an atom gives away electrons the atom becomes positively charged and is called a cation.

ii.When an atom gains electrons the atom becomes negatively charged and is called an anion.

Covalent Bonds

1.Covalent Bondsa.A covalent bond is a bond between atoms

that share electrons.b.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.

Covalent Bonds

a.When electrons are shared in a molecule of different elements the electrons are shared unequally.i.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.

Chemical Formulas

1.Chemical Formulasa.A chemical formula is symbols indicating the

elements a compound contains and the relative number of each element.

i. In a compound the number of elements in the same compounds are always found in the same proportion.

ii.For example, the chemical formula for water is H2O, which represents two hydrogen atoms and one oxygen atom for every H2O molecule.

Mixtures

1.Mixturesa.A mixture is material that contains

two or more substances that are not chemically combined. i.The substances in a mixture keep their

individual properties, and can be separated by a physical means.

Solutions

a.A solution is a mixture in which one substance is uniformly dispersed in another substance. i.An example of a solution is Kool-Aide. The

sugar is completely dissolved in the water.ii.Not all solutions are liquids. Gases and solids

can also form solutions. iii.An alloy is a solution of two or more metals,

such as brass (Cu & Zn) and bronze (Cu & Sn).