Chapter 19 – Molecules and Compounds Bonding and Molecules Atoms react /combine with other atoms to form molecules. Chemical bonds hold them together. Electrons in atoms are found in energy levels in the electron cloud. Outermost region contains the valence electrons and is the valence shell.

Maximum # of valence electrons that an atom can have is 8, except for the 1 st energy level that can only hold 2 electrons. Stable atoms have 8 valence electrons. When an atom has 8 valence electrons, it is said to have an octet of electrons. In order to reach 8 electrons, atoms will lose, gain, or share electrons.

 Atoms with fewer that 4 valence electrons will lose electrons.  Atoms with more than 4 will gain electrons.  Valence electrons are often represented using dot diagrams.

Elements in Group 1 have 1 valence electrons Elements in Group 2 have 2 valence electrons Elements in Group 13 have 3 valence electrons Elements in Group 14 have 4 valence electrons Elements in Group 15 have 5 valence electrons Elements in Group 16 have 6 valence electrons Elements in Group 17 have 7 valence electrons Elements in Group 18 have 8 valence electrons

MUST DRAW DOT DIAGRAMS IN RED BOX

 Types of chemical bonds: Ionic and covalent  Ionic bonds are formed when atoms lose or gain electrons.  Covalent bonds are formed when atoms share electrons. Covalently bonded atoms of the same type are diatomic molecules. Ex: o 2

19.2 – Chemical Formulas  When atoms combine, they form compounds. All compounds are neutral.  A compound made of ions is called an ionic compound.  A covalent compound is a compound that consists of covalently bonded atoms.

 The periodic table shows the charge of the ions of the elements based on the group.  An oxidation number indicates how many electrons are lost, gained, or shared when bonding occurs.

 To write a chemical formula:  Write the symbol of the positive ion with its charge  Write the symbol of the negative ion with its charge  Criss-Cross the charges of each and use the numbers as the subscript  MUST DRAW

 Ions with more than one element are called polyatomic ions.  Ex: Carbonate = CO 3

 To name compounds:  Write the name of the first element  Write the root name of the second element  Replace the ending with –ide

 To name a compound that contains polyatomic ions:  Write the name of the positive ion first  Write the name of the negative ion second.

 Greek prefixes are used in naming binary covalent compounds.  Ex: CO = Carbon Monoxide  The simplest whole electrons number ratios by which elements combine are written is called the empirical formula.  The actual number of atoms of each element in the compound is written is called the molecular formula.