Chapter 2: Molecules & Solutions

What is a molecule? two or more bonded Definition: a molecule is a group of two or more chemically bonded atoms. The two or more atoms joined together, can be the same element or different elements.  E.g. molecule of water = H 2 O, molecule of oxygen = O 2

Why do atoms bond with each other? Atoms bond with one another so that they can fulfill the octet rule. An atom will try to have the same electron configuration as that of noble gases (full last shell). E.g. two oxygen atoms How many electrons does each oxygen atom have? 6

What do the yellow dots represent? ****VALENCE ELECTRONS****

What atom is this? Will this atom bond with any other atom? When an atom’s last shell is complete/full, it will not want to react or bond easily with another atom. E.g. all noble gases ARGON NO

Valence Electrons: Bonding When 2 atoms form a bond, it is their valence electrons that “react” with one another to create that bond.

Potassium (K) & Chlorine (Cl) What is the best way for each atom to acquire a full last shell? Ionic K loses 1 e - Cl gains 1 e -

Chemical Formulae Symbols of elements in a chemical formula represent the elements present. Subscript numbers represent mole proportions of the proceeding elements.  CaCl 2  In this chemical formula, there are two atoms of chlorine and one atom of calcium. Note that if there is no subscript number, it means a subscript of 1.  CaCl 2  In this example, Ca has no subscript. It means that there is one atom of calcium. “ Ca 1 Cl 2 ”

CaCl 2

Number of molecules To show the number of molecules, a full sized number is located in front of the molecule. For example 4 molecules of carbon dioxide is designated as:  4CO 2 This means there are a total of 4 C atoms and 8 O atoms in the combination. A way to remember this is to think of it as 4 x (CO 2 ).

4 CO 2

Complex formulas Just as in Algebra, you can use parentheses to separate parts in a complex formula. One example is the formula for nitroglycerin, a highly explosive substance.  C 3 H 5 (NO 3 ) 3 This formula shows that nitroglycerin consists of 3 atoms of C, 5 atoms of H and then 3 NO 3 nitrate ions. If the parentheses were not used, you might have a formula like:  C 3 H 5 N 3 O 9 Is this formula correct?

How many atoms of sulfur (S) are there in Na 2 SO 4 ? a. 1 b. 2 c. 3 d. 4

How many atoms of oxygen (O) are there in Al 2 (SO 4 ) 3 ? a. 2 b. 3 c. 4 d. 12

Which of the following chemical formulae represents the following diagram? H 3 PO 4 C 6 H 11 OH S(CH 3 ) 4 (NH 2 ) 2 CH 3 CH 2 OH

Exceptions Lithium, Beryllium, Boron tend to lose their valence electrons to resemble Helium   Duet rule

Ions Atoms are normally neutrally charged because they contain as many protons as they have electrons.  Protons have a positive charge  Electrons have a negative charge E.g. Carbon has 6 protons = +6 (proton charge) Carbon has 6 electrons = - 6 (electron charge) 0 (neutral) ion Definition: an ion is an atom that has become electrically charged by losing or gaining one or more electrons.

Gaining and Losing Electrons When does an atom gain or lose electrons?  When an atom forms a bond it can gain or lose electrons to fulfill the octet rule.  E.g. Chlorine atom = ___ protons ___ electrons To fulfill the octet rule, chlorine will gain/lose __ e - protons - electrons Cl -

When an atom loses 1 or more electrons, it becomes positively charged = positive ion. When an atom gains 1 or more electrons, it becomes negatively charged = negative ion.  Exercise: if oxygen gains 2 electrons, what is its net charge? -2-2

Review Negative Ion - gained one or more electrons -more electrons than protons - negative net charge Positive Ion - lost one or more electrons -more protons than electrons - positive net charge

Ionic Charges Table IAIIAIIIAIVAVAVIAVIIAVIIIA LiBeBCNOFNe Lose 1 e Lose 2 e Lose 3 e Gain or lose 4 e Gain 3 e Gain 2 e Gain 1 e None / None Group Number Number of Valence electrons Example Element Tendency Charge

Cation: positively charged atom Anion: negatively charged atom