Now it is time to get the heart of the matter--or more specifically, how atoms form molecules and produce matter. We will begin by discussing the theory of the atom, what particles make up the atom and differences between different atoms. Remember, atoms are elements, they are simply the smallest particle of an element that can be identified as that element.

ATOMS--Dalton's Atomic Theory

• All matter (including elements) is composed of atoms; each atom is a very small, chemically indivisible particle
• the word 'atom' is from the Greek work "atmos" which means "cannot be cut apart"
• elements are different because they are composed of different types of atoms
• each type of atom has properties different from other atoms
• each type of atom has different properties, one of which is mass--the mass of a carbon atom weighs 12 times that of a hydrogen atom
• compounds are composed of two or more atoms chemically combined in fixed proportions
• for example, water, H₂O, always occurs in a ratio of 2 hydrogens : 1 oxygen--if the ratio were anything else it would not be water, H₂O₂ with a ratio of 2 : 2 is peroxide, certainly not water
• (law of constant composition)
• chemical reactions rearrange the atoms
• by rearranging the atoms and their ratios of chemical combination, the substances change
• chemical reactions cannot create or destroy matter, they can only rearrange it, therefore all atoms in a chemical reaction product, must have occurred within the reactants, albeit in a rearranged form
• (conservation of matter)
• If two elements, A & B, form more than one compound, the masses of B that can combine with a given mass of A are in a ratio of small whole numbers
• Law of Multiple Proportions

ATOMIC PARTICLES:

Atoms consist of three subatomic particles:

• electrons
• electrons are negatively charged particles and their properties are summarized in the following table
• protons
• protons are positively charged particles and their properties are summarized in the following table
• neutrons
• neutrons have no charge and their properties are summarized in the following table

ELECTRONS:

• J.J. Thompson determined charge:mass ratio of the electron, e^-, in 1897
• the charge is -1
• the mass is 5.486 x 10 ^-4 amu or 9.10939 x 10 ^-31 kg
• Robert Millikan measured the charge of e^-, 1909
• Thompson developed the "plum pudding" model of the atom
• this model had all atomic particles distributed evenly throughout the volume of the atom
• Rutherford developed the "nuclear" model of the atom
• based upon his experiment which showed that atoms contains regions of highly dense, positive material, called the nucleus
• the nucleus is very dense, 99.95% (or more) of the mass of the atom is in the nucleus which has a diameter of approximately 10 ^-15 m-- a matchbox of nucleus material would weigh 2½ billion tons! The density is approximately 1013 - 1014 g/cm3.
• Rutherford discovered this through his famous experiment with gold foil in which he shot alpha particles (fairly massive particles with a positive charge) through thin gold foil and found that many particles were strongly deflected and some bounced back at him! This could only happen if the gold foil atoms contained massive centers that had a positive charge, as exhibited in the figure below.

PROTONS:

• small, positively charged particles
• reside in the nucleus
• along with the neutron, make up most of the mass of the atom
• the number of protons is what defines the type of a particular atom

NEUTRONS:

• small particles with no charge
• reside in the nucleus
• along with the proton, make up most of the mass of the atom
• a differing number of neutrons is what defines an "isotope" of an atom

ATOMIC NUMBER:

• the atomic number has the symbol, Z, and is shown as a subscript to the element symbol
• the atomic number gives the number of protons in the nucleus (and the number of electrons if the species is neutral) of a particular atom

• the atomic number defines a specific type of atom since each different type of atom (representing each element) will have a different number of protons in the nucleus 

MASS NUMBER:

• the mass number has the symbol, A, and is shown as a superscript to the element symbol
• the mass number gives the mass of atom in amu, atomic mass number, and is approximately equal to the number of protons plus the number of neutrons

ELEMENTAL SYMBOLS:

                      ^AX_Z

Elemental symbols are typically written as above where X is the element symbol with the mass number as a superscript and the atomic number as a subscript.

ISOTOPES:

• Almost all atoms have "Isotopes"
• Elements with the same number of protons (atomic number) but differing number of neutrons – isotopes are the same elements (atoms) with different masses
• isotopes will have slightly different chemical and physical properties due to the difference in mass, which can be very helpful in characterizing substances
• for example, below are three isotope of carbon, C:

           ¹²C₆              ¹³C₆                ¹⁴C₆

        # p          6                         6                           6

        # e          6                         6                            6

        # n          6                         7                            8

• notice that the sum of # protons and # neutrons is equal to the mass number
  

• because nearly all elements have one or more isotopes, the mass of a naturally occurring element will be a "weighted average" of all the isotopes which occur naturally, for example:
  
  Carbon has two prominent isotopes which occur naturally (C-14 is present in very small quantities and is radioactive so does not stick around long and we won't count it here) shown below with their relative percent abundance
  
  C-12 12.0000 amu 98.9% abundant
  C-13 13.0000 amu 1.1 % abundant
  
  To determine the mass of naturally occurring carbon, we calculate the weighted average of the two isotopes by summing (fractional abundance)(mass of isotope) for each isotope:

ATOMIC MASSES:

Because atoms are too small to weigh individually, we have had to develop a relative mass scale for elements on the periodic table:

• amu – atomic mass unit – is one such relative mass scale--one amu equals exactly 1/12 the mass of an atom of carbon-12 isotope
• rounded masses:       C         12 amu
                                      Mg       24 amu
                                      Al         27 amu
                                       F          19 amu
• masses can be accurately calculated using a mass spectrometer but we won't worry about how that is done at this time. All of our atomic masses we will use will come from published values on a periodic table of the elements to be discussed next.
  

PERIODIC TABLE:

• a very good periodic table can be found on the internet at URL:  www.webelements.com. In addition to the table itself, there is a great deal of information about each element which is accessible at the above internet address
• there are a number of ways that the elements have been arranged--we will use the "Modern Periodic Table" which is derived from Mendeleev's version of the organization of the elements.
• Modern Periodic Table is based upon increasing atomic number of the elements and allows for organization of the elements as well as grouping of the elements according to chemical and physical properties
• Groups – are the vertical groupings or columns of elements in the table
• Periods – are the horizontal groupings or rows of elements in the table

• Three major groupings of types of elements:
• Metals -- generally hard, shiny, dense, conductive with a shiny luster
• examples would be copper, lead, sodium
• know which elements on the periodic table are metals -- in the periodic table in the front of your text, metals are in blue
• Non-metals -- do not exhibit the properties of metals, they are generally insulators, brittle, and often gaseous
• examples would be chlorine, neon, sulfur
• know which elements on the periodic table are non-metals -- in your text, non-metals are orange
• Metalloids – have properties in between those of metals and non-metals -- in your text, metalloids are purple
• they are boron, silicon, germanium, arsenic, antimony, tellurium, astatine, know these
• There are assorted named sub-groupings which you should be familiar with:
• Representative elements -- all Group A elements
• Transition elements – all Group B elements
  Group #                     Name               
        IA                            alkali metals
        IIA                           alkaline earth metals
        VA                           nitrogen group
        VIA                          oxygen group or chalcogens
        VIIA                         halogens
        VIIIA                        nobel gases
• two numbers on the periodic table which you must become very familiar with are:
• the atomic number--it is the smaller number and is always a whole number
• the mass number--it is the larger number and is a decimal number

MOLECULES AND IONS:

Compounds and elements are pure substances which are the basic building blocks of all matter The following chart should clarify the relationship between pure substances, elements, compounds, atoms and molecules :

Notice that both elements and compounds are pure substances. Atoms are the smallest particles that can be identified as a particular element, and molecules are the smallest particles that can be identified as a particular compound. Elements can also occur in a molecular form in which the same type of elements (atoms) are chemically combined, such as two oxygen atoms, O, which form molecular oxygen, O₂, when chemically combined. There are some elements, which you should know, that only occur naturally (under normal conditions) in their molecular forms. They are called diatomic molecules or, sometimes, molecular elements and they are: hydrogen, H₂; nitrogen, N₂; oxygen, O₂; fluorine, F₂; chlorine, Cl₂; bromine, Br₂; iodine, I₂.   Compounds are chemical combinations of elements (atoms) of different types, such as water, H₂O or carbon dioxide, CO₂. Let us look further at the formation of compounds, both molecular compounds and ionic compounds.

MOLECULES:

• molecules are formed from chemical combinations of atoms--atoms are combined in specific ratios to one another:
  
  eg. water is H₂O with a ratio of 2 : 1 in H : O
  H₂O₂ is not water -- it is hydrogen peroxide with a ratio of 2 : 2 in H : O
  
• molecular substances are compounds formed between different non-metal elements
• the molecules that make up molecular substances are individual units which act independently but are identical to one another
• ionic substances are formed between metals and non-metals and are quite different from molecular substances--we will discuss them in the next section
• masses of molecules or compounds are simply the summed masses of all atoms or elements which combine to form a molecule or compound and are represented by the chemical formula
  
  eg. alcohol             C₂H₆O                   ether             C₂H₆O
  vitamin C               C₆H₈O₆
  sugar                      C₁₂H₂₂O₁₁
  saccharin                C₇H₅NO₃S
  aspirin                    C₉H₈O₄
  cocaine                   C₁₇H₂₁NO₄
  
• The formulas give the type, ratio and number of atoms in the chemical combination but they say nothing of the actual structure, for example, alcohol and ether are very different although they have the same chemical formula. Their structural formulas (how the atoms hook together), shown below, are different.
  
• The mass of a compound is the sum of the masses of all the atoms that combine to form the compound:
  
  Mass of vitamin C is 176.08 amu                        6 C  =      72.00 amu
                                                                                   8 H  =        8.08 amu
                                                                                   6 O  =       96.00 amu
                                                                             C₆H₈O₆        176.08 amu
  
• A structural formula will show not only type, ratio and number of atoms in the chemical formula, but also which atoms are attached to which atoms

It would be wise, at this point, to obtain a model set of your own. The ability to actually build a 3-dimensional model will enable you to "see" the spatial nature of the compounds that we talk about.

IONS:

• ions are substances that have either a positive or negative charge
• ions come in two flavors
• cations
• have a positive charge of one or greater
• are generally derived from either metal elements or groups of elements from which one or more electrons have been removed
• cations (monotomic) are always smaller that the element from which they are derived
• anions
• have a negative charge of one or greater
• are generally derived from either non-metal elements or groups of elements to which one or more electrons have been added
• anions (monotomic) are always larger than the element from which they are derived

• monotomic ions are derived from single elements
• examples:                 Na         ®        Na⁺     +     1e^-            (e^- is lost)
                                     Ca         ®         Ca²⁺   +      2e^-            (e^- 's are lost)
                                     Cl  +  1e^-       ®      Cl^-                          (e^- is gained)
                                     O   +   2e^-     ®      O^2-                          (e^- 's are gained)
• polyatomic ions are derived from groups of elements which are generally non-metals
• examples:         CO₃^2-
                             PO₄^3-
                             SO₄^2-
                             NH₄⁺
  

• Ionic substances
• are formed between oppositely charged ions--cations and anions
• are held together by ionic bonds which are due to the electrostatic attractions between the opposite charges
• ionic compounds are always neutral species formed by combining the same number of positive and negative charges:
  
  for example:   Mg²⁺ and Cl^- produce MgCl₂ not MgCl or MgCl₃ or any other combination
  

Formulas and Relative Masses:

• Because ions combine to form neutral ionic substances and elements combine for form neutral molecular substances, if we know the relative masses of substances which combine, we can determine their relative combining ratio:

• Now, if we also know the relative atomic masses of the elements, and the masses of the elements that combine, we can calculate the ratio in which they combine:
  
  A compound containing only nitrogen and oxygen is analyzed. A sample weighing 2.20 g is found to contain 1.40 g of nitrogen. What is the formula of the nitrogen oxide?
  
  mass N combined                1.40 g
  mass O combined                0.80 g
  tot. mass of sample              2.20 g
  
  Ratio of N : O is 1.40/0.80 = 1.75 so in this compound N is 1.75 heavier than O
  
  We know that the mass of N is 14.00 amu and the mass of O is 16.00 amu and we also know that in our sample, the ratio of N/O must = 1.75
  
  therefore            2 x mass of N      =      28.00 amu     =     1.75
                                mass of O                  16.00 amu
  
  and the ratio or chemical formula is N₂O

Electrical Properties of Solutions:

When substances dissolve in water, the basic particles from which they are made (molecules in molecular substances and ions in ionic substances) are separated by the water molecules. We will discuss this in much more detail in Chapter 13--Solutions

• Molecular Substances
• The basic or smallest particle of a molecular substance is a molecule, and molecules are always "neutral", hence, when molecular substances dissolve, there are only neutral particles present.
• Because there are no charged particles in solution to be attracted to a charged electrode, molecular substances are non-conductors when dissolved in water
  
• Ionic Substances
• The basic or smallest particle of an ionic substance is an ion, and ions are charged particles, hence, when ionic substances dissolve, there are individual charged particles (both positive and negative) in the solution.
• Because there are both types of charged particles in the solution which can be attracted to either a positive or a negative electrode, ionic substances are conductors when dissolved in water.
• If positive and negative electrodes are attached to a light bulb at one end while the other ends are placed in a solution containing a dissolved ionic substance, the light bulb will light up (this will not be the case with a dissolved molecular substance because the individual particles are neutral)
• this is due to the mobility of the individual positive and negative ions in the solution which are attracted to the electrode of opposite charge and complete the electrical circuit.
  

Sample Problems:

• Fill* in the following chart with the appropriate values:
  
  *answers are in blue

• the atomic number always gives the number of protons
• if the element is neutral (no charge), the number of electrons will equal the number of protons
• if the element is charged the number of electrons and protons will be different
• the numerical value of the charge is the difference between the number of electrons and the number of protons
• the sign of the charge is positive if the number of protons is greater than the number of electrons and negative if the number of electrons is greater than the number of protons
  

• Indicate the correct formula for the ionic substance produced from the following ions:
  
• Mg²⁺   &     O^2-                     MgO
• Na⁺     &      O^2-                     Na₂O
• Al³⁺    &      Cl^-                      AlCl₃
  
• Ca²⁺    &      CO₃^2-                CaCO₃
  
• notice that polyatomic ions such as CO₃^2- are treated as single units although they are formed from multiple elements or atoms
  
• Na⁺    &     CO₃^2-                  Na₂CO₃
  
  
• NH₄⁺   &    O^2-                     (NH₄)₂O
  
• in this case we set the unit -- the NH₄ cation unit -- apart by parentheses and indicate that we require two of these units by the subscript "₂" outside the parentheses

• How many of each type of ion and each type of atom occurs in the smallest identifiable unit of the ionic compound Al₂(CO₃)₃?
  
• there are two aluminum ions (Al³⁺)
  there are three carbonate ions (CO₃^2-)--within each carbonate ion there is one carbon and three oxygens
  
• there are two aluminum atoms
  there are three carbon atoms
  there are nine oxygen atoms
  
  
• What is the charge of each ion occurring in the following ionic substances:
  
• there are some rules which will help you here:
  Group 1A elements (metals) always form +1 cations
  Group 2A elements (metals) always form +2 cations
  Group 6A elements (non-metals) generally form -2 anions
  Group 7A elements (non-metals) generally form -1 anions
  H generally forms a +1 cation
  
• KI                     K = +1   &    I = -1
• K is group 1A and I is group 7A and when they form a 1 : 1 ionic substance, it is neutral
  
• CuO                Cu = +2   &    O = -2
• we don't know the charge of Cu (in fact, Cu can have two different charges +1 & +2!) but we do strongly suspect that O is -2, and if O is -2 then Cu must be +2 in order for the CuO with a 1 : 1 ratio to be neutral
  
• AlPO₄            Al = +3   &     PO₄ = -3
• we know that Al = +3 (Group 3A metal), but we don't know PO₄ = ? PO₄^? must be a polyatomic anion (because this is an ionic substance and Al is the cation so PO₄ must be the anion)--since the ratio of Al : PO₄ is 1 : 1, then PO₄ = -3
  

• Underline all correct isotopes of uranium (atomic #92) in the following list:
  
  ²³⁴U₉₂                 ²³⁸U₉₂            ²³⁸Np₉₃        ²³⁵U₉₂                ²³⁷U₉₀

Below are listed some of the more common monotomic and polyatomic ions.   Know the ion names, symbols and charges--you will need this information in the next chapter.

Remember, each ion (whether monotomic or polyatomic) is considered an individual unit and remains so during chemical reactions (at least at this point on your path toward chemical knowledge!)