Periodic Table

             In the 1860’s several scientist realized that by listing the known elements in order of increasing atomic weights, similar elements with similar properties (i.e., melting points, boiling points, density, and chemical activity) appeared at fairly regular intervals.

             Dimitri Mendeleev was one of the first to publish a table using these properties to arrange the elements.  He also placed similar elements in the same vertical column.  He left open spaces on his chart for elements he predicted would someday be found but had not been discovered at that time.

             He developed the idea of the Periodic Law: “When elements are arranged in order of increasing atomic weight their properties are repeated periodically.”  This idea of repetition of properties is one of the most important milestones in chemistry.

             Today we know that atomic number is a better guide to correlating properties.  Mendeleev had suspected atomic weights were not the best criteria to use.  These early researchers did not know about protons and neutrons and isotopes. 

Groups and Periods on the Periodic Table

Group 1 – Alkali metals

-         are very good conductors of heat and electricity, are solid at room temperatures, are so soft they can be cut with a knife, have low densities and low melting points

-         most chemically active of the metals (must be stored under oil as they will react with air, give up one electron to reach a noble gas configuration, are often identified with a flame test

-         are used to produce chemicals, metals, soap, glass, ceramics, petroleum products

 Group 2 – Alkaline Earth metals  -  are very chemically reactive (give up 2 electrons during reactions) and are never found free in nature (same as alkali metals)

 Group 17 – Halogen family  -  named from “salt former”, these elements exhibit all 3 physical states at room temp.

Group 18 -  Noble gases  -  all are gases which are naturally occurring

 Transitition metals  -  have properties of metals (ductile, malleable, good conductors of heat and electricity)

 Lanthanides  -    many form alloys with magnetic properties

 Actinides  -    all are radioactive

Elements on the Periodic Table

 8 of the elements known make up 98.5 % of the earth’s crust by weight.

 Oxygen = 46.6 %, silicon = 27.7 %, and aluminum = 8 % with Fe, Ca, Na, K, and Mg making up the rest.

_ 75 of these natural elements are solid, 11 are gases (O, Cl, F, N, H, He, Ne, Ar, Kr, Xe, Rn) and 2 are liquids at room temperature (Hg and Br)

 Four general classes of elements:

a)  metals        - usually lose electrons in forming compounds

b)      nonmetals - usually gain electrons in forming compounds

c)      metalloids – sometimes act like metals and sometimes like nonmetals (ex. C, H, Si)

d)      noble gases – very unreactive, usually neither gain nor lose electrons

Physical properties of metals:

a)      luster – metals are shiny because they are able to reflect light (ex. Polished aluminum reflectors in headlights)

b)      conduction of heat – metals are good conductors of heat due to the freedom with which the electrons flow through the metal (ex. Cooking utensils are made of iron, copper, or aluminum because they spread heat throughout the material evenly and quickly)

c)      conduction of electricity – silver, copper, and gold are good conductors

d)      ductility – metals have the ability to be drawn into thin wires (ex. Wire can be made by drawing heated metals rods through a tube that is wide at one end and narrow at the other)

e)      malleability – property that allows metals to be flattened into thin sheets without shattering or crumbling (ex. Metals like aluminum can be flattened by being passed between rollers)

METALS                                                                                NONMETALS

Good conductors of heat and electricity…………………….poor conductors of heat and electricity

Shiny in appearance (have luster)…………………………...dull in appearance (lack luster-exceptions: iodine and 
                                                                                                       carbon graphite are crystalline and have a sheen)

Malleable and ductile………………………………………..shatter or crumble when struck

High tensile strength..................................................................cannot withstand stress

High melting and boiling points................................................low melting and boiling points

High densities............................................................................low densities

Sonorous (give a note when struck such as the
sound of a blacksmith hammering iron)..................................not sonorous

All solids except Hg…………………………………………exist in all three physical states

 Extension notes:

a)      in chemical reactions, metal atoms tend t donate electrons to nonmetals that accept them (nonmetals can also share electrons with other nonmetals)

b)      metalloids act as electron donors with nonmetals and act as electron acceptors with metals – are solid at room temperatures, are brittle, and are poor conductors of heat and electricity

c)      hydrogen is sometimes found in groups 1 and 17 because it can sometimes donate electrons and sometimes accept them.  Due to its physical characteristics and the type compounds it forms it is usually classed as a nonmetal.

Trends on the Periodic Table

 _ atom’s size decreases from left to right in a given period

            Atoms get smaller as you go from left to right in a period.  The electron cloud is pulled inward due to an increase in the nuclear charge (the attractive force of the positively charged nucleus for the negatively charged electron cloud.) The more protons, the stronger the nuclear charge.

            As you go left to right the added electrons only fill up a shell that already exist and are thus does not the size of the atom.

 _ atom’s size increases from top to bottom in groups

            As you go down a group new main shells are added each period with each new main shells larger than the one it surrounds. 

            Outer shell electrons are shielded from the nuclear charge by interior shells.  This shielding effect results in a larger cloud.

 _ as you go down a group the tendency to lose electrons increases

            The larger atoms (at bottom of group) have electrons so far from the nucleus (added main shells and shielding effects) that these electrons are more easily ionized.  The nuclear charge holding them is weak.

 _ ionization energies increase across periods and decrease down groups

            The first ionization energy is the amount of energy needed to remove the most loosely held electron (and thus forming a positive ion).  Metals have low ionization energies and readily form positive ions.  Nonmetals (due to small size, stronger nuclear charge, and many electrons in outer shells) have large ionization energies.

   _ electronegativity increases from left to right in a period and decreases from top to bottom

            Electronegativity is a number showing the ability an atom has to attract electrons (usually shared pairs of electrons) to itself when bound with another atom.

            The most active metals (lower left on chart) have lowest electronegativities (they tend to lose electrons).  Atoms of high electronegativity (upper right nonmetals) tend to gain electrons.

 _ negative ions are larger than their atoms and positive ions are smaller than their atoms

            Nonmetals gain one or more electrons to fill out their outer shell.  The cloud tends to swell outward as the nuclear charge holding each electron is now less. Negative ions are larger.

            Positive ions (formed usually from metals) grow smaller as they lose their outer shell of electrons. The effective nuclear charge is now stronger on the remaining electrons and they are pulled inward.

Oxidation States

             Chemist have found that certain elements are more stable, or less reactive than others.  These stable atoms have their outtermost shell (s and p subshell) filled to capacity with 8 electrons (2 electrons for hydrogen and helium).  Noble gases show little tendency to form compounds as they already have a filled outer shell. 

            When 2 or more atoms combine, each atom tends to get a complete outermost shell holding 8 electrons.  This outermost s and p shell is called the valence shell.  The Octet Rule or Rule of 8 maintains that atoms try to fill this outer valence shell by losing, gaining, or sharing electrons during reactions.

            The oxidation state is used to help identify how many electrons will be transferred or shared.  A positive oxidation state indicates that the atom will lose electrons.  A negative oxidation state indicates that the atom will gain or share electrons.

    back to top of page