Ch. 6 The Periodic Table

Mendeleev’s Periodic Table

• Elements in the

periodic table are arranged

by repeating properties• Arranged in order by

increasing atomic mass• Left spaces where he

predicted future elements

would fit

Mendeleev’s Periodic Table

However…..

• Mendeleev ordered his elements in order of their relative atomic mass, and this gave him some problems. For example, iodine (126.90) has a lower relative atomic mass than tellurium (127.60), so it should come before tellurium in Mendeleev's table - but in order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, thereby breaking his own rules

Moseley

• Soon after Rutherford's landmark experiment of discovering the proton in 1911, Henry Moseley (1887-1915) subjected known elements to x-rays. He was able to derive the relationship between x-ray frequency and number of protons. When Moseley arranged the elements according to increasing atomic numbers and not atomic masses, some of the inconsistencies associated with Mendeleev's table were eliminated. The modern periodic table is based on Moseley's Periodic Law (atomic numbers).

At age 28, Moseley was killed in action during World War I and as a direct result Britain adopted the policy of exempting scientists from fighting in wars.

Periodic Law

• In the modern periodic table, elements are arranged in order by increasing Atomic Number

• 7 horizontal rows called periods• Each period corresponds to a principle

energy level• Elements within a group (column) have

similar properties

Periodic Law

• States: When elements are arranged by increasing atomic number, there is a periodic repetition of their physical and chemical properties

Metals, Nonmetals, Metalloids

Metals

• 80% of the periodic table is metals

• Good conductors of heat and electricity

• Malleable- can be shaped• Ductile- can be drawn

into wires• Lustrous• Solids at room

temperature (except Hg)• Many react with HCl and

CuCl2

ZINC

MAGNESIUM

Nonmetals

• Poor conductors of heat and electricity• Not malleable• Not ductile• Dull

Metalloids

• Generally has some properties of a metal, some of a nonmetal

Alkali Metals

Lithium (Li) 1s22s1

Sodium (Na) 1s22s22p63s1

Potassium (K) 1s22s22p63s23p64s1

Alkaline Earth Metals

Beryllium (Be) 1s22s2

Magnesium (Mg)1s22s22p63s2

Calcium (Ca) 1s22s22p63s23p64s2

Halogens

Fluorine (F) 1s22s22p5

Chlorine (Cl)

1s22s22p63s23p5

Bromine (Br)

1s22s22p63s23p64s23d104p5

Noble Gasses

Neon (N)

1s22s22p6

Argon (Ar)

1s22s22p63s23p6

Krypton (Kr)

1s22s22p63s23p64s23d104p6

Trends in Atomic Size

• Atomic radius is ½ the distance between the nuclei of 2 like atoms

• As you move DOWN a group, atomic size increases

• As you move LEFT to RIGHT across a period size decreases

Why?

As you move across the period you gain electrons but you also gain protons. More + protons hold their electrons tighter

As you move down the group you gain more electrons which are in electron orbitals further away from the nucleus. The nucleus has less influence the further out you move.

Trends in Ionization Energy

• Ionization Energy is the energy required to remove 1 electron from a gaseous atom

• As you move DOWN a group ionization energy DECREASES

• As you move LEFT to RIGHT across a period ionization energy INCREASES

Why?

• As you move down a group you add more energy levels. The valence electrons are now further from the positive nucleus and it is easier to “lose” them.

• As you move across a period, you add more protons in the nucleus, but not more energy levels. There is less “electron shielding” between the positive nucleus and electrons…the positive nucleus pulls the electrons closer into it.

Why?

Trends in Electronegativity

• Electronegativity is the tendency of an atom to attract electrons in a compound

• As you move DOWN a group Electronegativity DECREASES

• As you move LEFT TO RIGHT across a period Electronegativity INCREASES

Why?

• Do any of the atoms in group 1A want to gain electrons????

• NO!...They are looking to give them away…please take them..I won’t fight you for them!

• Just ask yourself….

who really, really wants to gain electrons?

Fluorine….it is the T.Rex of the periodic table.---Small radius and just needs one more electron!

Fluorine

Fluorine

Atomic Radius

The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value.

We will use the following definition: atomic radius is the average distance from the center of the nucleus to the outer orbital.

Trends in Atomic Ion Radius

For METALS: as you lose electrons there is less electron-electron repulsion (remember like charges repel) and the radius gets smaller. You “lose” all the electrons in the outer energy level the resulting ion is MUCH smaller.

For NONMETALS: as you add electrons, you increase the electron-electron repulsion (they want to get far away from one another) and the radius “fills out” thus getting larger.

Summary of Periodic Trends