The Periodic Table

The how and why…The how and why…

Early in the 19th century, scientists began to seek ways of classifying elements.

1817, Dobereiner found the properties of metals calcium, barium and

strontium were very similar

he noted the atomic mass of strontium was about midway between those of

calcium and barium he grouped these three elements together and

called them a triad later he found other groups with similar

properties

Examples of Dobereiner’s Triads

Triad 1 Triad 1 Triad 1 Name At. Mass Name At. Mass Name At. Mass 1st Elem’t Calcium 40 Chlorine 35.5 Sulfur 32 3rd Elem’t Barium 137 Iodine 127 Tellurium

127.5 AVE. AVE. AVE. 2nd Elem’t Strontium 87.6 Bromine 79.9 Selenium 79.2

1863- John Newlands arranged elements in order of

increasing atomic mass noted their appeared a repetition of

similar properties ever eighth element arranged the 49 elements known into

seven groups of seven each

Called LAW OF OCTAVES

Newland’s Law of Newland’s Law of OctavesOctaves1 2 3 4 5 6 7 1 2 3 4 5 6 7

Li Be B C N O FLi Be B C N O FNa Mg Al Si P S ClNa Mg Al Si P S ClKK

Just 6 years later, 1869

Demitri Mendeleev proposed a similar idea

Felt similar properties occurred after periods(horizontal rows) of varying length.

placed seven elements in 1st two periods, seventeen elements in the next two

Mendeleev and a German chemist Lothar Meyer each working alone-

made an eight column table of the elements

However, Mendeleev had to leave some blank spots in order

to group all the elements with similar properties in the same column----

to explain these spots, he suggested there must be other elements which had not yet been discovered

he predicted properties and atomic masses of these unknown elements

TODAY- the other elements have been discovered

Mendeleev’s predictionsMendeleev’s predictions

Ekasilicon GermaniumEkasilicon Germanium

Predicted properties Actual PropertiesPredicted properties Actual Properties

Atomic mass= 72 Atomic mass= 72.6Atomic mass= 72 Atomic mass= 72.6High Melting point Melting pt. 958High Melting point Melting pt. 958 oo CCDensity = 5.5 g/cmDensity = 5.5 g/cm33 Density= 5.36 g/cm Density= 5.36 g/cm33

Dark gray metal Gray metalDark gray metal Gray metalSlightly dissolved in HCl Not dissolved in HClSlightly dissolved in HCl Not dissolved in HClWill form oxide EsOWill form oxide EsO22 Forms oxide GeO Forms oxide GeO22

Mendeleev arranged elements in order of increasing atomic masses

He showed the properties of the elements repeated as a function of their atomic

masses.

CALLED PERIODIC LAW Mendeleev called

FATHER OF THE PERIODIC TABLE

MENDELEEV KNEW HIS TABLE HAD PROBLEMS-

Tellurium and iodine seemed to be in the wrong columns

SWITCHING THEIR POSITIONS WOULD PLACE THEM IN THE RIGHT

COLUMNS BUT HE DIDN’T KNOW WHY...

Cobalt and Nickel also need to be reversed

Henry Mosely- found a reason In 1913- his X-ray experiments showed

the nucleus had a integral + charge, the ATOMIC NUMBER

he arranged the periodic table by ATOMIC NUMBER and everything fell into the right column

The Modern Statement of the Periodic Law is

the properties of the elements are a periodic function of their

atomic numbers.

THE MODERN TABLE Elements are still grouped by

properties. Similar properties are in the same

column. Order is in increasing atomic number. Added a column of elements Mendeleev

didn’t know about. The noble gases weren’t found because

they didn’t react with anything.

Horizontal rows are called periods There are 7 periods

Vertical columns are called groups.

Elements are placed in columns by similar properties.

Also called families

1A

2A 3A 4A 5A 6A7A

8A0

The elements in the A groups are called the representative elements

The group B are called the transition elements

These are called the inner transition elements and they belong here

Group 1A are the alkali metals Group 2A are the alkaline earth metals

Group 7A is called the Halogens Group 8A are the noble gases

Why? The part of the atom another atom

sees is the electron cloud. More importantly the outside

orbitals. The orbitals fill up in a regular

pattern. The outside orbital electron

configuration repeats. The properties of atoms repeat.

1s1

1s22s1

1s22s22p63s1

1s22s22p63s23p64s1

1s22s22p63s23p64s23d104p65s1

1s22s22p63s23p64s23d104p65s24d10 5p66s1

1s22s22p63s23p64s23d104p65s24d105p66s2

4f145d106p67s1

H1

Li3

Na11

K19

Rb37

Cs55

Fr87

He2

Ne10

Ar18

Kr36

Xe54

Rn86

1s2

1s22s22p6

1s22s22p63s23p6

1s22s22p63s23p64s23d104p6

1s22s22p63s23p64s23d104p65s24d105p6

1s22s22p63s23p64s23d104p65s24d10

5p66s24f145d106p6

Alkali metals all end in s1

Alkaline earth metals all end in s2

really have to include He but it fits better later.

He has the properties of the noble gases.

s2s1 S- block

Transition Metals -d block

d1 d2 d3s1

d5 d5 d6 d7 d8s1

d10 d10

The P-block p1 p2 p3 p4 p5 p6

F - block inner transition elements

f1 f5f2 f3 f4

f6 f7 f8 f9 f10 f11 f12 f14

f13

Each row (or period) is the energy level for s and p orbitals.

1

2

3

4

5

6

7

D orbitals fill up after previous energy level so first d is 3d even though it’s in row 4.

1

2

3

4

5

6

7

3d

f orbitals start filling at 4f

Ne is 1s22s22p6

so Al is [Ne] 3s23p1

1

2

3

4

5

6

7 4f

5f