The Periodic Table 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  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 Octaves Li Be B C N O F Na Mg Al Si P S Cl K

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 predictions Ekasilicon Germanium Predicted properties Actual Properties Atomic mass= 72 Atomic mass= 72.6 High Melting point Melting pt. 958 o C Density = 5.5 g/cm 3 Density= 5.36 g/cm 3 Dark gray metal Gray metal Slightly dissolved in HCl Not dissolved in HCl Will form oxide EsO 2 Forms oxide GeO 2

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 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 u Elements are still grouped by properties. u Similar properties are in the same column. u Order is in increasing atomic number. u Added a column of elements Mendeleev didn’t know about. u The noble gases weren’t found because they didn’t react with anything.

u Horizontal rows are called periods u There are 7 periods

u Vertical columns are called groups. u Elements are placed in columns by similar properties. u Also called families

1A 2A3A4A5A6A 7A 8A 0 u The elements in the A groups are called the representative elements

The group B are called the transition elements u These are called the inner transition elements and they belong here

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

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

Why? u The part of the atom another atom sees is the electron cloud. u More importantly the outside orbitals. u The orbitals fill up in a regular pattern. u The outside orbital electron configuration repeats. u The properties of atoms repeat.

1s11s1 1s 2 2s 1 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6 7s 1 H 1 Li 3 Na 11 K 19 Rb 37 Cs 55 Fr 87

He 2 Ne 10 Ar 18 Kr 36 Xe 54 Rn 86 1s21s2 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6

u Alkali metals all end in s 1 u Alkaline earth metals all end in s 2 u really have to include He but it fits better later. u He has the properties of the noble gases. s2s2 s1s1 S- block

Transition Metals -d block d1d1 d2d2 d3d3 s1d5s1d5 d5d5 d6d6 d7d7 d8d8 s 1 d 10 d 10

The P-block p1p1 p2p2 p3p3 p4p4 p5p5 p6p6

F - block u inner transition elements

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

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

f orbitals start filling at 4f Ne is 1s 2 2s 2 2p 6 so Al is [Ne] 3s 2 3p f 5f