Chapter-5
Periodic Classification of Elements
• Matter around us is present in the form of elements, compounds
and mixture.
• Elements are substances containing atoms of only one type eg.,
Na, Mg, Au, etc.
• There are 118 elements known to us. All these have different
properites.
Need for Periodic Classification :
• To make the study of these elements easy, these elements have been
divided into few groups in such a way that elements in the same group have similar properites. Now study of a
large number of elements is reduced to a few group of elements.
• Dobereiner's Traids : When elements were arranged in the
order of increasing atomic masses, groups of three elements (known as traids), having similar chemical
properites are obtaine.
The atomic mass of the middle elements of the triad was roughly the average of the atomic massess of the other
two elements.
| Elements | Atomic Mass |
|---|---|
| Ca | 40.1 |
| Sr | 87.6 |
| Ba | 137.3 |
• Limitations : Only three traids were recognized from the elements known at that time.
| Li | Ca | Cl |
| Na | Sr | Br |
| K | Ba | I |
• Newland's Law of Octaves :
Newland arranged the then known elements in the order of increasing atomic masses and found that the
properties of every 8th element is similar to that of the 1st elements.
He compared this to teh octaves found in music and called it the 'Law' of Octaves.
For example, the properites of lithium (Li) and sodium (Na) were found to be the same.
Newland's Octave
| Sa | Re | Ga | Ma | Pa | Dha | Ni |
|---|---|---|---|---|---|---|
| H | Li | Be | B | C | N | O |
| F | Na | Mg | Al | Si | P | S |
| Cl | K | Ca | Cr | Ti | Mn | Fe |
| Co and Ni | Cu | Zn | Y | In | As | Se |
| Br | Rb | Sr | Ce and La | Zr | - | - |
Limitations :
• It was applicable upto calcium (for lightere elements only).
• Properties of new discovered elements did not fit into the law
of octave.
• To fit elements into his table, Newlands put even two elements
together in one slot and that too in the column of unlike elements having very different properites.
Mendeleev's Periodic Table : When elements are arranged in the order of increasing atomic masses, the
element with similar properites occur at regular intervals. The properites of elements are a periodic
function of their atomic masses.
Meddeleev's periodic table in based on the chemical properites of elements. It contains 6 periods
(horizontal rows) and 8 groups (vertical columns).
Table. Mendeleev's Periodic Table
| Group | I | II | III | IV | V | VI | VII | VIII |
|---|---|---|---|---|---|---|---|---|
| Oxide Hydride |
R2O RH |
RO RH2 |
R2O3 RH4 |
RO2 RH4 |
R2O5 RH3 |
RO3 RH2 |
R2O7 RH |
RO4 |
| Periods x |
A B | A B | A B | A B | A B | A B | A B | Transition series |
| 1 | H 1,008 |
|||||||
| 2 | Li 6.939 |
Be 9.012 |
B 10.81 |
C 12.011 |
N 14.007 |
O 15.999 |
F 18.998 |
|
| 3 | Na 22.99 |
Mg 24.31 |
Al 29.88 |
St 28.09 |
P 30.974 |
S 32.06 |
Cl 35.453 |
|
| 4. First series : Second series : |
K 39.102 Cu 63.54 |
Ca 40.08 Zn 65.37 |
Sc 44.96 Ga 69.72 |
Ti 47.90 Ge 72.59 |
V 50.94 As 74.92 |
Cr 50.20 Se 78.96 |
Mn 54.94 Br 79.99 |
Fe Ce Ni 55.85 58.93 58.71 |
| 5. First series : Second series : |
Rb 85.47 Ag 107.87 |
Sr 87.62 Cd 112.40 |
Y 88.91 In 114.82 |
Zr 91.22 Sn 118.69 |
Nb 92.91 Sb 121.75 |
Mo 95.94 Te 127.60 |
Tc 99 I 126.60 |
Ru Rb Pd 101.07 102.91 106.4 |
| 6. First series : Second series : |
Ca 132.90 Au 196.97 |
Ba 137.34 Hg 200.97 |
La 138.91 Tl 204.37 |
Hr 178.49 Pb 207.19 |
Tn 108.95 Bi 208.98 |
W 183.85 |
Os Ir Pt 190.2 192.2 195.09 |
Modern Periodic Table
• Atomic number of an element is a more fundametal property than its
atomic mass.
• According to the Modern Periodic law : The properites of elements
are a periodic function of their atomic number.
• All the anomalies of Mendeleev's Classification disappear.
Modern Periodic Table
Merits of Mendeleev's Periodic Table
(i) Some gaps were left for the undiscovered elements like gallium (Ga), Scandium (Sc) and Germanium (Ge).
(ii) Predict properites of elements on the basis of their positions in the periodic table.
(iii) Accommodate noble gases when they were discovered without disturbing the original arrangement.
Limitations of Mendeleev's Classification
(i) Position of isotopes could not be explained.
(ii) No fixed position for hydrogen.
(iii) Wrong order of atomic masses of some elements could not be explained.
• Atomic Number : It is denoted by Z and equal to the number
of protons in the nucleus of an atom.
• Modern Periodic table has 18 vertical columns known as 'groups' and
7 horizontal rows known as 'periods'.
• Elements with same number of valence electrons are placed in the
same group. For example,
Li : 2, 1 Na : 2, 8, 1 K : 2, 8, 8, 1
Outermost or valence shell in the the three contains 1 electrons. These elements been placed in the same group.
• Number of shells increases as we go down the group.
• Elements with same number of occupied shells are placed in same
period. For example, Li (2, 1); Be (2, 2); B (2, 2), C (2, 4), N
(2 , 5). These elements have same number of shells (two).
• Each period marks a new electronic shell getting filled.
• Number of elements placed in a particular period depends upon the
fact that how electrons are filled into various shell.
• Maximum number of electrons that can be filled in a shell is given
by 2n2 where n is shell number.
-
E.g., K shell n = 1 or 2n2 = 2(1)2 = 2 First period has 2 elements.
L shell n = 2 or 2n2 = 2(2)2 = 8 Second period has 8 elements.
• Position of an elements in the periodic table tell us its chemical
reactivity.
• Valence electrons determine the kind and number of bonds formed by
the elements.
Trends in the Modern Periodic Table :
Valecncy is the combining power if an element with other atoms when it forms a chemical compound.
OR
Valecncy is equal to number of electrons gained or lost or shared to complete etc octet or doublet.
On moving from left to right in each period, the valencency of elements increases from 1 to 4 and then decreases to 0.
| Third period elements | Na | Mg | Al | Si | P | S | Cl | Ar |
| Valecncy | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
Valecncy remains the same down in a group.
Atom size : Atomi size refers to the radius of an atom. It may be visualized as the distance between the centre of the nucleus and the outermost shell.
• Atomic size or radius of an atom decreases as we move from left to right in a period because due to large +ve charge on the nucleus, the electrons are pulled in more close to hte nucleus and size decreases. E.g.,
| Third period elements | Na | Mg | Al | Si | P | S | Cl |
| Atomic radii (Pm) | 186 | 160 | 143 | 118 | 110 | 104 | 99 |
• Atomic size increases as we move down the group because new shells are being added and ths increases the distance between nucleus and outermost electron.
| Group I | Lithium | Li | 152 | | | Atomic radii (pm) |
| Sodium | Na | 186 | | | ||
| Potassium | K | 231 | | | ||
| Rubidium | Rb | 244 | | | ||
| Casium | Cs | 262 | ↓ | ||
| Francium | Fr | 270 |
Metallic Character :
• Metallic character means the tendency of an atom to lose electron.
• Metals occupy the left hand side of the periodic table.
• On moving left to right in a period, the metallic character of an element decreases because the effective nuclear charge increases. It means tendency to lose electron decreases.
• Metals are electoropsitive as they tend to lose electrons while forming bonds.
• Metallic character increases as we go down a group as the effective nuclear charge is decreasing.
Non-Metallic Character :
• Non-metals are electronegative as they tend to form bonds by gaining electrons.
• Non-metals occupies the right side of the periodic table.
• Non-metallic character decreases as we move down a group due to decreases in effective nuclear charge experienced by the valence electron thus the tendency to gain electron decreases.
• In the middle of periodic table we have semi-metals or metalloid because. they exhibit some proiodic of metals and non-metals.
• Oxides of metals are basic in nautre while oxides of non-metals are acidic in nature.
| S.No. | Property | Variation across Periods | Reason | Variation along Groups | Reason |
|---|---|---|---|---|---|
| 1. | Atomic Size | Decreases | Due to increases in nuclear charge, or resulting in stronger force of attraction which causes shrinking. | Increases | Due to addition of new shells, the distance between outermost electron and nucleus increases. |
| 2. | Metallic character | Decreases | Due to increases in effective nuclear charge, tendency to lose valence electrons decreases. | Increases | Decreases in effective nuclear charge experienced by valence electrons |
| 3. | Non-metallic character | Increases | Due to increases in effective nuclear charge, tendency to gain electrons increases. | Decreases | Due to decreases in effective nuclear charge experienced by valence electrons (due to addition of the new shells tendency to gain electorns decreases. |
