Early Models of Periodic Table
Dobereiner arranged a group of three elements with similar properties in the order of increasing atomic masses and called it a triad. He showed that the atomic mass of the middle element is approximately the arithmetic mean of the other two. But, Dobereiner could identify only following three triads from the elements known at that time.
Newlands’ Law of Octaves
|sa (do)||re (re)||ga (mi)||ma (fa)||pa (so)||da (la)||ni (ti)|
|Co and Ni||Cu||Zn||Y||In||As||Se|
|Br||Rb||Sr||Ce and La||Zr||______||_____|
When the elements are arranged according to increasing atomic masses, the physical and chemical properties of every eighth element are similar to that of the first.
Newlands compared these octaves to the series of eight notes of a musical scale.
Assumptions and Limitations:
1. The law was applicable for elements with atomic masses up to 40.
2. Properties of new elements discovered did not fit into the law of octaves.
3. In a few cases, Newlands placed two elements in the same slot to fit elements in the table.
4. He also grouped unlike elements under the same slot.
Mendeleev’s Periodic Table and Law
The physical and chemical properties of elements are periodic functions of their atomic weights.
Features of Mendeleev’s Periodic Table
● Twelve horizontal rows, which were condensed to 7, known as periods.
● Eight vertical columns known as groups.
● Groups I to VII subdivided into A and B subgroups.
● Groups VIII doesn’t have any subgroups and contains three elements in each row.
● Elements in the same group exhibit similar properties.
Achievements of Mendeleev’s Periodic Table
1. A systematic study of elements: Elements with similar properties were grouped together, that made the study of their chemical and physical properties easier.
2. Correction of atomic masses: Placement of elements in Mendeleev’s periodic table helped in correcting the atomic masses of certain elements. For example, the atomic mass of beryllium was corrected from 13.5 to 9. Similarly, atomic masses of indium, gold, platinum etc., were also corrected.
3. Prediction of properties of yet to be discovered elements: Eka-boron, eka-aluminium and eka-silicon were the names given to yet to be discovered elements. The properties of these elements could be predicted accurately from the elements that belonged to the same group. These elements, when discovered were named scandium, gallium, and germanium respectively.
4. Placement of noble gases: When discovered, they were placed easily in a new group called zero group of Mendeleev’s table, without disturbing the existing order.
Limitations of Mendeleev’s Periodic Table
1. Position of hydrogen: Hydrogen resembles both, the alkali metals (IA) and the halogens (VIIA) in properties, so, Mendeleev could not justify its position.
2. Position of isotopes: Atomic weight of isotopes differ, but, they were not placed in different positions in Mendeleev’s periodic table.
3. Anomalous pairs of elements: Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.
4. Placement of like elements in different groups: Platinum (Pt) and Gold (Au) has similar properties but were placed in different groups.
5. Cause of periodicity: He could not explain the cause of periodicity among the elements.
Modern Periodic Table
Modern Periodic Law
The physical and chemical properties of elements are the periodic function of their atomic numbers.
Cause of periodicity – It is due to the repetition of same outer shell electronic configuration at a certain regular interval.
Periods in Modern Periodic Table
Elements present in the same period have the same number of shells which is equal to the period number.
On moving from left to right in a given period, the number of electrons in the valence shell increases from one to eight while the number of shells remains the same.
Number of Elements in a Period
The first period contains only two elements 1Hand2He and is known as the shortest period.
The second period (3Lito10Ne) and the third period (11Na to 18Ar) contain 8 elements each and are known as short periods.
The fourth period (19K to 36Kr) and the fifth period (37Rb to 54Xe) contain 18 elements each and are called long periods.
The sixth period contains 32 elements (55Cs and 86Rn) and is also known as the longest period.
The seventh period is an incomplete period.
(After the recent discoveries of the new elements and their addition to the periodic table, the seventh period is officially complete)
Groups in Modern Periodic Table
The modern periodic table contains 18 vertical columns known as groups.
Group 1 elements are known as alkali metals.
Group 2 elements are known as alkaline earth metals.
Group 15 elements are known as pnicogens.
Group 16 elements are known as chalcogens.
Group 17 elements are known as halogens.
Group 18 elements are known as noble gases.
The elements in the first group, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) are called alkali metals.
They were given the name because they all react with water to form alkalis.
The alkali metals are all shiny, soft, highly reactive solids at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1.
Number of valence electrons = 1
Alkali Earth Metals
Classification of Modern Periodic Table
Trends in the Modern Periodic Table
Trends in Modern Periodic Table
Variation of Valency
Variation of Atomic Size
Atomic size or radii: It is defined as the distance from the centre o the nucleus to the valence shell of the atom.
Along the period – Atomic radius decreases because effective nuclear charge increases by one unit and it pulls valence electrons or the electron cloud closer to the nucleus.
Down the group – Atomic radius increases because new shells are added, hence, the distance between the nucleus and valence electrons or the electron cloud increases.
Variation of Metallic Properties
Along the period – Metallic character decreases because the tendency to lose valence electrons decreases due to increasing nuclear charge.
Down the group- As the distance between the nucleus and outermost electron increases, nuclear pull decreases. This increases the tendency of an atom to lose valence electron/s, hence metallic character increases.
Variation of Nonmetallic Properties
Along the period – Non-metallic character increases as the tendency to gain electrons in the valence shell increases due to increasing nuclear charge.
Down the group – As the distance between the nucleus and valence shell increases, nuclear pull decreases. This decreases the tendency of an atom to gain an electron its valence shell, hence non-metallic character decreases.
Variation of Electronegativity
Along the period – Electronegativity increases as the tendency to gain electrons in the valence shell increases due to increasing nuclear charge.
Down the group – As the distance between the nucleus and valence shell increases, nuclear pull decreases. This decreases the tendency of an atom to gain an electron, hence electronegativity decreases.
Introduction to Periodic Classification of Elements
Without the classification of elements, it would be extremely difficult and time-consuming to individually study the chemistry of all the elements. Hence, to simplify and systematize the study of elements and their compounds, they are classified into groups and periods.