Periodic Table - Classification of Elements and Periodic Properties mind map Class 11
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In this Unit, you have studied the development of the Periodic Law and the Periodic Table. Mendeleev’s Periodic Table was based on atomic masses. Modern Periodic Table arranges the elements in the order of their atomic numbers in seven horizontal rows (periods) and eighteen vertical columns. Atomic numbers in a period are consecutive, whereas in a group they increase in a pattern.
The periodic table is a tabular arrangement of elements that organizes them according to their atomic number, electron configuration, and recurring chemical properties. The table is organized into vertical columns (called groups) and horizontal rows (called periods). Each element in the periodic table is represented by its atomic symbol, atomic number, and its average atomic mass.
The periodic table was first proposed by Russian chemist Dmitri Mendeleev in 1869. He organized the 63 known elements of his time into eight groups with similar properties. Later, other scientists added more elements to the table as they were discovered or synthesized in laboratories. Today’s version of the periodic table contains 118 elements arranged into 18 vertical columns and 7 horizontal rows.
The arrangement of the elements in the periodic table provides insight into their chemical behavior. Elements within the same group usually have similar physical and chemical properties due to their shared electron configurations; for example, all alkali metals are highly reactive because they all have one electron in outermost shell.
Elements of the same group have similar valence shell electronic configuration and, therefore, exhibit similar chemical properties. However, the elements of the same period have incrementally increasing number of electrons from left to right, and, therefore, have different valencies.
Four types of elements can be recognized in the periodic table on the basis of their electronic configurations. These are s-block, p-block, d-block and f-block elements. Hydrogen with one electron in the 1s orbital occupies a unique position in the periodic table.
Metals comprise more than seventy eight per cent of the known elements. Nonmetals, which are located at the top of the periodic table, are less than twenty in number. Elements which lie at the border line between metals and non-metals (e.g., Si, Ge, As) are called metalloids or semi-metals. Metallic character increases with increasing atomic number in a group whereas decreases from left to right in a period.
Electronegativity also shows a similar trend. Electron gain enthalpies, in general, become more negative across a period and less negative down a group. There is some periodicity in valence, for example, among representative elements, the valence is either equal to the number of electrons in the outermost orbitals or eight minus this number.
Chemical reactivity is highest at the two extremes of a period and is lowest in the centre. The reactivity on the left extreme of a period is because of the ease of electron loss (or low ionization enthalpy). Highly reactive elements do not occur in nature in free state; they usually occur in the combined form. Oxides formed of the elements on the left are basic and of the elements on the right are acidic in nature. Oxides of elements in the centre are amphoteric or neutral.
The physical and chemical properties of elements vary periodically with their atomic numbers. Periodic trends are observed in atomic sizes, ionization enthalpies, electron gain enthalpies, electronegativity and valence. The atomic radii decrease while going from left to right in a period and increase with atomic number in a group. Ionization enthalpies generally increase across a period and decrease down a group.
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