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Classification of Elements and Periodicity in Properties MCQ Quiz
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Summary
Mendeleev’s initial table was based on atomic masses, but today’s Modern Periodic Table arranges elements by their atomic numbers, into seven rows (periods) and eighteen columns (groups or families). Elements within a period have consecutive atomic numbers, while those within a group follow a pattern of increasing atomic numbers. Elements in the same group share similar valence shell electronic configurations, resulting in comparable chemical properties. Conversely, elements in the same period possess incrementally increasing numbers of electrons from left to right, leading to differing valencies.
The Periodic Table classifies elements into four main types based on their electron configurations: s-block, p-block, d-block, and f-block elements. Hydrogen, with a single electron in its 1s orbital, occupies a unique position. The majority of known elements are metals, constituting over seventy-eight percent. Non-metals, located at the table’s upper reaches, number fewer than twenty. Elements like Silicon (Si), Germanium (Ge), and Arsenic (As), situated on the border between metals and non-metals, are termed metalloids or semi-metals. Metallic character generally increases within a group with increasing atomic number but decreases from left to right across a period.
The properties of elements vary periodically with their atomic numbers, influencing atomic sizes, ionization enthalpies, electron gain enthalpies, electronegativity, and valence. Atomic radii decrease from left to right across a period and increase down a group. Ionization enthalpies generally rise across a period and decrease down a group, mirroring trends in electronegativity. Electron gain enthalpies generally become more negative across a period and less negative down a group. Valence patterns also exhibit periodicity; for example, representative elements typically exhibit valences equal to the number of electrons in their outermost orbitals or eight minus that number.
Chemical reactivity varies across periods, with highest reactivity observed at the extremes and lowest at the center. Elements on the left extreme of a period exhibit high reactivity due to ease of electron loss (low ionization enthalpy). Highly reactive elements are rarely found in a free state in nature and usually occur in combination with other elements. Oxides of left-hand elements tend to be basic, while those of right-hand elements are acidic. Central element oxides are typically amphoteric or neutral in nature.
This overview underscores the significance of periodic trends in understanding the behavior and properties of elements, guiding their classification and practical applications in chemistry.
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