Periodic Classification of Elements
As known elements increased from 31 in 1800 to 63 in 1865 and then 118, scientists needed a logical property-based arrangement.
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Why Elements Need Periodic Classification
As known elements increased from 31 in 1800 to 63 in 1865 and then 118, scientists needed a logical property-based arrangement.
Open article →Dobereiner's Triads and Atomic-Mass Mean
Dobereiner's 1817 triads grouped three elements by mass, with the middle mass nearly equal to the mean of the other two.
Open article →Newlands' Law of Octaves
In 1866 Newlands arranged 56 elements by increasing atomic mass and compared every eighth element's recurrence to a musical octave.
Open article →Mendeleev's Periodic Table: Law, Prediction and Limits
Mendeleev's 1869 mass-based table grouped recurring properties, corrected masses and predicted unknown elements through gaps.
Open article →Moseley and the Modern Periodic Law
Moseley's work established atomic number as the sound basis for arranging elements and expressing periodic recurrence.
Open article →Seven Periods: Atomic-Number Ranges and Capacities
The seven periods contain 2, 8, 8, 18, 18, 32 and 32 places across atomic-number spans 1–118.
Open article →Eighteen Groups and Periodic Families
The 18 vertical groups form recurring families from alkali metals and alkaline earth metals to halogens and noble gases.
Open article →s, p, d and f Blocks of the Periodic Table
Subshell filling divides the table into s, p, d and f blocks corresponding to representative, transition and inner-transition elements.
Open article →Special Positions of Hydrogen and Noble Gases
Hydrogen resembles Groups 1 and 17, while filled-shell, monoatomic noble gases occupy stable Group 18 positions.
Open article →Periodicity and Atomic Radius Trends
Atomic radius uses the outer shell or half an internuclear distance and generally decreases across periods and increases down groups.
Open article →Ionic Radius, Charge and Periodic Trend
Cations are smaller and anions larger than their neutral atoms; ionic radii generally decrease across periods and increase down groups.
Open article →Ionisation Energy: Definition, Unit and Trend
Ionisation energy removes an electron from an isolated gaseous ground-state atom and is measured in kJ mol⁻¹.
Open article →Electron Affinity: Equation, Unit and Trend
Electron affinity is the energy released when an isolated gaseous atom gains an electron to form an anion, measured in kJ mol⁻¹.
Open article →Electronegativity, Pauling Values and Bond Character
Electronegativity attracts shared electrons; Pauling values and their difference indicate whether a bond is more covalent or ionic.
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