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How the periodic table is organized
Dmitri Mendeleev arranged elements by atomic mass in 1869, predicting gaps for undiscovered elements. Modern tables arrange by atomic number (proton count). Rows are called periods — elements in the same period have the same highest electron shell. Columns are called groups — elements in the same group have the same valence electron configuration and similar reactivity.
Period 1 has 2 elements. Periods 2 and 3 have 8 each. Periods 4 and 5 have 18 each. Periods 6 and 7 have 32 each (including lanthanides and actinides, typically shown separately below the main table to keep it compact).
Element categories and their properties
Alkali metals (Group 1): highly reactive, soft, low density, react vigorously with water. Alkaline earth metals (Group 2): reactive but less so than Group 1, higher melting points. Transition metals: hard, high melting points, often colored compounds, multiple oxidation states. Nonmetals: poor conductors, varied physical states, essential for organic chemistry (C, N, O, H).
Noble gases (Group 18): nearly chemically inert due to full valence shells. Halogens (Group 17): highly reactive nonmetals, form salts with metals. Lanthanides and actinides: f-block elements with complex electron configurations; actinides include radioactive elements used in nuclear applications.
Frequently asked questions
- How many elements are in the periodic table?
- There are 118 confirmed elements as of 2024. Elements 1-94 occur naturally on Earth; elements 95-118 are synthetic, produced in laboratories by nuclear reactions.
- How are elements organized in the periodic table?
- Elements are arranged by increasing atomic number (number of protons) in rows (periods) and columns (groups). Elements in the same group share similar chemical properties due to having the same number of valence electrons.
- What do the colors in the periodic table mean?
- Colors indicate element categories: alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloids, nonmetals, halogens, noble gases, lanthanides, and actinides. Each category has distinct chemical and physical properties.
- What is electronegativity?
- Electronegativity is a measure of how strongly an atom attracts electrons in a chemical bond. Fluorine has the highest electronegativity (3.98). Electronegativity increases across a period and decreases down a group.