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Chemistry is the scientific discipline involved with elements and compounds composed of atoms, molecules, and ions: their composition, structure, properties, behavior and the changes they undergo during a reaction with other substances.

In the scope of its subject, chemistry occupies an intermediate position between physics and biology. It is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level.

For example, chemistry explains aspects of plant chemistry (botany), the formation of igneous rocks (geology), how atmospheric ozone is formed and how environmental pollutants are degraded (ecology), the properties of the soil on the moon (astrophysics), how medications work (pharmacology), and how to collect DNA evidence at a crime scene (forensics).

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Chemical reactions

Chemical reactions: how they break and form bonds between atoms. Balanced reactions, reversibility, and equilibrium.

Chemical reactions introduction

Reactants and products in reversible and irreversible chemical reactions.

Chemical bonds

Chemical bonds hold molecules together and create temporary connections that are essential to life. Types of chemical bonds including covalent, ionic, and hydrogen bonds and London dispersion forces.

Intermolecular forces

Different types of intermolecular forces (forces between molecules).

Electronegativity and bonding

Electronegativity differences in bonding using the Pauling scale. Classifying bonds as covalent, polar covalent, or ionic.


Electronegativity is a measure of an atom's ability to attract shared electrons to itself. On the periodic table, electronegativity generally increases as you move from left to right across a period a

Covalent bonds

Covalent bonds involve the sharing of electron pairs between atoms. Electron pairs shared between atoms of equal or very similar electronegativity constitute a nonpolar covalent bond (e.g., H–H or C–H

Ionic bonds

Ionic bonds result from the electrostatic attraction between oppositely charged ions, which form when valence electrons are transferred from one atom to another.

The periodic table, electron shells, and orbitals

The Bohr model and atomic orbitals. Using an element's position in the periodic table to predict its properties, electron configuration, and reactivity.

Groups of the periodic table

The s-, p-, and d-block elements of the periodic table are arranged into 18 numbered columns, or groups. The elements in each group have the same number of valence electrons. As a result, elements in