A molecule is a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided, while still retaining its composition and chemical properties. The ratio of atoms bonded together to form molecules is fixed. For instance, every water molecule contains two hydrogen atoms and one oxygen atom. Molecules are held together by covalent bonds, or shared electron pairs. Because these bonds are directional, the atoms arrange themselves in particular positions relative to one another to maximize bond strength. As a result, each molecule has a definite and fairly rigid structure, or spatial distribution of its atoms.

What are molecules made of?

Molecules are the smallest units of compounds that can exist. They're formed when atoms interact and distribute themselves in space, resulting in a lower total energy compared to alternative arrangements. Atoms are composed of elementary particles called quarks and leptons, which include electrons. Quarks combine to form protons and neutrons, and these, along with electrons, constitute the atoms of elements like hydrogen, oxygen, and iron. Atoms link via chemical bonds to create molecules. The chemical bond involves the distribution of electrons between atoms. Not all substances are made of distinct molecules. For example, sodium chloride consists of sodium and chlorine ions arranged in a lattice.

How do molecules form from atoms?

Molecules form when atoms interact and distribute themselves in space, resulting in a total energy lower than in separate arrangements; this energy decrease is the bonding energy. Chemical bonds, mainly ionic and covalent, hold atoms together. Covalent bonds involve sharing electron pairs between atoms, while ionic bonds arise from electrostatic attraction between oppositely charged ions due to electron transfer. The ratio of atoms bonded together is fixed, differentiating compounds from mixtures. Atoms achieve an eight-outer-electron configuration like noble gases through losing, gaining, or sharing electrons. Electronegativity, the ability of an atom to attract electrons, influences bond formation; high electronegativity favors anion formation, while low electronegativity favors cation formation.

What is the difference between a molecule and an atom?

An atom is the basic building block of matter and chemistry. It cannot be divided into smaller parts through ordinary chemical processes. Atoms consist of positively charged protons, neutral neutrons, and negatively charged electrons. Each element contains only one type of atom. A molecule is a group of two or more atoms that form the smallest identifiable unit of a pure substance while retaining the substance's composition and chemical properties. Atoms bond together to form molecules.

How do molecules differ from one another?

Molecules differ from one another in several ways. Atomic Composition Molecules are comprised of atoms, and the types and numbers of atoms in a molecule define its composition. For example, a water molecule ((H_2O)) contains two hydrogen atoms and one oxygen atom, while hydrogen peroxide ((H_2O_2)) contains two hydrogen atoms and two oxygen atoms. Isomers Molecules with the same number and kinds of atoms can still differ if their atoms are bonded in different ways. These are called isomers. Ethyl alcohol ((CH_3CH_2OH)) and methyl ether ((CH_3OCH_3)) are isomers because they both contain one oxygen, two carbon, and six hydrogen atoms, but they are bonded differently. Spatial Arrangement The three-dimensional arrangement of atoms also creates differences. Molecular shape significantly impacts the properties of molecules; for instance, water molecules' bent shape leads to strong interactions, which might not occur if they were linear. Intermolecular Forces Molecules can bond through weak intermolecular forces, influencing whether substances exist as gases, liquids, or solids. These forces, also known as Van der Waals forces, contribute to the overall properties of molecular substances.

Why are molecules important in chemistry and daily life?

Molecules, the smallest units of a substance that retain its chemical properties, are crucial in chemistry and daily life. They determine how atoms combine in fixed ratios to form compounds, distinguishing them from mixtures. In daily life, biomolecules like carbohydrates, lipids, nucleic acids, and proteins are essential for all living organisms. They serve as energy sources, structural components, and genetic information storage. In chemistry, understanding molecules is vital for controlling chemical reactions, creating new materials, and developing drugs. The properties of molecules correlate with their structures, influencing chemical processes.

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Diagram of the caffeine molecule, showing colored spheres representing carbon, hydrogen, nitrogen, oxygen atoms, and methyl groups, with labeled bonds and a key for each element.

Diagram showing the formation of a polar covalent bond in a water molecule, with two hydrogen atoms bonding to one oxygen atom to form H2O, illustrating shared electrons and chemical reaction.

Diagram showing the formation of an ionic bond between sodium (Na) and chlorine (Cl) atoms. Sodium loses an electron to become Na+, chlorine gains an electron to become Cl-, resulting in sodium chloride (NaCl), or table salt.

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Figure 22: Scattering of light from molecules (see text).

At a Glance

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molecule, a group of two or more atoms that form the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance.

Characteristics of molecules

The division of a sample of a substance into progressively smaller parts produces no change in either its composition or its chemical properties until parts consisting of single molecules are reached. Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties. In this latter stage of fragmentation the chemical bonds that hold the atoms together in the molecule are broken.

Atoms consist of a single nucleus with a positive charge surrounded by a cloud of negatively charged electrons. When atoms approach one another closely, the electron clouds interact with each other and with the nuclei. If this interaction is such that the total energy of the system is lowered, then the atoms bond together to form a molecule. Thus, from a structural point of view, a molecule consists of an aggregation of atoms held together by valence forces. Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical, as in, for example, the oxygen molecule (O₂), they compose a homonuclear diatomic molecule, while if the atoms are different, as in the carbon monoxide molecule (CO), they make up a heteronuclear diatomic molecule. Molecules containing more than two atoms are termed polyatomic molecules, e.g., carbon dioxide (CO₂) and water (H₂O). Polymer molecules may contain many thousands of component atoms.

Molecular bonding

The ratio of the numbers of atoms that can be bonded together to form molecules is fixed; for example, every water molecule contains two atoms of hydrogen and one atom of oxygen. It is this feature that distinguishes chemical compounds from solutions and other mechanical mixtures. Thus hydrogen and oxygen may be present in any arbitrary proportions in mechanical mixtures but when sparked will combine only in definite proportions to form the chemical compound water (H₂O). It is possible for the same kinds of atoms to combine in different but definite proportions to form different molecules; for example, two atoms of hydrogen will chemically bond with one atom of oxygen to yield a water molecule, whereas two atoms of hydrogen can chemically bond with two atoms of oxygen to form a molecule of hydrogen peroxide (H₂O₂). Furthermore, it is possible for atoms to bond together in identical proportions to form different molecules. Such molecules are called isomers and differ only in the arrangement of the atoms within the molecules. For example, ethyl alcohol (CH₃CH₂OH) and methyl ether (CH₃OCH₃) both contain one, two, and six atoms of oxygen, carbon, and hydrogen, respectively, but these atoms are bonded in different ways.

Not all substances are made up of distinct molecular units. Sodium chloride (common table salt), for example, consists of sodium ions and chlorine ions arranged in a lattice so that each sodium ion is surrounded by six equidistant chlorine ions and each chlorine ion is surrounded by six equidistant sodium ions. The forces acting between any sodium and any adjacent chlorine ion are equal. Hence, no distinct aggregate identifiable as a molecule of sodium chloride exists. Consequently, in sodium chloride and in all solids of similar type, the concept of the chemical molecule has no significance. Therefore, the formula for such a compound is given as the simplest ratio of the atoms, called a formula unit—in the case of sodium chloride, NaCl.

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Molecules are held together by shared electron pairs, or covalent bonds. Such bonds are directional, meaning that the atoms adopt specific positions relative to one another so as to maximize the bond strengths. As a result, each molecule has a definite, fairly rigid structure, or spatial distribution of its atoms. Structural chemistry is concerned with valence, which determines how atoms combine in definite ratios and how this is related to the bond directions and bond lengths. The properties of molecules correlate with their structures; for example, the water molecule is bent structurally and therefore has a dipole moment, whereas the carbon dioxide molecule is linear and has no dipole moment. The elucidation of the manner in which atoms are reorganized in the course of chemical reactions is important. In some molecules the structure may not be rigid; for example, in ethane (H₃CCH₃) there is virtually free rotation about the carbon-carbon single bond.

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Characteristics of molecules

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Molecular bonding