Molecule

Jai Ganesh · Yesterday 19:54:35

Molecule

Gist

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. These atoms can be of the same element, such as an oxygen molecule (O2), or different elements, such as in a water molecule (H2O). Molecules are the smallest units of a substance that retain its chemical and physical properties.

It is the smallest unit of a pure substance that can exist and still retain the substance's chemical properties. For example, a water molecule (H2O) consists of two hydrogen atoms and one oxygen atom, while an oxygen molecule (O2) consists of two oxygen atoms.

Summary

A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions.

A molecule may be homonuclear, that is, it consists of atoms of one chemical element, e.g. two atoms in the oxygen molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, e.g. water (two hydrogen atoms and one oxygen atom; H2O). In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. This relaxes the requirement that a molecule contains two or more atoms, since the noble gases are individual atoms. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules.

Concepts similar to molecules have been discussed since ancient times, but modern investigation into the nature of molecules and their bonds began in the 17th century. Refined over time by scientists such as Robert Boyle, Amedeo Avogadro, Jean Perrin, and Linus Pauling, the study of molecules is today known as molecular physics or molecular chemistry.

Details

A molecule is 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.

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.

Determining molecular structure

The nuclear positions in a molecule are determined either from microwave vibration-rotation spectra or by neutron diffraction. The electron cloud surrounding the nuclei in a molecule can be studied by X-ray diffraction experiments. Further information can be obtained by electron spin resonance or nuclear magnetic resonance techniques. Advances in electron microscopy have enabled visual images of individual molecules and atoms to be produced.

Theoretically the molecular structure is determined by solving the quantum mechanical equation for the motion of the electrons in the field of the nuclei (called the Schrödinger equation). In a molecular structure the bond lengths and bond angles are those for which the molecular energy is the least. The determination of structures by numerical solution of the Schrödinger equation has become a highly developed process entailing use of computers and supercomputers.

Polar and nonpolar molecules

If a molecule has no net electrical charge, its negative charge is equal to its positive charge. The forces experienced by such molecules depend on how the positive and negative charges are arranged in space. If the arrangement is spherically symmetric, the molecule is said to be nonpolar. If there is an excess of positive charge on one end of the molecule and an excess of negative charge on the other, the molecule has a dipole moment (i.e., a measurable tendency to rotate in an electric or magnetic field) and is therefore called polar. When polar molecules are free to rotate, they tend to favour those orientations that lead to attractive forces.

Nonpolar molecules generally are considered lipophilic (lipid-loving), whereas polar chemicals are hydrophilic (water-loving). Lipid-soluble, nonpolar molecules pass readily through a cell membrane because they dissolve in the hydrophobic, nonpolar portion of the lipid bilayer. Although permeable to water (a polar molecule), the nonpolar lipid bilayer of cell membranes is impermeable to many other polar molecules, such as charged ions or those that contain many polar side chains. Polar molecules pass through lipid membranes via specific transport systems.

Molecular weight

The molecular weight of a molecule is the sum of the atomic weights of its component atoms. If a substance has molecular weight M, then M grams of the substance is termed one mole. The number of molecules in one mole is the same for all substances; this number is known as Avogadro’s number (6.022140857 × {10}^{23}). Molecular weights can be determined by mass spectrometry and by techniques based on thermodynamics or kinetic transport phenomena.

Additional Information

A molecule is defined as a group of two or more atoms that are bonded together through chemical interactions. This fundamental structure forms the basis of much of the matter in the universe, including the air we breathe and the water in Earth's oceans. Molecules can consist of atoms from the same element, such as oxygen gas (O2), or from different elements, such as methane (CH4) or glucose (C6H12O6). The way atoms bond to form molecules can occur through covalent or ionic bonding, affecting their properties and behaviors.

Covalent bonds involve the sharing of electrons between atoms, while ionic bonds result from one atom donating electrons to another, creating charged ions. Molecules can be categorized as polar or nonpolar based on the distribution of electrons; for example, water is polar, while methane is nonpolar. This classification influences how molecules interact with each other, particularly regarding their solubility in water, which is crucial in biological systems. Understanding molecules is essential for exploring chemistry and biology, as they play a vital role in the structure and function of living organisms.

Molecule

A molecule consists of two or more atoms that have bonded together. A great deal of matter is made up of molecules, including Earth’s oceans and the air that people breathe. Scientists estimate that the first molecule was formed approximately 250,000 to 300,000 years after the big bang. They believe that the first molecule to form was the helium hydride ion (HeH+), a positively charged ion containing hydrogen and helium, which together account for the vast majority of atoms created during the big bang.

Over time, atoms of other elements began to form, including oxygen and carbon, as well as heavier elements such as gold and silicon. Hydrogen atoms began bonding with these other elements to form different molecules, such as water (H2O) and various hydrocarbons. The most abundant molecules in biological systems are those that contain carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

Brief History

Perhaps the biggest contribution to the discovery of the molecule was made by English scientist John Dalton (1766–1844). During the early 1800s, Dalton developed what was later known as his atomic theory of matter. The principles of this theory included the following proposals: (1) that all matter was composed of atoms; (2) that atoms could not be cut, subdivided, created, or destroyed; (3) that all atoms of a specific element were identical in mass; (4) that compounds were created from combinations of whole numbers of atoms; and (5) that chemical reactions were caused by atoms either separating or combining.

Dalton’s second and third principles were later disproved, the second when scientists discovered that atoms could, in fact, be subdivided into even smaller subatomic particles called protons, neutrons, and electrons. The third principle was refuted by the discovery of isotopes, which are atoms of the same element that have different masses and properties.

Overview

In their most basic form, molecules are simply two or more atoms bonded together through chemical interactions. It does not matter whether those two atoms are of the same element or not; two individual oxygen atoms can bond together to create the molecule O2 (oxygen gas), while four hydrogen atoms and one carbon atom can bond together to create the molecule CH4 (methane). Some other examples of common molecules are C6H12O6 (glucose), PO4 (phosphate), and H2O2 (hydrogen peroxide). Molecules come in many different shapes, sizes, and arrangements, characteristics that dramatically affect how they react with one another.

Whenever molecules are composed of more than a single element, they are referred to as molecular compounds. Examples include methane, glucose, phosphate, and hydrogen peroxide, as shown above, as well as calcium oxide (CaO) and potassium sulfide (K2S). As a result, all compounds are molecules, but not all molecules are compounds.

Individual atoms form larger molecules through chemical bonding. The two main types of bonding that occur between atoms are covalent bonding and ionic bonding. Covalent bonding occurs when two atoms share two, four, or six electrons between them. (Two shared electrons form a single covalent bond, and certain atoms have the ability to form double or even triple bonds.) Examples of covalent bonds include those found in diatomic carbon (C2) or molecular oxygen (O2).

Because molecules such as C2 and O2 are formed from atoms of the same element, there is no difference in the amount of pull that each one has on their electrons. For two atoms to bond together, their outermost electrons, called valence electrons, need to be either shared or transferred from one to the other. The atom with the stronger pull on its electrons is said to be more electronegative. As mentioned above, covalent bonding occurs whenever two atoms, of either the same element or different ones, share their valence electrons. In the case of C2 and O2, because both atoms are from the same element, there is no difference in electronegativity, and the electrons can be shared evenly as a result.

Ionic bonding is a process in which one atom gives up one or more electrons to another. Under these circumstances, the atom giving up, or donating, its electrons will almost always have a lower amount of electronegativity than the atom receiving, or accepting, the electrons. This process creates two oppositely charged ions: the donor atom becomes a positively charged ion, or cation, and the acceptor atom becomes a negatively charged ion, or anion. The two ions are then held together by the attraction between their opposite charges, thus creating an ionic compound.

Individual molecules can be classified as either polar or nonpolar. In a polar molecule, the electrons are not evenly shared between the individual atoms. Water is a perfect example of this. Oxygen is more electronegative than hydrogen, so the oxygen atom pulls the electrons of the hydrogen atoms closer to it, leaving the positively charged nucleus of each hydrogen atom exposed. As a result, one end of the molecule—the oxygen atom—develops a slight negative charge, while the exposed nuclei of the hydrogen atoms at the other end cause them to develop a slight positive charge.

In nonpolar molecules, electrons are shared equally among the individual atoms, and unshared electrons remain largely in place around their respective atoms. Methane is an example of a nonpolar molecule. It consists of one carbon atom and four hydrogen atoms, the latter bonded to the former via equivalent and equidistant carbon-hydrogen bonds to form a symmetrical tetrahedral molecule. As a result, no polar regions form on the molecule.

Nonpolar molecules tend to be hydrophobic, meaning they do not mix well with water, due to the polar nature of the water molecule. Likewise, polar molecules tend to be hydrophilic, meaning that they do mix well with water, because the negative regions of the polar molecule are attracted to the positive regions of the water molecule, and vice versa.

In biological systems, whether a molecule is hydrophobic or hydrophilic is very important. For instance, the membrane of a living cell is composed primarily of a lipid bilayer that is hydrophilic on each outer surface and hydrophobic in the middle, which helps separate the outside of the cell from the inside. In the physical world, water’s polar nature allows for the creation of hydrogen bonds, which are the main reason that water remains in a liquid rather than gaseous state at room temperature.

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Molecule

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