Definition of Molecule
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. 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.
- C- Carbon Atom
- H- Hydrogen Atom
- N- Nitrogen Atom
- O- Oxygen Atom
- CH3– Methyl Radical
Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical as in.
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.
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 ).
Further more, 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 (CH2CH2OH) 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 (H3 CCH3 ) there is virtually free rotation about the carbon-carbon single bond.
There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi atom combinations are called molecules.
A molecule is the smallest part of a substance that has the physical and chemical properties of that substance. In some respects, a molecule is similar to an atom.
A molecule, however, is composed of more than one atom. Some elements exist naturally as molecules.
For example, hydrogen and oxygen exist as two-atom molecules. Other elements also exist naturally as diatomic molecules. Molecules are compounds in which the elements are in definite, fixed ratio, For example: water, glucose, ATP.
Examples of Molecules
A molecule is two or more atoms bonded together to form a single chemical entity. Each atom carries a certain number of electrons that orbit around the nucleus.
The nucleus consists of protons and neutrons, of different numbers in different elements. The electrons that orbit the nucleus exist in various clouds, or valence shells.
These shells prefer to have specific numbers of electrons, depending on the shell. Sometimes, one atom will give away electrons to another atom.
These atoms both change in electrical charge and become ions. One will be positive and one will be negative. These opposite electrical effects attract each other and form ionic bonds. These bonds to not make a molecule, and the ions can be easily separated. However, sometimes atom share electrons.
Carbon Based Molecule
The Carbon is probably the most important element for all living organisms. Carbon has a unique ability to form 4 covalent bonds, which can lead to long chains of molecules.
All organic molecules contain carbon, and the ability to manipulate carbon bonds was probably a very early development in the evolution of life.
All of the types of molecules described below contain carbon, with a wide variety of other atoms covalently bonded to the carbon. Carbon, when it forms double bonds with other carbon atoms, can rotate around the bond.
This can create molecule that are flexible, and vary in shape. The wide variety of differently shaped carbon molecules in the biological world produces unique interactions.
Adenosine Triphosphate (ATP)
A molecule that nearly every organism uses is adenosine triphosphate or ATP. Adenosine is molecule of multiple carbon rings, as represented by the right side of the molecule below.
The left side is a chain of phosphate groups, which are a phosphorus atoms covalently bonded to oxygen atoms. When the bonds between theses phosphate groups are broken, energy is released.
Usually ATP functions as a coenzyme, transferring the energy from the bond to an enzyme, which can use the energy to speed a chemical reaction. Two molecules are present after the break, a free-floating phosphate group and adenosine diphosphate or ADP.
Through the processes of glycolysis (the breakdown of glucose) and respiration (the use of oxygen to further break down glucose), ATP is produced, which can then be used for energy in other cellular processes.
History of Molecule
According to Merriam-Webster and the Online Etymology Dictionary, the word “molecule” derives from the Latin “moles” or small unit of mass.
Molecule (1794) – “extremely minute particle”, from French molecule (1678), from New Latin molecula, diminutive of Latin moles “mass, barrier”. A vague meaning at first; the vogue for the word (used until the late 18th century only in Latin form) can be traced to the philosophy of Descartes.
The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties.
This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
Types of Molecule
Followings are the types of molecule
A diatomic atom is composed of only two atoms, of the same or different chemical elements. Examples of diatomic molecules are O2 and
Heteronuclear Diatomic Molecules
A heteronuclear diatomic molecule consists of two of atoms of the same element combined. There are seven diatomic Elements: Hydrogen (H2), Nitrogen (N2), Oxygen ( O2), Fluorine (F2), Chlorine (Cl2), –Iodine (I2) and Bromine (Br2) . These seven elements are so reactive that they can be found very often bonded with another atom of the same type.
Homonuclear Diatomic Molecules
A homonuclear diatomic molecule consists of two atoms of different elements chemically combined. Examples of homonuclear diatomic molecules are: carbon monoxide, hydrochloric acid (HCl) , and Hydrogen Fluoride (HF).
Shapes of Molecules
The three dimensional shape or configuration of a molecule is an important characteristic. This shape is dependent on the pre ferred spatial orientation of covalent bonds to atoms having two or more bonding partners.
Three dimensional configurations are best viewed with the aid of models. In order to represent such configurations on a two-dimensional surface (paper, blackboard or screen), we often use perspective drawings in which the direction of a bond is specified by the line connecting the bonded atoms.
In most cases the focus of configuration is a carbon atom so the lines specifying bond directions
will originate there.
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