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.
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.
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.
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 fieldof 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.
Atoms
Atom, smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. As such, the atom is the basic building block of chemistry.
Most of the atom is empty space. The rest consists of a positively charged nucleus of protons and neutrons surrounded by a cloud of negatively charged electrons. The nucleus is small and dense compared with the electrons, which are the lightest charged particles in nature. Electrons are attracted to any positive charge by their electric force; in an atom, electric forces bind the electrons to the nucleus.
Because of the nature of quantum mechanics, no single image has been entirely satisfactory at visualizing the atom’s various characteristics, which thus forces physicists to use complementary pictures of the atom to explain different properties. In some respects, the electrons in an atom behave like particles orbiting the nucleus. In others, the electrons behave like waves frozen in position around the nucleus. Such wave patterns, called orbitals, describe the distribution of individual electrons. The behaviour of an atom is strongly influenced by these orbital properties, and its chemical properties are determined by orbital groupings known as shells.
Most matter consists of an agglomeration of molecules, which can be separated relatively easily. Molecules, in turn, are composed of atoms joined by chemical bonds that are more difficult to break. Each individual atom consists of smaller particles—namely, electrons and nuclei. These particles are electrically charged, and the electric forces on the charge are responsible for holding the atom together. Attempts to separate these smaller constituent particles require ever-increasing amounts of energy and result in the creation of new subatomic particles, many of which are charged.
All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately 50 million atoms of solid matter lined up in a row would measure 1 cm (0.4 inch). A convenient unit of length for measuring atomic sizes is the angstrom (Å), defined as 10−10 metre. The radius of an atom measures 1–2 Å. Compared with the overall size of the atom, the nucleus is even more minute. It is in the same proportion to the atom as a marble is to a football field. In volume the nucleus takes up only 10−14metres of the space in the atom—i.e., 1 part in 100,000. A convenient unit of length for measuring nuclear sizes is the femtometre (fm), which equals 10−15 metre. The diameter of a nucleus depends on the number of particles it contains and ranges from about 4 fm for a Light nucleus such as carbon to 15 fm for a heavy nucleus such as lead. In spite of the small size of the nucleus, virtually all the mass of the atom is concentrated there. The protons are massive, positively charged particles, whereas the neutrons have no charge and are slightly more massive than the protons. The fact that nuclei can have anywhere from 1 to nearly 300 protons and neutrons accounts for their wide variation in mass. The lightest nucleus, that of hydrogen, is 1,836 times more massive than an electron, while heavy nuclei are nearly 500,000 times more massive.
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Atoms are the basic units of matter. They are so small that they cannot be seen with the naked eye, even with the most powerful microscopes. Atoms are made up of three types of particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus of the atom, while electrons orbit the nucleus.
The number of protons in an atom determines its chemical element. For example, all atoms that have 1 proton in their nucleus are hydrogen atoms. All atoms that have 2 protons in their nucleus are helium atoms, and so on.
The number of neutrons in an atom can vary, even for atoms of the same element. These atoms are called isotopes. For example, hydrogen has three isotopes: protium, deuterium, and tritium. Protium has no neutrons, deuterium has one neutron, and tritium has two neutrons.
The number of electrons in an atom is equal to the number of protons in the nucleus. This is because atoms are electrically neutral, so the positive charge of the protons must be balanced by the negative charge of the electrons.
Electrons are arranged in shells around the nucleus. The first shell can hold up to two electrons, the second shell can hold up to eight electrons, the third shell can hold up to 18 electrons, and so on.
The electrons in the outermost shell are called valence electrons. These electrons are involved in chemical bonding.
Chemical bonding is the force that holds atoms together to form molecules. There are four main types of chemical bonds: ionic bonds, covalent bonds, metallic bonds, and hydrogen bonds.
Ionic bonds are formed when one atom donates an electron to another atom. The atom that donates the electron becomes a positively charged ion, while the atom that receives the electron becomes a negatively charged ion. The oppositely charged ions are attracted to each other, forming an ionic bond.
Covalent bonds are formed when two atoms share electrons. The electrons are shared equally between the two atoms, forming a covalent bond.
Metallic bonds are formed when atoms of a Metal share electrons with each other. The electrons are free to move throughout the metal, giving metals their characteristic properties, such as conductivity and malleability.
Hydrogen bonds are weak bonds that form between hydrogen atoms and other atoms that are electronegative, such as oxygen and nitrogen. Hydrogen bonds are important in many biological molecules, such as proteins and DNA.
Molecules are groups of two or more atoms that are held together by chemical bonds. The properties of a molecule depend on the type of atoms that are present and the type of bonds that hold them together.
Molecular structure is the arrangement of atoms in a molecule. The molecular structure of a molecule can be determined by X-ray crystallography or by nuclear magnetic resonance (NMR) spectroscopy.
Molecular geometry is the shape of a molecule. The molecular geometry of a molecule is determined by the number of valence electrons in the molecule and the type of bonds that hold the atoms together.
Polarity is a property of molecules that have a positive end and a negative end. Polar molecules are attracted to water molecules, which are also polar.
Bond order is a measure of the strength of a chemical bond. The bond order of a covalent bond is equal to the number of pairs of electrons that are shared between the two atoms.
Resonance is a phenomenon that occurs when two or more Lewis structures can be drawn for a molecule. The resonance structures represent different ways of distributing the electrons in the molecule.
Hybridization is a concept that is used to explain the bonding in molecules. Hybridization is the mixing of atomic orbitals to form new orbitals that are used in bonding.
Valence bond theory is a theory that explains chemical bonding in terms of the overlap of atomic orbitals.
Molecular orbital theory is a theory that explains chemical bonding in terms of the formation of molecular orbitals from atomic orbitals.
Lewis structures are diagrams that represent the bonding in molecules. Lewis structures show the valence electrons of the atoms in a molecule and how they are shared between the atoms.
Formal charge is a concept that is used to assign charges to atoms in a molecule. Formal charge is equal to the number of valence electrons in an atom minus the number of electrons that are assigned to the atom in the Lewis structure.
VSEPR theory is a theory that explains the molecular geometry of molecules. VSEPR theory states that the shape of a molecule is determined by the number of electron pairs around the central atom.
VSEPR notation is a way of representing the molecular geometry of a molecule. VSEPR notation uses letters to represent the different types of electron pairs around the central atom and numbers to represent the number of electron pairs of each type.
Electronegativity is a property of atoms that measures their ability to attract electrons. Electronegativity is a measure of the tendency of an atom to attract electrons to itself when it
Here are some frequently asked questions and short answers about the following topics:
- What is a chemical element?
A chemical element is a pure substance consisting of one type of atom. The atoms of a chemical element all have the same number of protons in their nucleus.
- What is a molecule?
A molecule is a group of two or more atoms that are held together by chemical bonds. The atoms in a molecule can be of the same element or of different Elements.
- What is a compound?
A compound is a substance that is made up of two or more elements that are chemically combined. The elements in a compound are held together by chemical bonds.
- What is a mixture?
A mixture is a substance that is made up of two or more substances that are not chemically combined. The substances in a mixture can be elements, compounds, or other mixtures.
- What is a solution?
A solution is a type of mixture that is made up of a solute and a solvent. The solute is the substance that is dissolved in the solvent. The solvent is the substance that does the dissolving.
- What is a suspension?
A suspension is a type of mixture that is made up of a solid and a liquid. The solid particles in a suspension are large enough to settle out of the liquid over time.
- What is a colloid?
A colloid is a type of mixture that is made up of two or more substances that are not evenly mixed. The particles in a colloid are too small to settle out of the mixture, but they are too large to dissolve in the solvent.
- What is a heterogeneous mixture?
A heterogeneous mixture is a mixture that is not uniform in composition. The different substances in a heterogeneous mixture can be easily distinguished from each other.
- What is a homogeneous mixture?
A homogeneous mixture is a mixture that is uniform in composition. The different substances in a homogeneous mixture cannot be easily distinguished from each other.
- What is a physical change?
A physical change is a change in the state or properties of a substance, but not in its chemical composition.
- What is a chemical change?
A chemical change is a change in the chemical composition of a substance.
- What is a Chemical Reaction?
A chemical reaction is a process that changes one or more substances into new substances.
- What is a reactant?
A reactant is a substance that is involved in a chemical reaction.
- What is a product?
A product is a substance that is formed in a chemical reaction.
- What is a Catalyst?
A catalyst is a substance that speeds up The Rate of a Chemical Reaction, but is not itself consumed in the reaction.
- What is an inhibitor?
An inhibitor is a substance that slows down the rate of a chemical reaction.
- What is an equilibrium?
An equilibrium is a state of balance in which the rate of the forward reaction is equal to the rate of the reverse reaction.
- What is a spontaneous reaction?
A spontaneous reaction is a reaction that occurs without the need for an input of energy.
- What is a non-spontaneous reaction?
A non-spontaneous reaction is a reaction that does not occur without the need for an input of energy.
- What is an endothermic reaction?
An endothermic reaction is a reaction that absorbs heat from its surroundings.
- What is an exothermic reaction?
An exothermic reaction is a reaction that releases heat to its surroundings.
- What is a buffer?
A buffer is a solution that resists changes in pH.
- What is a pH scale?
A pH scale is a scale that measures the acidity or alkalinity of a solution.
- What is an acid?
An acid is a substance that donates protons in a chemical reaction.
- What is a base?
A base is a substance that accepts protons in a chemical reaction.
- What is a salt?
A salt is a compound that is formed when an acid reacts with a base.
- What is a precipitate?
A precipitate is a solid that forms when two solutions are mixed.
- What is a double displacement reaction?
A double displacement reaction is a reaction in which the ions of two compounds swap places to form two new compounds.
- What is a redox reaction?
A redox reaction is a reaction in which electrons are transferred from one
Which of the following is not a type of molecule?
(A) Atom
(B) Compound
(C) Ion
(D) ElementWhich of the following is not a property of atoms?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of molecules?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a type of bond?
(A) Ionic bond
(B) Covalent bond
(C) Metallic bond
(D) Hydrogen bondWhich of the following is not a type of compound?
(A) Ionic compound
(B) Covalent compound
(C) Metallic compound
(D) Molecular compoundWhich of the following is not a type of element?
(A) Metal
(B) Nonmetal
(C) Metalloid
(D) GasWhich of the following is not a property of elements?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of compounds?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a type of reaction?
(A) Synthesis reaction
(B) Decomposition reaction
(C) Single replacement reaction
(D) Double replacement reactionWhich of the following is not a type of mixture?
(A) Homogeneous mixture
(B) Heterogeneous mixture
(C) Solution
(D) SuspensionWhich of the following is not a property of mixtures?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of solutions?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of suspensions?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a type of physical change?
(A) Melting
(B) Boiling
(C) Freezing
(D) CondensationWhich of the following is not a type of chemical change?
(A) Combustion
(B) Decomposition
(C) Synthesis
(D) Double replacementWhich of the following is not a property of physical changes?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of chemical changes?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a type of energy?
(A) Kinetic energy
(B) Potential energy
(C) Chemical energy
(D) Nuclear EnergyWhich of the following is not a property of energy?
(A) Mass
(B) Charge
(C) Size
(D) ShapeWhich of the following is not a property of kinetic energy?
(A) Speed
(B) Direction
(C) Magnitude
(D) TimeWhich of the following is not a property of potential energy?
(A) Position
(B) Mass
(C) Charge
(D) ShapeWhich of the following is not a property of chemical energy?
(A) Bond energy
(B) Lattice energy
(C) Activation energy
(D) Heat of formationWhich of the following is not a property of nuclear energy?
(A) Mass defect
(B) Binding energy
(C) Half-life
(D) RadioactivityWhich of the following is not a type of force?
(A) Gravitational Force
(B) Electrical force
(C) Magnetic force
(D) Nuclear forceWhich of the following is not a property of forces?
(A) Magnitude
(B) Direction
(C) Time
(D) ShapeWhich of the following is not a property of gravitational force?
(A) Mass
(B) Charge
(C) Distance
(D) ShapeWhich of the following is not a property of electrical force?
(A) Charge
(B) Distance
(C) Shape