Several concepts are essential for the study of chemistry; some of them are:
Atom
An atom is the basic unit of chemistry. It consists of a positively charged core (the atomic nucleus) which contains protons and neutrons, and which maintains a number of electrons to balance the positive charge in the nucleus.
Element
A chemical element is a substance which is composed of a single type of atom. A chemical element is characterized by a particular number of protons in the nuclei of its atoms. This number is known as the atomic number of the element.
A molecule is the smallest indivisible portion of a pure chemical substance that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. Molecules can exist as electrically neutral units unlike ions. Molecules are typically a set of atoms bound together by covalent bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in lone pairs
A chemical reaction can be symbolically depicted through a chemical equation. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons.
The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its mechanism. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many reaction intermediates with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the kinetics and the relative product mix of a reaction. Many physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the Woodward-Hoffmann rules often come handy while proposing a mechanism for a chemical reaction.
According to the IUPAC gold book a chemical reaction is a process that results in the interconversion of chemical species". Accordingly, a chemical reaction may be an elementary reaction or a stepwise reaction. An additional caveat is made, in that this definition includes cases where the interconversion of conformers is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').
An atom is the basic unit of chemistry. It consists of a positively charged core (the atomic nucleus) which contains protons and neutrons, and which maintains a number of electrons to balance the positive charge in the nucleus.
Element
A chemical element is a substance which is composed of a single type of atom. A chemical element is characterized by a particular number of protons in the nuclei of its atoms. This number is known as the atomic number of the element.
For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium.
Ninety–four different chemical elements or types of atoms based on the number of protons exist naturally. A further 18 have been recognised by IUPAC as existing artificially only.
Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of neutrons; such atoms are termed isotopes. In fact several isotopes of an element may exist.
Compound
A compound is a substance with a particular ratio of atoms of particular chemical elements which determines its composition
Compound
A compound is a substance with a particular ratio of atoms of particular chemical elements which determines its composition
For example, water is a compound containing hydrogen and oxygen in the ratio of two to one. Compounds are formed and interconverted by chemical reactions.
Substance
A chemical substance is a kind of matter with a definite composition and set of properties. Strictly speaking, a mixture of compounds, elements or compounds and elements is not a chemical substance, but it may be called a chemical.
Substance
A chemical substance is a kind of matter with a definite composition and set of properties. Strictly speaking, a mixture of compounds, elements or compounds and elements is not a chemical substance, but it may be called a chemical.
Most of the substances we encounter in our daily life are some kind of mixture; for example: air, alloys, biomass, etc.
Molecule
A molecule is the smallest indivisible portion of a pure chemical substance that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. Molecules can exist as electrically neutral units unlike ions. Molecules are typically a set of atoms bound together by covalent bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in lone pairs
Mole and amount of substance
Mole is a unit to measure amount of substance (also called chemical amount). A mole is the amount of asubstance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.[39] The number of entities per mole is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30)×10^23 mol−1 (2007 CODATA). One way to understand the meaning of the term "mole" is to compare and contrast it to terms such as dozen. Just as one dozen eggs contains 12 individual eggs, one mole contains 6.02214179(30)×10^23 atoms, molecules or other particles. The term is used because it is much easier to say, for example, 1 mole of carbon, than it is to say 6.02214179(30)×10^23 carbon atoms, and because moles of chemicals represent a scale that is easy to experience.
The amount of substance of a solute per volume of solution is known as amount of substance concentration, or molarity for short.
Mole is a unit to measure amount of substance (also called chemical amount). A mole is the amount of asubstance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.[39] The number of entities per mole is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30)×10^23 mol−1 (2007 CODATA). One way to understand the meaning of the term "mole" is to compare and contrast it to terms such as dozen. Just as one dozen eggs contains 12 individual eggs, one mole contains 6.02214179(30)×10^23 atoms, molecules or other particles. The term is used because it is much easier to say, for example, 1 mole of carbon, than it is to say 6.02214179(30)×10^23 carbon atoms, and because moles of chemicals represent a scale that is easy to experience.
The amount of substance of a solute per volume of solution is known as amount of substance concentration, or molarity for short.
Molarity is the quantity most commonly used to express the concentration of a solution in the chemical laboratory. The most commonly used units for molarity are mol/L (the official SI units are mol/m3).
Ions and salts
An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. Positively charged cations (e.g. sodium cation Na+) and negatively charged anions(e.g. chloride Cl−) can form a crystalline lattice of neutral salts (e.g. sodium chloride NaCl).
Ions and salts
An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. Positively charged cations (e.g. sodium cation Na+) and negatively charged anions(e.g. chloride Cl−) can form a crystalline lattice of neutral salts (e.g. sodium chloride NaCl).
Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide(OH−) and phosphate (PO43−).
Ions in the gaseous phase are often known as plasma.
Acidity and basicity
A substance can often be classified as an acid or a base. There are several different theories which explain acid-base behavior. The simplest is Arrhenius theory, which states than an acid is a substance that produces hydronium ions when it is dissolved in water, and a base is one that produces hydroxide ions when dissolved in water.
Ions in the gaseous phase are often known as plasma.
Acidity and basicity
A substance can often be classified as an acid or a base. There are several different theories which explain acid-base behavior. The simplest is Arrhenius theory, which states than an acid is a substance that produces hydronium ions when it is dissolved in water, and a base is one that produces hydroxide ions when dissolved in water.
According to Brønsted–Lowry acid-base theory, acids are substances that donate a positive hydrogen ion to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion. A third common theory is Lewis acid-base theory, which is based on the formation of new chemical bonds. Lewis theory explains that an acid is a substance which is capable of accepting a pair of electrons from another substance during the process of bond formation, while a base is a substance which can provide a pair of electrons to form a new bond.
According to concept as per Lewis, the crucial things being exchanged are charges.[unreliable source?] There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept
Acid strength is commonly measured by two methods.
One measurement, based on the Arrhenius definition of acidity, is pH, which is a measurement of the hydronium ion concentration in a solution, as expressed on a negative logarithmic scale.
Thus, solutions that have a low pH have a high hydronium ion concentration, and can be said to be more acidic. The other measurement, based on the Brønsted–Lowry definition, is the acid dissociation constant (Ka), which measure the relative ability of a substance to act as an acid under the Brønsted–Lowry definition of an acid. That is, substances with a higher Ka are more likely to donate hydrogen ions in chemical reactions than those with lower Ka values.
Electron atomic and molecular orbitals
Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the multipolebalance between the positive charges in the nuclei and the negative charges oscillating about them.[42] More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.
A chemical bond can be a covalent bond, an ionic bond, a hydrogen bond or just because of Van der Waals force. Each of these kind of bond is ascribed to some potential. These potentials create the interactions which hold atoms together in molecules or crystals. In many simple compounds, Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (VSEPR), and the concept of oxidation number can be used to explain molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory is less applicable and alternative approaches, such as themolecular orbital theory, are generally used. See diagram on electronic orbitals.
Reaction Main article: Chemical reaction
When a chemical substance is transformed as a result of its interaction with another or energy, a chemical reaction is said to have occurred. Chemical reaction is therefore a concept related to the 'reaction' of a substance when it comes in close contact with another, whether as a mixture or a solution; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well with the system environment which may be a designed vessels which are often laboratory glassware. Chemical reactions can result in the formation or dissociation of molecules, that is, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. Oxidation, reduction, dissociation, acid-base neutralization and molecular rearrangement are some of the commonly used kinds of chemical reactions.
Electron atomic and molecular orbitals
Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the multipolebalance between the positive charges in the nuclei and the negative charges oscillating about them.[42] More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.
A chemical bond can be a covalent bond, an ionic bond, a hydrogen bond or just because of Van der Waals force. Each of these kind of bond is ascribed to some potential. These potentials create the interactions which hold atoms together in molecules or crystals. In many simple compounds, Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (VSEPR), and the concept of oxidation number can be used to explain molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory is less applicable and alternative approaches, such as themolecular orbital theory, are generally used. See diagram on electronic orbitals.
Reaction Main article: Chemical reaction
When a chemical substance is transformed as a result of its interaction with another or energy, a chemical reaction is said to have occurred. Chemical reaction is therefore a concept related to the 'reaction' of a substance when it comes in close contact with another, whether as a mixture or a solution; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well with the system environment which may be a designed vessels which are often laboratory glassware. Chemical reactions can result in the formation or dissociation of molecules, that is, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. Oxidation, reduction, dissociation, acid-base neutralization and molecular rearrangement are some of the commonly used kinds of chemical reactions.
A chemical reaction can be symbolically depicted through a chemical equation. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons.
The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its mechanism. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many reaction intermediates with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the kinetics and the relative product mix of a reaction. Many physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the Woodward-Hoffmann rules often come handy while proposing a mechanism for a chemical reaction.
According to the IUPAC gold book a chemical reaction is a process that results in the interconversion of chemical species". Accordingly, a chemical reaction may be an elementary reaction or a stepwise reaction. An additional caveat is made, in that this definition includes cases where the interconversion of conformers is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').
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