Category Science

Ionic bonds happen when one atom gains one or more electrons from another atom. The electrons in an atom have a negative charge and are equal in number to the positively charged protons in the nucleus. When an atom gains or loses electrons, the balance of charges is broken, so the atom becomes either positively or negatively charged. It is called an ion. An atom that has gained electrons has a negative charge and is called an anion. One that has lost electrons has a positive charge and is called a cation. As opposite charges attract each other, the two atoms that have gained and lost electrons are pulled together into a bond.

When two atoms combine, they form a compound or molecule in a chemical bond, which links them together. This bond can be ionic or covalent. In an ionic bond, one atom donates an electron to the other to stabilize it. In a covalent bond, the atoms are shared by the electrons.

In the chemistry world, an ionic bond is made from atoms with different electronegativity values. It is considered a polar bond if the attraction is between two oppositely charged ions. This works much in the same way as magnets that attract each other. If two atoms have different electronegativity values, they will make an ionic bond.

The combination of sodium (Na) and chloride (Cl) forms NaCl or common table salt, and this is an example of an ionic bond. Sulfuric acid is also an ionic bond, combining hydrogen and sulfur oxide, and it is written as H2SO4.

Ionic bonds take more energy to break than covalent bonds, so ionic bonds are stronger. The amount of energy needed to break a bond is known as bond dissociation energy, which is basically the force it takes to break bonds of any type.

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It is useful to think of an atom as having electrons circling in layers around its nucleus. These layers are known as “shells”. Each layer can only have a certain number of electrons before a new shell must be started. Atoms that have as many electrons as possible in the outer shell (or some other particular numbers) are said to be stable. They do not easily form bonds with other atoms. Atoms that are not stable try to become so by sharing electrons with, or borrowing electrons from, or giving electrons to, another atom. The number of electrons that an atom needs to give or gain to achieve a stable outer shell is called its valency.

The combining capacity of an atom is called its valency. Actually it can be defined as the number of electrons that an atom may lose (or) gain during a chemical reaction (or) the number of electrons shared. The numbers of electrons in outermost shell (or) valency shell of an atom are called valency electrons. 

The valency of an element is the number of atoms lost or gained by the atom of an element. Valence electrons are the number of electrons present in the outermost shell of an atom valency of an element depends on the valence electrons valency of atoms having 1, 2, 3 valence electrons = number of valence electrons valency of atoms having 5, 6, 7 valence electrons

= 8 – number of valence electrons

Covalent bond the interatomic linkage that results from the sharing of an electron pair between two atoms. The binding arises from the electrostatic attraction of their nuclei for the same electrons. A covalent bond forms when the bonded atoms have a lower total energy than that of widely separated atoms.

Molecules that have covalent linkages include the inorganic substances hydrogen, nitrogen, chlorine, water, and ammonia (H2, N2, Cl2, H2O, NH3) together with all organic compounds. In structural representations of molecules, covalent bonds are indicated by solid lines connecting pairs of atoms; e.g.

A single line indicates a bond between two atoms (i.e., involving one electron pair), double lines (=) indicate a double bond between two atoms (i.e., involving two electron pairs), and triple lines (?) represent a triple bond, as found, for example, in carbon monoxide (C?O). Single bonds consist of one sigma (?) bond, double bonds have one ? and one pi (?) bond, and triple bonds have one ? and two ? bonds.

The idea that two electrons can be shared between two atoms and serve as the link between them was first introduced in 1916 by the American chemist G.N. Lewis, who described the formation of such bonds as resulting from the tendencies of certain atoms to combine with one another in order for both to have the electronic structure of a corresponding noble-gas atom.

Covalent bonds are directional, meaning that atoms so bonded prefer specific orientations relative to one another; this in turn gives molecules definite shapes, as in the angular (bent) structure of the H2O molecule. Covalent bonds between identical atoms (as in H2) are nonpolar—i.e., electrically uniform—while those between unlike atoms are polar—i.e., one atom is slightly negatively charged and the other is slightly positively charged. This partial ionic character of covalent bonds increases with the difference in the electronegativity’s of the two atoms. 

A compound is a substance that is created when two or more elements are bonded by a chemical reaction. It is difficult to split a compound back into its original elements. Compounds do not necessarily take on the characteristics of the elements that form them. For example, sodium is a metal and chlorine is a gas. Together they form a compound called sodium chloride, which is not like either of them. In fact, sodium chloride is the chemical name for the salt that we put on our food.

In chemistry, a compound is a substance that results from a combination of two or more different chemical elements, in such a way that the atom s of the different elements is held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule.

A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).

Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure.

The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine.

1: Pure water is a compound made from two elements – hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always. Each molecule of water contains two hydrogen atoms bonded to a single oxygen atom.

2: Pure table salt is a compound made from two elements – sodium and chlorine. The ratio of sodium ions to chloride ions in sodium chloride is always.

3: Pure methane is a compound made from two elements – carbon and hydrogen. The ratio of hydrogen to carbon in methane is always.

4: Pure glucose is a compound made from three elements – carbon, hydrogen, and oxygen. The ratio of hydrogen to carbon and oxygen in glucose is always.

Elements do not usually exist on their own. In the natural world, they are found in combination with other elements. By understanding how elements combine, scientists have been able to make new combinations, creating molecules that are not found in nature. These combinations are not made simply by mixing two or more substances together. Brown sugar and salt can be stirred together, for example, but this does not create a new substance. Each little particle is either a grain of sugar or a grain of salt — they have remained separate. Mixtures can usually be separated again, but when elements are chemically joined together, they are said to be bonded and have created a new substance.

Bonding is caused by a chemical reaction. Most chemical reactions need some form of energy to start them. Usually, this energy is supplied in the form of heat. Many compounds are made by heating two or more substances together until their molecules are moving so fast that they react with each other.

Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.

 A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).

The formation of slaked lime (calcium hydroxide, Ca (OH)2) when water is added to lime (CaO) is exothermic. This reaction occurs when water is added to dry Portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N2H4), require energy input when they are formed from the elements. The decomposition of limestone (CaCO3) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur. The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.

Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favorability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.

A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning wood. Wood has low entropy. When wood burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

When we cook food, chemical reactions take place as het energy is supplied to the ingredients. New compounds are formed, so that the cooked dish usually has a different appearance, texture and taste from the mixed raw ingredients.

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