VALENCY AND IONS VALENCY Definition 1: It is the capacity of an atom to combine with the other atom. Definition 2: The number of electrons that

an atom wants to lose, gain or share is called its valency. IONS An ion is an atom, or group of atoms, that has a net positive or cation ion

with a positive charge If a negative charge. neutral atom loses one or more electrons it becomes a cation. Na

11 protons 11 electrons Na+ 11 protons 10 electrons anion ion with a negative charge if a

neutral atom gains one or more electrons it becomes an anion. 17 protons Cl 17 protons 17 electrons Cl-

18 electrons A monatomic ion contains only one atom. Na+, Cl-, Ca2+, O2-, Al3+, N3- A polyatomic ion contains more than one atom. OH-, CN-, NH4+, NO3-

? 13 protons, 10 (13 3) electrons How many protons and electrons are in 78 234Se ? 34 protons, 36 (34 + 2) electrons

Some Ions In Periodic Table CHEMICAL BONDS Chemical Bonding

All known elements are arranged in the increasing order of their atomic number in the modern periodic table. In an atom, the outermost shell is known as valence shell and electrons placed in this shell are called as valence shell electrons. The

valence shell electrons take part in the chemical bonding and chemical reactions with other atoms. All elements tend to get the octet configuration which makes them stable just like Nobel gases. There are two possible ways to get this configuration.

One is either sharing their valence electrons with other elements or by the complete transfer of electrons to form ions.

The sharing or transfer of electrons creates some attraction force between elements that is called as chemical bond. The formation of chemical bond

completes the octet configuration of element and makes them stable in a molecule. The sharing of electrons between bonded atoms forms a covalent bond whereas complete transfer of electrons forms ionic bonds. Lets discuss what chemical bond is and how different types of chemical bonds are formed Types of Chemical Bonding

As a matter of convenience we usually divide chemical bonds into different types. There are two major classes of bonding. Ionic bonding which results from electrostatic

interaction among ions; and can be formed by the transfer of one or more electrons from one atom or group of atoms to another. Covalent bonding which results from sharing one or more electron pairs between two atoms. Electrovalent compounds are compounds formed by completed

transfer of electrons from a metallic atom to a non-metallic atom resulting in the formation of cation and anion. Electrovalent (ionic) Bonding When an atom donates one, two or three electrons from its valence shell to another atom, which has the ability

to accept these electrons, it is known as electro-valency. As a result of electro-valency, both these atoms achieve the structure of an inert gas. Electrovalent bond is the attractive force between the oppositely charged ions, which comes into existence by the transference of electrons. Ionic Bonding

An ionic bond is a strong mutual attraction of oppositely charged ions. Such bonds do not usually form by the direct transfer of an electron from one atom to another; rather atoms that have already become ions stay close together because of their opposite charges.

Ions may consist of single atom or multiple atoms, in which a group of atoms is called a "polyatomic ion". Examples of polyatomic anions include: carbonate ion, which is composed of carbon and oxygen; and sulfate ion, which is composed of sulfur and oxygen. An example of a polyatomic cation is ammonium ion, which consists of nitrogen and hydrogen. Cation are usually metal atoms and anions are either nonmetals or polyatomic ions. The attraction of the two charges holds the atoms or molecules together. Electrostatic forces

hold ionic bonds together. Ionic bond is also defined as: "An ionic bond is the force of attraction between oppositely charged ions in a compound."

Examples of Ionic Bonding Ionic compounds are made up of ions. For example, sodium chloride consists of sodium ions and chloride ions. A strong electrostatic force of attraction between the oppositely charged ins holds them together. This force of attraction is called an ionic bond.

Two examples illustrate the formation of ionic bonds. They are Sodium chloride Hot sodium metal reacts with chlorine gas, Cl2 to produce a white solid. The white solid is the compound sodium chloride. In these reaction, the sodium atom loses an electron to

become a sodium ion, Na+. The electron is taken by a chlorine atom to become a chloride ion, Cl- . There is a transfer of an electron from the sodium atom to chlorine atom. Formation of sodium ion Formation of chloride ion Formation of sodium

chloride + The positive sodium ion Na+ and the negative chloride ion Cl- are attracted by an electrostatic force. This force of attraction is very strong and it is the ionic bond.

Structure of Sodium Chloride Magnesium Fluoride Magnesium fluoride MgF2 is an ionic compound. The magnesium atom gives up two electrons to form a magnesium ion Mg2+. The two electrons are

transferred to two fluorine atoms to form two fluoride ions F-. The magnesium fluoride has the formula MgF2. Each unit of magnesium fluoride consists of one magnesium ion and two fluoride ions. Structure of Magnesium Fluoride Ionic Compound

An ionic compound is one, which consists of an ionic bond. An ionic compound usually consists of a metal and a non metal. Ionic compounds generally consist of a combination of metals with non metals. Strong metallic elements form ionic compounds with non-metallic elements. The rules for forming ionic compounds are the same as the rules for forming ionic bonds.

What is an Ionic Compound? Ionic bond is formed by the transference of one or more electrons from one atom to other. This type of bond usually comes into existence between a metal and a non metal element. The metallic atom loses one or more electrons and becomes a positive ion or cation,

while the non metal counterpart gains the electrons lost by the metal atom and becomes negatively charged, to form an anion. The negative ion and the positive ions, thus formed attract each other and stay together by electrostatic attraction. Thus, an ionic bond is defined as the electrostatic force of attraction holding the oppositely charged ions. Formation of ionic bond involves the following steps:

Formation of cation from the metal atom. Formation of anion from a non metal atom.

Association of oppositely charged ions due to electrostatic attraction. Since every atom has its own oxidation state, it is very much necessary to know the oxidation number exhibited by a metal (positive) or a non metal(negative), to write an ionic compound. It is also necessary to remember that the charge

on an ionic compound should always be zero. Therefore, the positive charge, should always cancel out the negative charge, and vice versa. Let us list out the oxidation states of some of the most popular elements: ELEMENT STATE Hydrogen Oxygen

Chlorine Fluorine Bromine Calcium Sulphur Nitrogen SYMBOL H O

Cl F Br Ca S N OXIDATION +1 -2

-1 -1 -1 +2 -2 -3

From the table, and by looking at the periodic table, it is generally understood that: All the first group elements IA, alkali metals have a +1 oxidation state. All second group elements (IIA) and IIIA (13) group elements have +2 and +3 oxidation states respectively.

Among non-metals from the nitrogen and oxygen group, oxidation states of -3 and -2 are highly prevalent. There are some discrepancies in this case, where oxidation states of elements or non metals present in these groups show a different oxidation state. Halogens, Cl, F, Br and I always show -1 oxidation state.

COVALENT BONDING Here we are going to discuss about covalent bonding. Can we imagine a world without oxygen, nitrogen, water or carbon dioxide? Without any of these there would be no life at all. All the above mentioned compounds are covalent compounds formed by sharing of electrons. Non-metal atoms share the valence electrons and form molecules. These shared pairs of electrons contributed by each of the atoms

involved in bonding are located between the nuclei of atoms. Covalent bonds arise from the sharing of electrons between two atoms. Simple examples are found in water and carbon dioxide. Each single covalent bond by definition consists of two shared electrons.

Double covalent bonds possess four shared electrons and triple covalent bonds, six shared electrons. The greater the multiplicity of the covalent bond, the greater the energy of the bond and the closer together of the

two bonded atomic nuclei. Covalent bonding may take place between atoms of the same element as in a hydrogen molecule or a chlorine molecule. It may also take place between atoms of different elements such as: (i) Hydrogen and chlorine combine to form hydrogen chloride (HCl). (ii) Carbon and oxygen combine to form carbon dioxide (CO2).

Covalent bonds are formed between non-metal atoms. Each of the atoms involved in bonding contribute one, two, three or more electrons to form the shared pair. EXAMPLES Formation of Hydrogen Molecule H

H electron present in K shell of the hydrogen atom Here the shared pair consists of two electrons, one electron being contributed by each of the hydrogen atoms. This is called a shared pair. Depending on the number of electron

pairs shared between atoms which participate in bonding, covalent bonds are classified as follows: Examples of Covalent Bonding There are many examples of compounds having covalent bonds, including the gases in our atmosphere, common fuels and most of the

compounds in our body. The molecules and ions just mentioned are composed entirely of nonmetals atoms. A point that needs special emphasis is that in molecules or ions made up only of nonmetals atoms, the also are attached by covalent bonds. Methane molecule (CH4) The electronic configuration of carbon is 2,4. It needs 4 more electrons in its outer shell to be like the noble gas neon. To do this one

carbon atom shares four electrons with the single electrons from four hydrogen atoms. The methane molecule has four C-H single bonds. Water molecule (H2O) one oxygen atom joins with two hydrogen atoms. The water molecule has two O-H single bonds.

-- - Carbon dioxide (CO2) one carbon atom joins with two oxygen atoms. The carbon dioxide molecule has two C=O bonds. Isotopes

The number of protons in the nucleus is known as the atomic/proton number of the particular element. It is conventionally represented by the symbol Z. It is the proton/atomic number (Z) that determines the number of electrons, specific electron structure and the specific identity of an element in terms of its physical and chemical properties. Since an atom having no charge, means the number of electrons must be equal to the number of protons that is the atomic number. Hence, in an atom the

atomic number is also equal to the number of electrons. All atoms have a certain value of mass number which is derived as follows. Hence, the sum of number of proton and neutron is known as mass number. The number of neutrons, N, is known as the number of neutrons in an atom. Thus, A=Z+N

The mass of protons and neutrons have approximately same, hence the atomic mass of an atom is equal to A. On the basis of atomic number and mass number, elements can be classified as isotopes, isobars and isotones. Lets discuss about isotopes with some common examples. ISOTOPES DEFINITION "Different kinds of atoms of the same element which have the same atomic

number but different mass numbers or atomic masses (or atomic weights) are called isotopes of that element." As isotopes have the same atomic number, we may define it as: "Atoms of the same element with same number of protons, but a different number of neutrons in their respective nuclei." CHARACTERISTICS OF ISOTOPES

The isotopes of any element will have the same number of valence electrons or valency, resulting in identical chemical properties. The isotopes' physical properties are different mainly due to the neutron number variation, present in the

nucleus. Properties such as melting point, boiling point, density etc., which depend upon the atomic mass should be different for different isotopes because the isotopes of an element have different masses. Cl - 35.5 Cu = 63.5

Atomic Mass of Isotopes Atomic mass = Mass of neutrons + Mass of of an protons isotope Isotopes of Hydrogen

ISOTOPES OF CHLORINE ISOTOPES OF CARBON Since isotopes of an element have the same atomic number, each of these isotopes contains equal numbers of protons in the nucleus and an equal number of electrons

revolving in different orbits around the nucleus. Now, since they have different mass numbers, they have a different number of neutrons in their nuclei. Thus, the number of protons (p), neutrons (n) and electrons (e) in an isotope with atomic number, Z and mass number A is given by these following relations.

Number of protons = p = Z Number of neutrons = n = A - Z Number of electrons = e = Z USES OF ISOTOPES

There are several applications of Isotopes. Radioisotopes are used as radioactive tracers. Radioactive isotopes have a property by which they can be easily detected and estimated quantitatively. They are also used in studying the reaction mechanisms of complicated reactions like photosynthesis, hydrolysis of esters, etc. Radioactive isotopes are used as a tracer to

diagnose many diseases. This is a very important use of radioisotope. The presence and location of a brain tumor, to detect the circulation of blood, to check the pumping action of blood, function of thyroid gland, etc. can be found with the help of radioisotopes. Apart from the radioisotopes, Isotopes of

carbon have been used in carbon dating, a phenomenon for detecting the age of wood. Similarly, isotopes of Hydrogen, Nitrogen and oxygen finds use in biological systems.


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