Atomic Structure Online Test 8th Science Lesson 12 Questions in English

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Every substance in our surrounding is made up of unique elements. There are 118 elements identified worldwide so far.

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Copper, Iron, Gold, Titanium, Germanium and Silver are some of the elements found in the nature. Elements like Technetium, Promethium, Neptunium and Plutonium are synthesised in the laboratories.

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Out of these 118 elements, 92 elements occur in the nature and the remaining elements are synthesised in the laboratories.

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Each element is made up of similar, minute particles called atoms. For example, the element gold is made up of similar atoms which determine its characteristics. The word atom is derived from the Greek word atomas. Tomas means smallest divisible particle and atomas means smallest indivisible particle

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Our Tamil poet Avvaiyar has mentioned about atoms in her poem while describing Thirukkural (அணுவைத் துளைத்து ஏழ் கடலைப்புகட்டிக் குறுகத் தரித்த குறள்).

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Ancient Greek philosophers like Democritus have spoken about atoms.

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The first scientific theory about atom was given by John Dalton. Followed by him, J.J. Thomson and Rutherford have given their theory about atom.

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John Dalton provided a basic theory about the nature of matter. He proposed a model of atom known as Dalton’s atomic theory in 1808 based on his experiments.

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Atoms of different elements have different sizes and masses and possess different properties. Atoms can neither be created nor be destroyed. i.e., atom is indestructible.

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Dalton’s theory explains most of the properties of gases and liquids. Atom is no longer considered as the smallest indivisible particle.

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If atoms have the same element but have different masses it is known as Isotopes.

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If atoms have the different elements but have same masses it is known as Isobars.

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In 1878, Sir William Crookes, while conducting an experiment using a discharge tube, found certain visible rays travelling between two metal electrodes.

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The Crookes rays are also known as Cathode rays. The discharge tube used in the experiment is now referred as Crookes tube or more popularly as Cathode Ray Tube (CRT).

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The electrode which is connected to the negative terminal of the battery is called the cathode (negative electrode). The electrode connected to the positive terminal is called the anode (positive electrode). There is a side tube which is connected to a pump. The pump is used to lower the pressure inside the discharge tube.

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Electricity, when passes through air, removes the electrons from the gaseous atoms and produces ions. This is called electrical discharge.

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The fact that air is a poor conductor of electricity is a blessing in disguise for us. Imagine what would happen if air had been a good conductor of electricity. All of us would have got electrocuted, when a minor spark was produced by accident.

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In television tube cathode rays are deflected by magnetic fields. A beam of cathode rays is directed toward a coated screen on the front of the tube, where by varying the magnet field generated by electromagnetic coils, the beam traces a luminescent image

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The nature of the cathode rays does not depend on the nature of the gas filled inside the tube or the
cathode used

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When invisible radiation falls on materials like zinc sulphide, they emit a visible light (or glow). These materials are called fluorescent materials.

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The presence of positively charged particles in the atom has been precisely predicted by Goldstein based on the conception that the atom being electrically neutral in nature, should necessarily possess positively charged particles to balance the negatively charged electrons.

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Goldstein repeated the cathode ray experiment by using a perforated cathode. On applying a high voltage under low pressure, he observed a faint red glow on the wall behind the cathode. Since these rays originated from the anode, they were called anode ray.

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Anode rays are also called as canal rays or positive rays. Anode rays were found as a stream of positively charged particles.

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The mass of the particle is the same as the atomic mass of the gas taken inside the discharge tube.

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When hydrogen gas was taken in a discharge tube, the positively charged particles obtained from the hydrogen gas were called protons. Each of these protons are produced when one electron is removed from one hydrogen atom. Thus, a proton can be defined as a hydrogen ion (H⁺ ).

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At the time of J.J. Thomson, only two fundamental particles (proton and electron) were known. In the year 1932, James Chadwick discovered another fundamental particle, called neutron. But, the proper position of these particles in an atom was not clear till Rutherford described the structure of atom.

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The mass of proton is 1.6 × 10^–24 grams. Its relative charge is +1.

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J.J. Thomson, an English scientist, proposed the famous atom model in the year 1904, just after the discovery of electrons.

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Thomson proposed that the shape of an atom resembles a sphere having radius of the order of 10^{– 10} m.

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The positively charged particles are uniformly distributed with electrons arranged in such a manner that the atom is electrically neutral. Thomson’s atom model was also called as the plum pudding model or the watermelon model. The embedded electrons resembled the seed of watermelon while the watermelon’s red mass represented the positive charge distribution. The plum pudding atomic theory assumed that the mass of an atom is uniformly distributed all over the atom.

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In order to understand valency of elements clearly, we need to learn a little about Rutherford’s atom model here. According to Rutherford, an atom consists of subatomic particles namely, proton, electron and neutrons.

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Protons and neutrons are found at the centre of an atom, called nucleus. Electrons are revolving around the nucleus in a circular path, called orbits or shells.

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An atom has a number of orbits and each orbit has electrons. The electrons revolving in the outermost orbits are called valence electrons.

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The arrangement of electrons in the orbits is known as electronic configuration. Atoms of all the elements will tend to have a stable electronic configuration, that is, they will tend to have either two electrons (known as duplet) or eight electrons (known as octet) in their outermost orbit.

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When molecules are formed, atoms combine together in a fixed proportion because each atom has different combining capacity. This combining capacity of an atom is called valency. Valency is defined as the number of electrons lost, gained or shared by an atom in a chemical combination so that it becomes chemically inert.

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Helium has two electrons in the outermost orbit and so it is chemically inert. Similarly, neon is chemically inert because, it has eight electrons in the outermost orbit. sodium atom (Atomic number: 11) has one electron in its outermost orbit and in order to have stability it loses one electron and becomes positively charged.

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All non-metals will have 4 to 7 electrons in the outermost orbit of their atoms. In order to attain stable electronic configuration, they need few electrons. They accept these electrons from other atoms in a chemical reaction and become negatively charged. These atoms which accept electrons are said to have negative valency.

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Chlorine atom (Atomic number: 17) has seven electrons in its outermost orbit. By gaining one electron it attains stable electronic configuration. Thus, chlorine has negative valency.

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Neutron is a neutral particle, that is, it carries no charge. It has mass equal to that of a proton, that is 1.6 × 10^–24 grams.

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Since valency of chlorine is one, the number of chlorine atoms with which one atom of an element can combine is called valency. In sodium chloride (NaCl) molecule, one chlorine atom combines with one sodium atom. So, the valency of sodium is one. But, in magnesium chloride (MgCl2) valency of magnesium is two because it combines with two chlorine atoms.

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Copper, mercury, tin and iron are elements with variable valency.

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The mass of Electron (e) is 9.1 × 10^–28 grams. Its relative charge is –1.

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In an atom, the number of protons is equal to the number of electrons and so the atom is electrically neutral. But, during chemical reactions unstable atoms try to attain stable electronic configuration (duplet or octet) either by gaining or losing one or more electrons.

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When an atom gains an electron, it has a more number of electrons and thus it carries negative charge. At the same time when an atom loses an electron it has more number of protons and thus it carries positive charge. These atoms which carry positive or negative charges are called ions.

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The number of electrons gained or lost by an atom is shown as a superscript to the right of its symbol. When an atom loses an electron, ‘+’ sign is shown in the superscript and ‘–’ sign is shown if an electron is gained by an atom. Sometimes, two or more atoms of different elements collectively loss or gain electrons to acquire positive or negative charge. Thus, we can say, an atom or a group of atoms when they either loss or gain electrons, get converted into ions or radicals.

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Ions are classified into two types. They are: cations and anions.

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If an atom loses one or more electrons during a chemical reaction, it will have more number of positive charge on it. These are called cations (or) positive radicals. Sodium atom loses one electron to attain stability and it becomes cation. Sodium ion is represented as Na+.

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A chemical compound is a substance formed out of more than one element joined together by chemical bond. Such compounds have properties that are unique from that of the elements that formed them.

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In naming a compound containing a metal and a non-metal, the name of the metal is written first and the name of the non-metal is written next after adding the suffix-ide to its name. Examples: NaCl – Sodium chloride Ag Br – Silver bromide.

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A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formulae. Every chemical equation has two components: reactants and products. Reactants are the substances that take part in a chemical reaction and the products are the substances that are formed in a chemical reaction. Before writing the balanced equation of a chemical reaction, skeletal equation is written.

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According to law of conservation of mass, the total mass of all the atoms forming the reactants should be equal to that of all the atoms forming the products. This law will hold good only when the number of atoms of all types of elements on both sides is equal. A balanced chemical equation is one in which the total number of atoms of any element on the reactant side is equal to the total number of atoms of that element on the product side.

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There are many methods of balancing a chemical equation. Trial and error method (direct inspection), fractional method and odd number-even number method are some of them.

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When two or more elements occur same number of times, the metallic element is balanced first in preference to non-metallic element. If more than one metal or nonmetal is present then a metal or non-metal with higher atomic mass (refer periodic table to find the atomic mass) is balanced first.

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A balanced chemical equation gives us both qualitative and quantitative information.

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A balanced chemical equation gives us qualitative information’s such as the names, symbols and formulae of the reactant molecules taking part in the reaction and those of the product molecules formed in the reaction. We also can get quantitative information like the number of molecules/ atoms of the reactants and products that are taking part in the reaction.

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The limitation of chemical equation are i) Physical state of the reactants and the products. ii) Heat changes (heat liberated or heat absorbed) accompanying the chemical reaction. iii) Conditions such as temperature, pressure, catalyst etc., under which the reaction takes place. iv) Concentration (dilute or concentrated) of the reactants and products. V) Speed of the reaction.

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By studying quantitative measurements of many reactions, it was observed that the reactions taking place between various substances are governed by certain laws. They are called as the ‘Laws of chemical combinations. They are given below. 1. Law of conservation of mass 2. Law of constant proportion 3. Law of multiple proportions 4. Gay Loussac’s law of gaseous volumes.

Explanation

The law of conservation of mass which relates the mass of the reactants and products during the chemical change was stated by a French chemist Lavoisier in 1774. It states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

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The law of conservation of mass means that mass can neither be created nor be destroyed during any chemical reaction. This law is also known as Law of indestructibility of mass.

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Law of constant proportions was proposed by the scientist Joseph Proust in 1779. He states that in a pure chemical compound the elements are always present in definite proportions by mass.

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Similarly, the mode of preparation of compounds may be different but their composition will never change. It will be in a fixed ratio. Hence, this law is also known as ‘Law of definite proportions.