BASIC SCIENCE DIGITAL TEXT

RAHUL S R PHYSICAL SCIENCE 18221971008

STANDARD 8

BASIC SCIENCE

In this book we are going to learn about the basic constituents of matter. We willstudy in detail about elements, compounds, atoms and molecules.

TABLE OF CONTENTS

Pure substance: elements and compounds

6-7

Elements: Name and Symbols

7-11

Atoms and Molecules

11-15

Method of representing elements and compounds

15-17

Dalton’s Atomic Theory

17-18

Particles Smaller Than Atoms

18-22

Atomic Number and Mass Number

Isotopes, Isobarand Isotone

23-26

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BASIC CONSTITUENTS OF MATTER

The earth in which we live is rich in diverse substances. Most of these substances which occur in different physical states, are mixtures and a few others are pure substances. Classify the substances familiar to you into mixtures and pure substances. Do you see such substances in the above picture? Matter can be classified into two broad categories: mixtures and pure substances. Mixtures are physical combinations of two or more substances. The term “physical combination” refers to mixing together different substances that do not chemically react with each other. The physical appearance of the substances may change, but the atoms in the substances do not.

In comparison, a pure substance is a form of matter that has a constant composition and constant properties throughout the sample. Elements and compounds are both example of pure substances. Now let’s do an experiment. Fill one quarter of a boiling tube with sugar and close it with cotton. Then heat the boiling tube strongly (Figure 1). What can be observed? What is seen on the sides of the boiling tube? Write down below. Figure (1) • K • , What are the substances obtained by heating sugar?

Now you know that carbon and water are the components of sugar. Carbon obtained by heating sugar cannot be further divided. But, in 1806 Sir Humphry Davy discovered that when electricity is passed through water it splits into hydrogen and oxygen. Sugar, a pure substance can be further divided into carbon, hydrogen and oxygen. Similarly, another pure

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substance, water can be divided into hydrogen and oxygen. At the same time, pure substances like carbon, hydrogen, oxygen, gold and silver cannot be further divided into its components by chemical reactions. Now you realize that pure substances are of two types. We can study more aboutthis.

Pure Substance: Elements and Compounds Pure substances are classified into elements and compounds. The pure substances which cannot be further decomposed through chemical processes are called elements. Some of the examples of elements are hydrogen, oxygen etc. Find some more examples for elements. 1. 2. 3. Compounds are pure substances formed from two or more elements through chemical combination. Compounds can be converted into its constituent elements through suitable chemical processes. Some examples of compound are sugar, water etc.

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Find some examples for water. 1. 2. 3.

Elements: Name and Symbols: So far, scientists have discovered or created 118 different types of atoms. Scientists have given a name to each different type of element and organized them into a chart called the periodic table. As you can see in the table below, each square contains one of the elements.

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Periodic Table: Each element not only has its own name, it also has its own symbol. Scientists use abbreviations called chemical symbols to represent the elements. Many of these symbols are the first one or two letters of the modern name of the element. The first letter of a chemical symbol must always be a capital letter, and the second letter, when there is a second letter, must always be a lowercase letter. Table below shows some examples of elements and their symbols. Elements

Symbols

Hydrogen

H

Oxygen

O

Calcium

Ca

Aluminium

Al

Activity 1: Find out more examples from periodic table, the elements having symbols from first one or two letters of their name.

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As seen in Table below, the symbols for some of the elements consist of the first letter of the name and another letter (not the second letter) that comes later in the name. Elements

Symbols

Zinc

Zn

Magnesium

Mg

Zirconium

Zr

Chlorine

Cl

Arsenic

As

Activity 2: Find out more examples from periodic table, the elements having symbols from first letter of their name and another letter that came later in the name. For other elements, the symbols were already used for other elements. When trying to decide on a symbol for silver, for example, the symbol S was already used for Sulphur, and the symbol Si was already used for silicon. Since silver has been known to man for over a thousand years, it had a Latin name from ancient times. The old Latin name for silver was argentum, so it was decided

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that the symbol for silver would be Ag. There are a number of symbols chosen in this same manner, as seen in Table below. Examples of Elements Whose Symbol Comes from Latin Elements

Ancient Name

Symbols

Silver

Argentum

Ag

Potassium

Kalium

K

Gold

Aurum

Au

Sodium

Natrium

Na

Lead

Plumbum

Pb

Copper

Cuprum

Cu

Naming of elements: In the past, the elements were named after places, countries, continents, characteristics, properties, scientists, planets, satellites etc. Elements

The basis of naming

Polonium

Poland

-

Country

Curium

Marie Curie -

Scientist

Chromium

Chrome

Colour

10

-

Indium

Indigo

-

Colour

Chlorine

Chloro

-

Colour

Neptunium

Neptune

-

Planet

Europium

Europe

-

Continent

When we look into the periodic table you can find many such interesting facts! Check the periodic table and find out about the basis for the name of each element.

ATOMS AND MOLECULES: Let us see which is the smallest particle of an element. The element carbon is made up of extremely small particles. Think of breaking a piece of carbon into smaller and smaller particles. When it is broken up in this manner, we get the smallest particle which retains all the characteristic properties of carbon at the end. This smallest particle is called an atom of carbon. The other elements are also made up of their atoms. Atom is the smallest particle which shows all the characteristic properties of an element. Atoms of some gaseous elements like helium, neon etc., can exist as free single atoms. (figure 2) 11

Figure 2

Atoms are very small in size and smaller than anything we can imagine or compare with. Atomic radius is measured in nanometers (nm). 1nm= 10-9 m. But atoms of some other gaseous elements like hydrogen, oxygen etc., can exist only as a combination of two atoms (figure 3). Atoms of some elements are seen to exist as a combination of more than two atoms (figure 4). Example P4 with four phosphorous atoms and S8 with 8 Sulphur atom.

Hydrogen molecule

Oxygen molecule

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(Figure 3)

(figure4) Phosphorous molecule

The smallest particles which can exist independently are called molecules. Molecules are the smallest particles which can exist independently.

Method of Representing Atoms and Molecules:

You have studied the method of assigning symbols for the elements. Now, let us see how the atoms and molecules are represented. The symbol of helium is 'He'. When we write 'He', it represents one atom of the element helium. What does 2He represent? ......................................

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Yes, the number on the left side of the element represents the number of atoms present. Hence 2He means two helium element is present. Molecules can be divided into three: • Monoatomic molecule - Molecules with only one atom are called monoatomic molecules. • Diatomic molecule

- Molecules with two atoms each are called

diatomic molecules. • Polyatomic Molecules - Molecules with more than two atoms are called polyatomic molecules. Elements like helium, neon, argon etc., are seen in nature as single atoms. They are monoatomic molecules. Their molecules can also be represented as He, Ne, Ar etc. But elements like hydrogen exist as diatomic molecules. Hydrogen atom is represented as H, and how about its molecule?............. Yes, hydrogen molecule can be represented as H2. In monoatomic molecules, the number given on the left side of the symbol indicates the number of molecules and atoms. In polyatomic molecules, the subscript on the right side of the symbol indicates the number of

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atoms within one molecule. The total number of molecules is indicated on the left side. Hence polyatomic molecule like oxygen can be represented as O3.

Activity 3: Classify the following molecules into monoatomic, diatomic and polyatomic. H2, Cl2, P4, O2, S8, He, Ar

Method of Representing Compound: We all know that molecules are formed from the same type of atoms. Likewise, compound is formed by the combination of two or more different atoms. Given below is the structure of water molecule:

• How many hydrogen atoms are present in water molecule? 15

• How many oxygens atom is present in water molecule? We can see that there are two hydrogen and one oxygen atom in water molecule. Hence, we represent water molecule as H2O. Whatever be the source of a compound, a fixed ratio is maintained between the atoms of the different elements present in it. In water molecule the ratio of hydrogen and oxygen atom will be 2:1. This type of representation is known as the chemical formula of the compound. The ratio of each atom is represented as subscript in the chemical formula.

Activity 4: Complete the following table: Molecules

Atoms Present

Carbon dioxide

Carbon- 1

Chemical Formula

Oxygen- 2 Water Ammonia

H2 O Nitrogen- 1 Hydrogen- 3

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Sodium chloride

NaCl

In this chapter we already learn about elements, compounds and about atoms and molecule. And we now know that molecules can be subdivided into minute particles called atoms. An atom is the basic building block of chemistry . It is the smallest unit of matter that has the characteristic properties of a chemical element. So, it is necessary to understand more about atoms to understand the world around us! John Dalton proposed the atomic theory in 1807 for the study on atoms and for finding on how matter has been formed.

Dalton’s Atomic Theory: The views put forward by John Dalton were based on logical thinking. These were not supported by any experimental observations or scientific evidences. Yet this theory could logically explain the basis idea regarding

the

formation

of

substances and its characteristics.

John Dalton 17

Major Ideas of Atomic Theory • All matter is made up of tiny, indivisible particles called atoms. • All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different element exhibit different properties and vary in mass and size. • Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles. • Atoms of different elements can combine with each other in fixed whole-number ratios in order to form compounds. • Atoms can be rearranged, combined, or separated in chemical reactions

Particles Smaller than Atoms: Even when the atomic theory existed, studies on matter and inquiries on the possibilities of dividing atom had been continuing. The observation and results of numerous experiments conducted by scientists have contradictory to that of atomic theory.

1. J J Thomson – Cathode ray Experiment: J J Thomson’s experiments helped in changing the ideas about an atom that existed at that time and in framing new concepts. He conducted the 18

experiment in a cathode ray tube. For the cathode ray tube J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flow through the air from the negative electrode to the positive electrode. In the experiment, a beam of particles called cathode rays originated from the cathode. When the cathode rays were passed through the charged plates, they got deflected towards the positively charged plate. This indicated that the cathode ray particles were negatively charged. These negatively charged particles were later termed as electrons. He also made it clear that these particles possess mass and energy.

2. Ernest Rutherford – Gold Foil Experiment:

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The discovery of electron energized the search for other particles in an atom. Even though the presence of positive charge in substances has been identified earlier, it is Rutherford who proved it authentically.

Ernest Rutherford

In this experiment a piece of gold foil was hit with alpha particles, which have a positive charge. Most alpha particles went right through. This showed that the gold atoms were mostly empty space. Some particles had their paths bent at large angles. A few even bounced backward. The only way this would happen was if the atom had a small, heavy region of positive charge inside it. This central part inside an atom where all the positive charge is concentrated Is called the nucleus. On the basis of his further experiments, in 1920, it was proved that particles that were responsible for the positive charge are protons. It is determined that the mass of a proton is equal to the mass of a hydrogen atom. Along with that, he also predicted the presence of a chargeless particle in the nucleus. 20

Gold Foil Experiment

3. James Chadwick – Discovery of neutron: In 1932, the physicist James Chadwick conducted an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium. The resulting radiation showed high penetration through a lead shield, which could not be explained via the

(James Chadwick) particles known at that time. Then he proved scientifically that, inside the nucleus of an atom, there are neutral particles having mass equal to that of protons. This chargeless particle is the proton. 21

Now we understand that atoms can further subdivide into proton, electron and neutron. The difference in the number of these particles make an element different from that of others.

Features of Electron, Proton and Neutron

Electron

Proton

Negative charge

Positive charge

Neutron Chargeless

Mass is very less. (1/1837 Mass equal to that of

Mass equal to that of

part of the mass of hydrogen atom.

hydrogen atom.

hydrogen atom.) Seen

outside

the Seen in the nucleus.

Seen in the nucleus.

nucleus.

We have studied about the fundamental particles of an atom. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J J Thomson was the first to propose an atomic model. Let’s take a look.

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ATOMIC NUMBER AND MASS NUMBER: The atomic number of an element is determined by the number of protons in it, and it is used to differentiate one element from another. The mass number of an element is determined by the number of protons and neutrons combined. The modern periodic table is arranged in such a way that all the elements have an increasing atomic number, and subsequently, increasing mass number. We can now study in detail about this. Well, as you know, an atom consists of electrons, protons and neutrons. What is atomic number? • The total number of protons in the nucleus of an atom gives us the atomic number of that atom. • It is represented with the letter ‘Z.’ • All the atoms of a particular element have the same number of protons, and hence the same atomic number. • Atoms of different elements have different atomic numbers.

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For example, all carbon atoms have the atomic number of 6, whereas all atoms of Oxygen have 8 protons in their nucleus. What is mass number? • The number of protons and neutrons combine to give us the mass number of an atom. • It is represented using the letter ‘A.’ • As both protons and neutrons are present in the nucleus of an atom, they are together called nucleons. • The number of protons remains the same in all atoms of an element, the number of neutrons can vary. • The weight of an electron is almost negligible. Thus, the atomic mass of an atom is almost the same as its mass number. For example, an atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is 12. ➢ If you know the atomic number of an atom the number of which all particles can be found? Give reasons. …………………………………………………………………………………………………………………….

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Atomic Number

= Number of protons = Number of electrons

Mass Number

= Number of protons + Number of neutrons

Number of neutrons = Mass Number – Atomic Number

Notation of Atom: To write the notation of an atom, we need to know the symbol of the element, the atomic number and the mass number. The mass number of the atom goes above the symbol and the atomic number is written as a subscript.

So, the notation of carbon is,

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Activity 5: An atom has an atomic number of 9 and a mass number of 19. • Determine the number of protons present • Determine the number of neutrons present • Determine the number of electrons present

ISOTOPES: Isotopes are atoms with the same number of protons, but differing numbers of neutrons. A number of protons in the atom is the atomic number of that atom. A particular chemical element has a fixed number of protons. Hence, the

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atomic number of the atoms of the same chemical element is similar to each other. Therefore, isotopes are atoms of the same chemical element. The total number of protons and neutrons is known as the atomic mass. Isotopes have different atomic masses. The chemical behaviors of isotopes of a chemical element are identical, but the physical properties are different from each other. Almost all the chemical elements have isotopes. There are 275 known isotopes of 81 stable chemical elements. Atoms of the same element having the same atomic number but different mass number are called isotopes.

Protium, Deuterium and Tritium are isotopes of hydrogen. See how they are represented using symbols:

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Let’s look the isotopes of carbon:

ISOBARS: Isobars are atoms of different chemical elements having equal values for atomic mass. Atomic mass is the sum of protons and neutrons in the nucleus of an atom. A proton or a neutron is known as a nucleon. Therefore, isobars have the same number of nucleons. The atomic numbers of these isobars are different from each other because different chemical elements have different atomic numbers.

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Isobars are atoms of different chemical elements having equal values for atomic mass.

ISOTONES: Isotones are atoms of different elements having an equal number of neutrons in the atomic nucleus. Isotones have different atomic numbers (number of protons in the nucleus is different from each other) as well as different atomic masses. It can be expressed as below; Atomic number = Z Atomic mass = A

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Number of neutrons = N For all isotones in one series, A≠Z but (A-Z) = N (N is equal for all the isotones in one series).

In all these elements the number of neutrons is equal. Hence, they are isotones. Isotones are atoms of different elements having an equal number of neutrons in the atomic nucleus.

Difference Between Isotope, Isobar and Isotone:

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Atomic Number: Isotopes -

Isotopes have the same atomic number.

Isobars -

Isobars have different atomic numbers.

Isotones - Isotones have different atomic numbers.

Atomic Mass: Isotopes -

Isotopes have a different atomic mass.

Isobars -

Isobars have the same atomic mass.

Isotones - Isotones have different atomic mass.

Number of Neutrons: Isotopes -

Isotopes have different numbers of neutrons.

Isobars - Isobars have different numbers of neutrons. Isotone - Isotones the same number of neutrons. In this chapter you have familiarized the concepts regarding constituents of matter which include elements, compounds, atom, molecules and the detailed study about the structure of atoms. More details regarding the structure of atom can be studied in higher classes.

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Extended activities: 1. Classify the following into elements and compounds. Ammonia, sugar, nitrogen, mercury, sodium chloride, water, copper sulphate, sodium, carbon. 2. Choose the correct symbol of the elements given below from the box. (Calcium, carbon, sodium, potassium, Iron, silver) (S, C, Na, Ca, Si, Ag, In, P, K, Fe) 3. The chemical formula of calcium bisulphate is Ca (HSO4)2. Write down the number of atoms of each element present in this molecule. 4. Na. This is the symbol of an element. Which statement given below is wrong. Justify? • Symbol of Nitrogen • Indicates one atom.

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5. We know about elements and compounds. Find out the odd one from the following. Give reason? a. (A) Hydrogen (B) Oxygen (C) Nitrogen (D) Carbon dioxide b. (A) Sodium (Na), (B) Potassium (K) (C) Magnesium (Mg), (D) Iron (Fe) c. Write the molecular formula of carbon dioxide? d. What is the Latin name of sodium? 6. Arrange the following from smallest to largest. Molecule, substance, atom 7. Symbol of Nitrogen is ‘N’. a. What represent symbols of 2N and N2? b. How many atoms are in 2NH3? c. How can we represent 5 water molecules? 8. Find the monoatomic molecule from given elements. a. (A) Hydrogen (B) Oxygen (C) Nitrogen (D) Helium b. Write the symbols of Hydrogen and Helium? c. Write chemical formula of Oxygen molecule?

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9. If number of electrons in an atom is 8 and number of protons is also 8, then what is the atomic number of the atom? 10. Explain with examples: (i) Atomic number

(ii) Mass number

(iii) Isotopes and

(iv) Isobars.

11. Isotopes of an element have (a) the same physical properties (c) different number of neutrons (b) different number of neutrons (d) different atomic numbers.

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Notes: ……………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………….. ……………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………….. ……………………………………………………………………………………………………………………… ………………………………………………………………………………………………………………………

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