Ans: An atom is the smallest particle of an element which can take part in a chemical reaction.
2. What is atomic number?
Ans: The number of electrons or the number of protons in an atom is called atomoic number.
3. What is an atom?
Ans: An atom is the smallest particle of an element which can take part in a chemical reaction.
4. What is atomic number?
Ans: The number of electrons or the number of protons in an atom is called atomoic number.
5. What is atomic mass?
Ans: The sum of protons and electrons in an atom is called atomic mass.
6. What is electronic configuration?
Ans: The distribution of electrons in different orbits is called electronic configuration.
7. What is a molecule?
Ans: A molecule is the smallest particle of a substance which exists independently.
8. What does the Modern period law states?
Ans: Modern period law states that the properties of elements are the period function of their atomic number.
9. What is chemical change?
Ans: A permanent change in which new substances are formed is called chemical change.
10. What are periods?
Ans: The horizontal rows in a periodic table are called periods.
11. What is compound?
Ans: A compound is the pure substance formed by the chemical combination of two or more elements in a fixed proportion.
12. What is physical change?
Ans: Physical change is a temporary change in which no new substance is formed.
13. What are groups?
Ans: The vertical columns of the period table are called groups.
14. What is Mendeleev period law?
Ans: Mendeleev period law states that “the physical and chemical properties of the elements are the periodic functions of their atomic masses”.
15. What is proton?
Ans: Proton is the smallest particle that has a unit of positive electric charge.
16. What is neutron?
Ans: Neutron is the chargeless particle which is situated at the nucleus of an atom.
17. What is molecular weight?
Ans: The sum of the atomic weights of all the atoms of the same or different elements present in a molecule is called molecular weight.
18. What is period table?
Ans: The systematic arrangement or classification of all the elements on the basis of their similar properties in the tabular form is called periodic table.
19. An atom is electrically neutral. Why?
Ans: An atom is electrically neutral because the number of protons is equal to the number of electrons.
20. What is classification of elements?
Ans: The grouping of elements according to their similar characteristics is called classification of elements.
21. What are the differences between atom and molecule?
Ans: The differences between atom and molecule are;
| Atom | Molecule | |
|---|---|---|
| Definition | An atom is the smallest particle of an element which can take part in a chemical reaction. | A molecule is the smallest particle of an element or a compound that can exist independently. |
| Weight | The atomic weight of an atom is the sum of total number of protons and neutrons present in the atom. It is called atomic weight. | The molecular weight of a molecule is the sum of atomic weight of the atoms present in the molecule. It is called molecular weight. |
22. What are the differences between neutron and proton?
Ans: The differences between neutron and proton are;
| Neutron | Proton | |
|---|---|---|
| Definition | A neutron is a neutral sub-atomic particle which is present inside the nucleus of an atom. | A proton is a positively charged sub-atomic particle which is present inside the nucleus of an atom. |
| Charge | A neutron is neutral, so it has no charge. The charge in neutron is equal to 0 coulombs. | A proton is positively charged. The charge in a proton is equal to 1.60×10-19 coulombs. |
23. What are the differences between neutron and electron?
Ans: The differences between neutron and electron are;
| Neutron | Electron | |
|---|---|---|
| Definition | A neutron is a neutral sub-atomic particle which is present inside the nucleus of an atom. | An electron is a negatively charged sub-atomic particle which is present outside the nucleus of an atom; it is in the shell of an atom. |
| Denotation | A neutron is usually denoted by no. | An electron is usually denoted by n-. |
| Charge | A neutron is neutral, so it has no charge. The charge in a neutron is equal to 0 coulombs. | An electron is negatively charged. The charge in an electron is equal to – 1.60 ×10-19 coulombs. |
| Mass | The mass of a neutron is equal to 1 amu; which is equal to 1.67 × 10-24grams. | The mass of an electron is equal to 1/1837 amu; which is equal to 9.1 × 10-28 grams. |
24. What are the differences between proton and electron?
Ans: The differences between proton and electron are;
| Proton | Electron | |
|---|---|---|
| Definition | A proton is a positively charged sub-atomic particle which is present inside the nucleus of an atom. | An electron is a negatively charged sub-atomic particle which is present outside the nucleus of an atom; it is in the shell of an atom. |
| Denotation | A proton is usually denoted by p+. | An electron is usually denoted by n-. |
| Charge | A proton is positively charged. The charge in a proton is equal to 1.60×10-19 coulombs. | An electron is negatively charged. The charge in an electron is equal to – 1.60 ×10-19 coulombs. |
| Mass | The mass of a proton is equal to 1 amu; which is equal to 1.67 × 10-24 grams. | The mass of an electron is equal to 1/1837 amu; which is equal to 9.1 × 10-28 grams. |
25. What are the differences between molecule of an element and molecule of a compoud?
Ans: The differences between molecule of an element and molecule of a compound are;
| Molecule of an element | Molecule of a compound | |
|---|---|---|
| Definition | A molecule of an element is the molecule that is formed by the combination of only one type of atoms. | A molecule of compound is the molecule that is formed by the combination of two or more different types of atoms. |
| Example | N2 is the molecule of nitrogen. O2 is the molecule of oxygen. And so on. | NH3 is the molecule of ammonia. H2O is the molecule of water. And so on. |
26. What are the differences between element and compound?
Ans: The differences between element and compound are;
| Element | Compound | |
|---|---|---|
| Definition | Element is defined as the purest substance that contains only one type of atoms. | Compound is defined as the purest substance that contains two or more different types of atoms. |
| Nature | An element cannot be broken down into simpler substances by any chemical methods. | A compound can be separated into simpler substances by any chemical methods. |
27. What are the differences between electropositive radical and electronegative radical?
Ans: The differences between electropositive radical and electronegative radical are;
| Electropositive radical | Electronegative radical | |
|---|---|---|
| Definition | Electropositive radical is defined as a radical having positive charges. | Electronegative radical is defined as a radical having negative charges. |
| Type | Electropositive radical is also called basic radical or metallic radical. | Electronegative radical is also called acidic radical or non-metallic radical. |
28. What are the differences between Mendeleev’s periodic table and modern periodic table?
Ans: The differences between Mendeleev’s periodic table and modern periodic table are;
| Mendeleev’s periodic table | Modern periodic table | |
|---|---|---|
| Definition | The table which is obtained by arranging elements on the basis of their increasing atomic mass is called Mendeleev’s periodic table. | The table which is obtained by arranging elements on the basis of atomic number is called modern periodic table. |
| Statement of law | Mendeleev’s law states, “Physical and chemical properties of elements are the periodic function of their atomic mass.” | Modern periodic law states, “Physical and chemical properties of elements are the periodic function of their atomic mass.” |
| Periods and groups | In Mendeleev’s periodic table, there are 7 horizontal rows; called periods and 8 vertical columns; called groups. | In modern periodic table, there are 7 horizontal rows; called periods and 18 vertical columns; called groups. |
| Nature | Mendeleev’s periodic table could not explain the valency or metallic characteristics. | Modern periodic table explained the gradual change in valency and the metallic characteristics of different elements; metals, non-metals and metalloids. |
29. What are the differences between physical change and chemical change?
Ans: The differences between physical change and chemical change are;
| Physical change | Chemical change | |
|---|---|---|
| Definition | The change in which the chemical composition of a substance remains the same is called physical change. | The change in which the chemical composition of a substance alters i.e. altered is called chemical change. |
| Properties | Physical change in a substance does not form any new substance, only the physical properties of the substance changes. | Chemical change in a substance forms new substance with entirely different chemical properties from before. |
| Nature | Physical change is a temporary change; it can be reversed easily. | Chemical change is a permanent change; it cannot be reversed easily. |
| Example | Melting of ice and boiling of water are some of the examples of physical change. | Rusting of iron and burning of wood are some examples of chemical change. |
30. What is the difference between radical and compound?
Ans: The difference between radical and compound is;
| Radical | Compound | |
|---|---|---|
| Definition | Radical is defined as an atom or group of atoms that act as a single unit having positive or negative charges on it. | Compound is defined as the purest substance that contains two or more different types of atoms. |
31. What is the difference between chemical reaction and chemical equation?
Ans: The difference between chemical reaction and chemical equation is;
| Chemical reaction | Chemical equation | |
|---|---|---|
| Definition | The exchange, combination or decomposition that occurs in the molecules of matters during a chemical change is called chemical reaction. | The symbolic representation of the chemical change in a chemical reaction in terms of symbols or formulae is called chemical equation. |
32. What are the differences between radical and compound?
Ans:
| Radical | Compound | ||
| 1. | Definition | Radical is defined as an atom or group of atoms that act as a single unit having positive or negative charges on it. | Compound is defined as the purest substance that contains two or more different types of atoms. |
33. What are the differences between chemical reaction and chemical equation?
Ans:
| Chemical reaction | Chemical equation | ||
| 1. | Definition | The exchange, combination or decomposition that occurs in the molecules of matters during a chemical change is called chemical reaction | The symbolic representation of the chemical change in a chemical reaction in terms of symbols or formulae is called chemical equation. |
34. Calculate the molecular weight of chalk. Write two advantages of Mendeleev's periodic table.
Ans: Here,
We have,
Atomic weight an atom = number of proton + number of neutron
Molecular weight of an compound = sum of masses of the combining atom
Molecular formula of chalk is CaCO3
Again,
Atomic weight of calcium (Ca) = 20 + 20 = 40 amu
Atomic weight of carbon (C) = 6 + 6 = 12 amu
Atomic weight of oxygen (3O) = 3 (8 + 8) = 48 amu
Finally,
Molecular weight of chalk (CaCO3) = 40 + 12+ 48 = 100 amu
Hence, molecular weight of chalk is 100 amu.
Advantages of Mendeleev's periodic table are as followings:
I. Systematic study of elements
In Mendeleev's periodic table, elements were classified in such a way that the elements with similar properties came to lie in the same vertical column. This helped in systematic study of elements.
II. Prediction of new elements
Mendeleev's periodic table helped in discovery of new element.
35. Draw the well labelled atomic stucure of the element having atomic number 18 and also write the valency of same element.
Ans: Argon is the element having atomic number 18. Its valency is 0, because its outer shell is already filled with 8 electrons and it does not combine with other atoms.
36. Calculate atomic weight of CaCO3. Define periodic table and why it is constructed?
Ans: Here,
Atomic weight of calcium (Ca) = 20 + 20 = 40 amu
Atomic weight of carbon (C) = 6 + 6 = 12 amu
Atomic weight of oxygen (3O) = 3 (8 + 8) = 48 amu
We have,
Molecular weight of CaCO3 = 40 + 12+ 48 = 100 amu
Hence, molecular weight of CaCO3 is 100 amu.
The systematic arrangement of all the elements on the basis of their properties is called periodic table.
Periodic table is constructed due to following reasons:
I. It helps us to memorize the characteristics of large number of elements in the form of few groups only.
II. New elements of known properties can be predicted.
37. What are the subatomic particles? What does 2n2 rule indicate?
Ans: Atoms are made up of three fundamental particles: electron, proton and neutron. These are called subatomic particles.
2n2 rule indicates maximum number of electrons in different shell (K, L, M, N.....) of an atom.
38. What is octet? Mention the number of groups and period in modern periodic table.
Ans: Presence of 8 electrons in valence shell of an atom is termed as octet. There are 18 groups and 7 periods present in the modern periodic table.
39. Differentiate between electron and proton by two points. What is electric charge?
Ans: Differences between electrons and protons are as followings.
| Protons | Electrons |
|---|---|
| They are positively charged particles. | They are negatively charged particles. |
| They are found in nucleus. | They revolve around nucleus. |
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.
40. Write the number of electrons, protons and neutrons of an element having atomic number 19. What is valency and write the valency of that atom.
Ans: The element having atomic number 19 is potassium (K)
Number of electrons = 19
Number of protons = 19
Number of neutrons = 20
The combining capacity of an element or radical with other element or radical to form molecule or compound is called valency.
Valency of potassium is 1.
41. Define octet rule. What is the molecular formula of Aluminium oxide?
Ans: The natural tendency of an atom of an element to make 8 electrons in its valence shell by gaining, losing or sharing electrons between the combining atoms during the formation of molecules is called octet.
The molecular formula of magnesium oxide is MgO.
42. Who was the inventor of periodic table? What is compound? Why water is called compound?
Ans: The inventor of modern periodic table is Henery Moseley.
A substance made from the chemical reaction of two or more atoms of elements in a fixed proportion by weight is called compound.
Water is called as compound because it is made from chemical reaction of hydrogen and oxygen atom in a fixed proportion by weight.
43. What is orbit? Generally, atom is assumed that it is chargeless, why?
Ans: The path traced by electrons around the nucleus is called orbit.
In an atom, positively charged protons and neutral neutrons are found in nucleus while negatively charged electrons revolve around nucleus in a fixed path i.e. orbit. As number of protons and number of electrons in an atom are equal and positive charge of proton and negative charge of electron cancel each other, atom is assumed chargeless.
44.
Study the given table and answer the following questions.
i. Write the name of elements X and Y.
ii. Why zero group elements inert gas and elements of group VIIA are most reactive in periodic table?
Ans: | Group | IA | IIA | IIIA | IVA | VA | VIA | VIIA | O |
| Period2 | Li | Be | X | C | N | O | F | Ne |
| Period3 | Na | Mg | Al | Si | P | S | Cl | Y |
i. Write the name of elements X and Y.
ii. Why zero group elements inert gas and elements of group VIIA are most reactive in periodic table?
i. X - Boron (B)
Y - Argon (Ar)
ii. Zero group elements have 8 electron is their valence shell (except Helium has 2 electrons in it valence shell). As their valence shell is already filled, they do no need to lose or gain electron i.e. they do not take part in chemical reaction. Hence, due to their stable electronic configuration, zero group elements are called inert gas.
Group VIIA elements have 7 electrons in their valence shell. They need only one electron to fulfill their octet. So, they easily gain one electron from other atom during chemical reaction and acquire unit negative charge. Hence, group VIIA elements are very active.
45. Write any one advantage of periodic table. Draw atomic structure of chlorine atom.
Ans: Advantage of periodic table-
It helps us to memorize the characteristics of large number of elements in the form of few groups only.
Fig: Atomic structure of chlorine
46. Write any one difference between mixture and compound. Mention the charge of proton and electron.
Ans: Difference between mixture and compound is as following:
| Mixture | Compound |
| A mixture is a material made up of two or more different substances which are mixed but are not combined chemically. | A substance made from the chemical reaction of two or more atoms of elements in a fixed proportion by weight is called compound. |
Proton contains positive charge while electrons contain negative charge.
47. Calculate the number of protons, neutrons and electrons of the element having atomic mass 39. Write the molecular formula of ammonia.
Ans: The element having atomic mass 39 is Potassium (K).
Number of electrons = 19
Number of protons = 19
Number of neutrons = 20
The molecular formula of ammonia is NH4.
48. By what name Group IA elements are denoted? The valency of nitrogen is 3, clarify it.
Ans: Group IA elements are also known as alkali metals. The valency of nitrogen is 3. This means that combining capacity of nitrogen is 3. The number of electron present is valence shell of nitrogen is 5. So, it gains 3 more electrons from other atom during chemical reaction to form stable electronic configuration.
49. The element chlorine has atomic number 17 and atomic mass 35, calculate the number of protons, neutrons and electrons. Write one difference between octet and duplet rule.
Ans: Number of electrons = Atomic number = 17
Number of protons = Atomic number = 17
Number of neutrons = Atomic mass — Number of protons
= 35-17
= 18
Difference between octet rule and duplet rule is as following:
| Octet rule | Duplet rule |
| The natural tendency of an atom of an element to make 8 electrons in its valence shell by gaining, losing or sharing electrons between the combining atoms during the formation of molecules is called octet. | The natural tendency of an atom of an element to make 2 electrons in its valence shell by gaining, losing or sharing electrons between the combining atoms during the formation of molecules is called duplet. |
50. Who is the developer of modern periodic table? On which basis it is made? Valency of argon is zero. Why?
Ans: The developer of modern periodic table isHenery Moseley. Modern periodic table is made on the basis of atomic number of elements.
Argon has 8 electrons in its valence shell. As its valence shell is already filled, it has stable electronic configuration. It does no need to lose or gain electron i.e. it does not take part in chemical reaction. Hence, argon has valency 0.
51. What is chemical equation? Hydrogen and oxygen react to give water, write the word equation and chemical equation for it.
Ans: A chemical reaction which is expressed by using molecular formulae of reactants and products is called chemical equation.
Word equation-
Hydrogen + Oxygen → Water
Chemical equation-
2H2 + O2 → 2H2O
52. Write the molecular formula of magnesium chloride. What is inert gas? Write with an example.
Ans: The molecular formula of magnesium chloride is MgCl2.
Those elements which have 8 electrons in their valence shell (except Helium, it has 2 electrons) and have no tendency to combine with other elements are termed as inert gas.
For example- Helium, Neon and Argon.