# STRUCTURE OF THE ATOM NCERT 9 SAMPLE PAPER

Q1.
a. Define electronic configuration. Arrange the various energy shells in order of increasing energy. b. What is the maximum number of electrons that can be accommodated in a:

1. Penultimate shell
2. ii. N shell

Solution

a. The distribution of electrons in various shells of an atom is known as electronic configuration. In order of increasing energy: K < L < M < N b. Maximum number of electrons that can be accomodated in a penultimate shell = 18    Maximum number of electrons that can be accomodated in N shell = 32

Q2. State the observations in alpha-particle scattering experiment which led Rutherford to make the following conclusions: (i)Most of the space in an atom is empty. (ii)Whole mass of an atom is concentrated in its centre. (iii) Centre is positively charged.

Solution

(i)Most of the alpha particles passed through the gold foil without getting deflected.

(ii)Very few particles were deflected from their path by 180o indicating that the whole mass of the atom is present in its centre in a very small volume.

(iii)Few particles were deflected at small angles from their path and very few were deflected from their path by 180o indicating that the positive charge of the atom occupies very little space and is located at the centre.

Q3. Why were canal rays also called as anode rays?

Solution

Canal rays were also called anode rays since they were emitted from anode (electrode connected to positive terminal of high voltage source) in the gas discharge experiment using perforated cathode.

Q4. Write the distribution of electrons in helium and hydrogen atoms.

Solution

Distribution of electrons in helium atom (atomic number=2) is 2. It has 2 electrons in its first energy level. Distribution of electrons in hydrogen atom (atomic number=1) is 1. It has 1 electron in its first energy level.

Q5. What are the properties of electrons?

Solution

Properties of Electrons An electron, which is denoted as e, is the fundamental particle with a negative charge. Its properties are independent of the gas in the discharge tube. The mass of an electron is negligible and its charge is -1. The mass of an electron is extremely small. It is  of the mass of a hydrogen atom (9.108 × 10-28 g) and so its mass is considered almost negligible. (Hydrogen is an atom of the lowest mass). Since the mass of a hydrogen atom is 1 u, we can say that the relative mass of an electron is  u. The absolute mass of an electron is 9 × 10-28 grams. Its charge is 1 unit negative charge, i.e. 1.602 × 10-19 coulombs. The tiny electrons revolve around the nucleus of the atom in specific orbits or shells.

Q6. In the experiment performed by J.J. Thomson, how was it concluded that cathode rays consist of negatively charged particles?

Solution

Cathode rays were attracted to anode (positively charged electrode). Going by the principle that opposite charges attract each other, it was concluded that atoms contained negatively charged particles which were later named as electrons.

Q7. What determines the chemical properties of an element?

Solution

Atomic number i.e. the number of protons present in an atom of the element determines the chemical properties of an element.

Q8. Define an electron. What is the charge on it? How is an electron represented? Where is it located in the atom?

Solution

An electron may be defined as the negatively charged particle of an atom. An electron has a unit negative charge i.e. -1. It is represented as e. It is located outside the nucleus of the atom.

Q9. Nikhil is not keeping happy and is unable to focus on studies. His friend Ankur observed the same and inquired Nikhil. Ankur was told by Nikhil that his younger brother is not keeping good health. His neck has swollen. The fact that Nikhil’s family was not including iodized salt in their diet came to light. Ankur immediately suggested certain measures and insisted that Nikhil’s brother must see the doctor without any delay.· What values are displayed by Ankur?· What can be possibly the disease from which Nikhil’s brother is suffering?· Name the element whose isotope is used in the treatment for the disease.

Solution

(i) Caring for his friend, empathy, awareness. (ii) Goiter (iii) Iodine

Q10. (a)The composition of nuclei of two atomic species X and Y are given below:   X Y Protons 17 17 Neutrons 18 20 Find the mass number of X and Y. State the relationship between X and Y. (b)The K shell is completely filled and L shell of an atom is half filled.Find the number of electrons present in it. State the name of this element.

Solution

(a) Mass number = No. of protons + No. of neutrons So, mass number of X = 17 + 18 = 35 Mass number of Y = 17 + 20 = 37 X and Y are isotopes of same element since number of protons is same (same atomic number) in both. (b) Numbers of electrons = 6 since: K L 2 4 Name of the element = Carbon with atomic number 6.

Q11. An element ‘X’ has a mass number 4 and atomic number 2. Find the valency of ‘X’.

Solution

Electronic configuration of ‘X’ = 2 i.e. K shell is completely filled so, there is no valence electron and hence valency is zero.

Q12. Which of the following are isotopes and which are isobars? Argon, Protium, Calcium, Deuterium. Explain why the isotopes differ in physical properties?

Solution

Isotopes – Protium and Deuterium; both are isotopes of hydrogen. Isobars – Argon and calcium; both have mass equal to 40.   Since isotopes have identical electronic configuration containing same number of valence electrons, they have similar chemical properties, but because the masses are slightly different hence, the physical properties (density, melting pt., boiling pt., etc) are different.

Q13. Explain the formation of protons from gas discharge tube as stated by Goldstein.

Solution

Discovery of the Proton A German scientist, E. Goldstein in 1886, modified the discharge tube and passed an electric current through it. He found that the positively charged rays were emitted from the anode in the discharge tube. These rays were called canal rays.     When an electric field was applied, these rays deflected towards the negatively charged plate. Thus, Goldstein concluded that an atom contains positively charged particles along with the electrons. These positively charged particles were named as protons by a British scientist, Ernest Rutherford. Canal rays were also called anode rays since they were emitted from anode (electrode connected to the positive terminal of high voltage source) in the gas discharge experiments using a perforated cathode.

Q14. Does gas at ordinary pressure conduct electricity? Explain.

Solution

No, gas does not conduct electricity at ordinary pressure. At ordinary pressure, gas is not ionized, hence there is no carrier of current.

Q15. Explain if electrons and protons in an atom is 4 then: (i) What is the atomic number of the atom? (ii) What is the charge on the atom?

Solution

(i) Atomic number = Number of proton = Number of electrons The atom has 4 electrons and 4 protons. Therefore, the atomic number of the atom will be 4. (ii) Atom as a whole is electrically neutral. There are 4 electrons and 4 protons in the atom. It has same number of negatively charged electrons and positively charged protons so there is no change on atom.

Q16. How was the extra mass of 6 units in Carbon (atomic mass = 12 u) with no. of protons = 6 explained?

Solution

A carbon atom contains 6 protons and 6 electrons. Since electrons have negligible mass so, the atomic mass of carbon should be only 6 u, which is the mass of 6 protons. But, the actual atomic mass of carbon is 12 u. This problem was solved by the discovery of another subatomic particle called neutron. It is found in the nucleus of the atom and has mass of 1 u. Hence, atomic mass of C = mass of 6 protons + mass of 6 neutrons = 6 + 6 = 12 u.

Q17. What determines the mass number of an atom?

Solution

The number of protons and neutrons present in the nucleus of an atom determines the mass number of that atom.

Q18. What is the charge of a neutron and where is it located in an atom?

Solution

There is no charge on neutron (Neutral) and it is located in the nucleus.

Q19. The atomic number and mass number of an element are 16 and 32 respectively. (a)Find the number of protons, electrons and neutrons in it. (b)State its valency. (c)Is this element a metal or a non – metal? Justify your answer.

Solution

(a) No. of protons = atomic number = 16 No. of electrons = no. of protons = 16 No. of neutrons = Mass number – atomic number                       = 32 – 16                       = 16   (b) Electronic configuration of element: 2, 8, 6 So, Valency = 8 – no. of valence electrons                  = 8 – 6                  = 2   (c)The element is a non metal because it has 6 valence electrons and hence belongs to group 16.

Q20. Explain atomic number and atomic mass number.

Solution

Atomic Number and Atomic Mass Number The number of protons present in the nucleus of an atom is the atomic number of that atom. It is represented by the symbol Z. Atomic number (Z) = Number of protons (p) All atoms of an element have the same atomic number. The number of protons and electrons in an atom is equal. Thus, the atom of an element is electrically neutral. Atomic mass number is defined as the sum of the number of protons and neutrons contained in the nucleus of an atom of that element. It is denoted by the symbol A. Atomic mass number (A) = Number of protons (p) + Number of neutrons (n) The atomic number, atomic mass number and symbol of an element are written as –

Q21. According to Bohr model of an atom, what is the location of electrons and protons in an atom?

Solution

The electrons revolve around the nucleus in various orbits (shell) and the protons are present in the nucleus.

Q22. Explain isotopes and isobars with the help of suitable examples.

Solution

Isotopes Atoms of the same element differing in the number of neutrons in their nuclei are known as isotopes. Thus, isotopes of an element have the same atomic number but different atomic mass numbers. Isotopes are identified by their mass numbers. For example, the isotopes of carbon are referred to as carbon-12, carbon-13 and carbon-

14. Isotopes of an element have similar chemical properties but different physical properties.    Isobars The atoms of different elements having different atomic numbers but the same mass number are known as isobars. Examples of Isobars Isobars Number of protons Number of neutrons Mass number Chlorine-37 17 20 37 Argon-37 18 19 Cerium-76 32 44 76 Selenium-76 34 42 Iron-58 26 32 58 Nickel-58 27 31

Q23. One of the observations during α-particle scattering is that most of the fast moving α-particles passed straight through the gold foil non-deviated. What does this observation explain?

Solution

Most of the space around the nucleus is empty which allows the alpha particles to pass through them non-deviated. The nucleus is very small in size and is located at the centre of the atom and hence very few alpha particles deflected by an angle of 180o.

Q24. What do you understand by combining capacity of the atoms? Explain with examples.

Solution

An atom of each element has a definite combining capacity, called its valency. The combining capacity of the atoms i.e. their tendency to react and form molecules with atoms of the same or different elements was thus explained as an attempt to attain a stable octet state. This is done by sharing, gaining or losing electrons. For example, lithium and sodium atoms contain one electron each in their outermost shell; therefore each one of them can lose one electron to have 8 electrons in their outermost shell. So, they are said to have valency = 1.

Q25. Explain the properties of nucleus concluded from the Rutherford’s model of atom.

Solution

Properties of nucleus: 1. Nucleus is a small positively charged mass present at the centre of the atom. 2. It carries almost the entire mass of the atom. 3. It contains neutrons and protons. 4. The volume of the nucleus is negligibly small as compared to the volume of the atom. 5. The size of the nucleus is very small as compared to the size of the atom. 6. The electrons revolve around the nucleus in circular orbits.

Q26. Can electrons be arranged in second shell or L shell without filling the first shell or K shell?

Solution

No, because electrons are filled first in shells having lower value of energy, the energy of first shell is lower than second shell. So, electrons will be arranged first in the K shell i.e. the first shell.

Q27. Who discovered canal rays? Describe the experiment of the discovery of canal rays in brief.

Solution

Discovery of the Proton A German scientist, E. Goldstein in 1886, modified the discharge tube and passed an electric current through it. He found that the positively charged rays were emitted from the anode in the discharge tube. These rays were called canal rays.    When an electric field was applied, these rays deflected towards the negatively charged plate. Thus, Goldstein concluded that an atom contains positively charged particles along with the electrons. These positively charged particles were named as protons by a British scientist, Ernest Rutherford. Canal rays were also called anode rays since they were emitted from anode (electrode connected to the positive terminal of high voltage source) in the gas discharge experiments using a perforated cathode.

Q28. State three features of the nuclear model of an atom put forward by Rutherford.

Solution

(i)There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. (ii)The electrons revolve around the nucleus in well defined orbits. (iii)The size of the nucleus is very small as compared to the size of the atom.

Q29. What are energy levels? How are they designated?

Solution

Electrons revolve around the nucleus in a limited number of orbits. These orbits or shells are called energy levels. These orbits or shells are represented by the letters K, L, M, N, … or the numbers, n = 1,2,3,4 respectively.

Q30. What does the formation of cathode rays and anode rays tell us about the atoms?

Solution

The formation of cathode rays and anode rays tells us that atom is not indivisible and is made up of smaller subatomic particles. Formation of cathode rays tells us that atoms contain negatively charged particles (electrons) inside them and formation of anode rays tells us that atoms contain positively charged particles (protons) in them.