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1. What is an Atom?
Atoms are the building blocks of matter. All the matters in our surroundings are made of matters. Different kinds of matter exist because there are different kinds of atoms present in it.
Atoms consist of electrons, protons and neutrons.
We will discuss the charged particles, and how they were discovered later in this section.
What are charged particles in a matter?
- Whenever we rub two objects together, they become electrically charged. This is because atoms contain charged particles in them.
- Michael Faraday for the first time established a relationship between electricity and matter. He stated that electricity is made up of particles called “atoms of electricity’.
- G.J. Stoney suggested the name ‘electron’ for the atoms of electricity.
- The real credit for the discovery of the electron goes to J.J Thomson.
The Discovery of the electron
J.J Thomson discovered electrons during his Cathode Ray experiment with cathode ray tubes.
- The cathode rays are emitted from the cathode irrespective of the nature of the metal electrode or the nature of the gas contained in the tube.
- Thomson concluded that the negatively charged particles, now called electrons as an integral part of all atoms.
- Electrons have both definite mass and definite electric charge.
- The mass of an electron is nearly 1/1837 of the mass of the hydrogen atom. However, for all practical purposes, it may be taken as negligible. The charge of an electron has been assigned a value of -1
The discovery of proton
- The existence of protons in the atoms was given by E. Goldstein
- When electricity was passed at high voltage through a gas at a very low pressure in a discharge tube, streams of heavy particles were given at anode (positive electrode). These streams of particles are called anode rays.
- These particles were positively charged and the mass and charge of the anode ray particles depend on the nature of the gas taken in the discharge tube.
- A proton’s mass is equal to a hydrogen atom’s mass. therefore the relative mass of a proton is 1u.
- The charge of a proton is +1
DISCOVERY OF THE NUCLEUS (RUTHERFORD’S EXPERIMENT):
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom.
Rutherford designed an experiment for this. In this experiment, fast-moving alpha (α-particles
were made to fall on a thin gold foil.
He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about
1000 atoms thick.
a-particles are doubly-charged helium ions. Since they have a mass of 4u, the fast-moving α-
particles have a considerable amount of energy.
The following observations were made from his experiment:
- Most of the α-particles nearly 99%) passed through the gold foil undeflected.
- A few of them got deflected through small angles.
- Very few (about one in 1000,000) did not pass through the foil at all but suffered large deflections (more than 90 degrees) or even returned back in the direction from which they came.
Conclusion of Rutherford’s scattering experiment
- Most of the space inside the atom is empty because most of the α-particles pass through the gold foil without getting deflected.
- Very few particles were deflected from their path, indicating the atom’s positive charge occupies very little space.
- A very small fraction of α-particles were deflected by very large angles, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
- The strong deflections, bouncing or even bouncing back of α-particles from the foil were explained to be the result of a direct collision with the positively charged nucleus of the atom.
The features of Rutherford’s nucleus model of the atom
- There is a tiny positively charged centre in an atom called the nucleus. nearly all the mass of an atom resides in the nucleus. The positive charge of the nucleus is due to the protons.
- The electron revolves around the nucleus in well-defined orbits and forms the outside surface (extranuclear part) of the atom.
- The size of the nucleus is very small compared to the size of the atom.
- Between the nucleus and the outer electron, there is space except for other electrons.
How will you define an atomic number?
The Atomic number can be defined as the total number of protons contained in the nucleus of an atom.
An atomic number is the identity of an atom, changing atomic number means changing the atom. The atomic number is denoted by ‘Z’.
Atomic number = Nuclear charge or number of protons.
The discovery of Neutrons
In 1932, the scientist James Chadwick conducted an experiment where he bombarded a thin sheet of beryllium with α-particles. He observed the emission of electrically neutral particles with a mass slightly greater than that of protons. As a result, these were named neutrons.
The discovery of neutrons made it clear that the nucleus of the atom consists of protons and neutrons.
Since the electron has negligible mass, the entire mass of the atom is mainly due to protons and neutrons present in the nucleus.
The Fundamental particles of an atom
|Particles||Mass||Charge (unit Charge is 1.062 x 10-19 coulombs)|
|In Gram||In u (amu)|
|Electron||9.110 x 10-28||0.00055||-1|
|Proton||1.673 x 10-24||1.00728||+1|
|Neutron||1.675 x 10-24||1.00866||No charge|
Isotopes and Isobars
What are isotopes? What is the main cause of the isotopes of an element?
Those elements that have the same atomic number but different mass numbers are referred to as isotopes. Isotopes occur due to the presence of a different number of neutrons in elements having the same atomic number as mass number is the sum of the number of neutrons and protons. Many but not all elements have isotopes.
The isotopes of Hydrogen are Protium (has one proton and no neutrons), deuterium (has one proton and one neutron) and tritium (has one proton and two neutrons). The chemical properties of isotopes are the same because they have the same number of protons and hence the same number of electrons which determines the chemical properties of an element. However, the physical properties of the isotopes of an element may be slightly different due to differences in their mass numbers.
Isobars, on the other hand, are atoms having the same mass number but a different atomic number. For example, the atomic number of carbon and nitrogen is 6 and 7 respectively. Carbon- 14 an isotope of carbon has a mass number of 14 which is the same as that of nitrogen hence carbon-14 and nitrogen are isobars. Some isotopes are found useful in various fields.
(i) Use of isotope of uranium known as Uranium-235 as fuel in nuclear reactors
(ii) An isotope of cobalt known as cobalt-60 is used in the treatment of Cancer.
(iii) An isotope of iodine (Iodine-131) finds application in the treatment of diseases like goitre.
MODELS OF ATOM
The description of Thomson’s atomic model is one of the many scientific models of the atom. It
was proposed by J.J Thomson in t904 just after the discovery of electrons. However, at
that time the atomic nucleus was yet to be discovered. So, he proposed a model based on
known properties available at that time. The known properties are:
• Atoms are neutrally charged
• Negatively charged particles called electrons are present in an atom.
• Hence Thomson assumed that an atom is a uniform sphere of positive charges with electrons embedded in it.
Features of Rutherford’s proposed model of an atom
- There is a positively placed nucleus in an atom. Nearly all the mass resides in the nucleus
- Electrons revolve around the nucleus in well-defined orbits.
- The size of the nucleus is tiny compared to the size of an atom
Drawbacks of Rutherford’s Model
(i) According to electromagnetic theory, if a charged particle (electron) revolves around the
positively charged nucleus, the electron would continuously lose energy and will finally fall
into the nucleus
(ii) This will make the atom highly unstable
Bohr’s Model of Atom:
- The nucleus is situated at the centre of the atom.
- Only certain special orbits known as discrete orbits of electrons are allowed inside the atom
- The orbits or shells are numbered as l, 2, 3, 4… etc. or are designated as K, L, M, N… etc. shells.
- While revolving in discrete orbits, the electrons do not radiate energy.
- Energy is emitted or absorbed by an atom when an electron moves from one orbit to another
How are electrons distributed in different orbits?
The distribution of electrons in an atom is called Electronic Configuration. Formula 2n2 helps in the determination of the maximum number of electrons present in an orbit, here n :::: orbit number.
Electrons are negatively charged subatomic particles arranged like a cloud of negative charges outside the nucleus of an atom. The arrangement depends upon their potential energies in different orbits. The different energy levels are known as 1, 2, 3, and 4… and the corresponding shells are known as K, L, M, N and so on.
- 1st energy level- K shell/orbit
- 2nd energy level- L shell/orbit
- 3rd energy level- M shell/orbit and so on.
Electronic configuration of atoms (Elements):
The distribution of electrons in various shells in an atom is called Electronic Configuration
Atomic structures of Lithium to Neon:
The electrons present in the outermost shell of an atom are known as valence electrons (or
valency electrons) because they decide the valency (combining capacity) of the atom.
For Example: – The atomic number of sodium is Il, which means the sodium atom has Il
electrons in it.
So the electronic configuration K (2), L (8), M (l).
In the sodium atom, the M shell is the outermost shell or valence shell.
There is an I electron in the outermost shell of the sodium atom; therefore, the sodium atom has an I valence electron. Those electrons of an atom which take part in chemical reactions are called valence electrons.
Valence electrons are located in the outermost shell of an atom. To find out the number of valence electrons in an atom of the element, we should write down the electronic configuration of the element by using its atomic number.
The outermost shell will be the valence shell and the number of electrons present in it will give us the number of valence electrons.
Valency of a metal No. of valence electrons in the atom
Valency of a non-metal (8 — No. of valence electrons in its atom)
Structure of Atom Question and Answers
You can access all the Questions and answers from the chapter “Structure of atoms” in your science textbook by Manipur Board Below
Q1. Who Proposed the atomic theory?
Answer: John Dalton proposed the atomic theory.
Q2. From which electrode do the cathode rays originate?
Answer: The cathode rays originate from the cathode
Q3. What happens to the cathode rays when they are subjected to an electric field?
Answer: The cathode rays bent when they are subjected to an electric field
Q4. What is the charge of an electron?
Answer: The charge of an electron is -1
Q5. Where is the mass of an atom concentrated?
Answer: The mass of an atom is concentrated in the nucleus
Q6. What are the particles present in the nucleus of an atom?
Answer: Proton, neutrons and electrons are the particles present in the nucleus of an atom.
Q7. Where are the electrons in an atom found?
Answer: Electrons are found in shells or orbitals that surround the nucleus of an atom.
Q8. Which atom contains only two fundamental particles?
Answer: Protium(an isotope of hydrogen) contains only two fundamental particles.
Q9. What led Rutherford to discover the existence of the nucleus?
Answer: Since electrons have negligible mass, the entire mass of the atom is regarded as the mass of protons in the nucleus. This means that the nucleus must contain protons equal to the mass of the atom. But the number of protons is equal to the atomic number. This means that atomic mass would be equal to the atomic number. However, for all the atoms except hydrogen, the atomic mass is more than the atomic number. This led Rutherford to discover the existence of the nucleus.
Q10. Define atomic number and mass number
Answer: Atomic number- Atomic number is the number of protons in the nucleus of an atom, which is a characteristic of the chemical properties of an element and determines its place in the periodic table.
Mass number- The mass number is the total number of protons and neutrons in a nucleus.
Q11. What are isotopes? Give an example.
Answer: Those elements that have the same atomic number but different mass numbers are referred to as isotopes.
Example: The isotopes of hydrogen are protium (has one proton and no neutron), deuterium (has one proton and one neutron) and tritium (has one proton and two neutrons).
Q12. What are cathode rays and how do they differ from positive rays?
Answer: Cathode rays are the beams of negatively charged electrons travelling from the negative end of an electrode to the positive end within a vacuum, across a potential difference between the electrodes.
Difference between cathode rays and anode rays(positive rays)
|Cathode Rays||Anode Rays|
|They are stream of negatively charged electrons||They have positively charged material particles|
|Cathode rays deflect towards the positive plate of an electric field||Anode rays deflect towards the negative plate of an electric field.|
Q13. Give experimental evidence to show that:
- The entire mass of an atom is practically concentrated in the nucleus.
- The nucleus of an atom is positively charged.
Answer: (I) Alpha particles are positively charged particles. In Rutherford’s gold foil experiment, some of the alpha particles are deflected at certain angles and very few of them suffered large deflections. This means that there is a heavy positively charged mass concentrated and occupies a very small volume at the centre of the atom called the nucleus.
(ii) We know that equal charges repel each other. The positively charged alpha particles suffered deflection and even bounced back after meeting the small mass concentrated at the nucleus due to the force of repulsion shows that the nucleus of an atom is positively charged.
Q14. Derive a relationship between atomic number, mass number and number of neutrons in an atom.
Answer: Mass number=Atomic number+Number of neutrons.
Q15. How many protons, electrons and neutrons are there in the following atoms?
147N, 126C, 146C, 148O, 168O, 4019K, 4018Ar
Which of these are Isotopes and isobars?
|No. of Protons||No. of electrons||No. of neutrons|
126C and 146C are isotopes
148O and 168O are isotopes
147N, 146C and 148O are isobars
4019K and 4018Ar are isobars
Q16. Which subatomic particle was not present in Thomson’s model of the atom?
Q17: State one drawback of Rutherford’s model of an atom.
Answer: According to the electromagnetic theory, if a charged particle (electron) revolves around the positively charged nucleus, the electron would continuously lose energy and will finally fall into the nucleus. This will make the atom highly unstable.
Q18. Why is an atom neutral despite the presence of charged particles in it?
Answer: In an atom, the number of electrons outside the nucleus is equal to the number of positively charged particles in the nucleus. Hence the atom is neutral.
Q19. How did Neils Bohr explain the stability of the atom?
According to Neils Bohr:
I) Electrons could revolve around the nucleus in only certain orbits or certain energy levels, each Orbit having a different radius. The electrons which are in orbits close to the nucleus have low energy while those in orbits farther from the nucleus have higher energy.
ii) When an electron is revolving in a particular orbit or energy level around the nucleus, the electrons do not lose energy even though it has accelerated motion around the nucleus. Since the electrons do not lose energy while revolving in certain permitted orbits they do not fall into the nucleus and hence the atom remains stable
Q20. Draw a sketch of Bohr’s model of an atom with three shells.
Q21. Write the distribution of electrons in oxygen and sodium atoms.
Answer: Electronic configuration of Oxygen (8) =
K – 2
L – 6
Electronic configuration of Sodium (11)=
K – 2
L – 8
M – 1
Q22. An atom has 2 and 7 electrons in the K and L shells respectively. What would be the atomic number of the element? Name the element
Answer: The atomic number of the element is 9.
The element is fluorine.
Q23. The nucleus of an atom has 7 protons and 8 neutrons. What would be the
- Atomic number
- Mass number
- The number of electrons and
- The number of valence electrons?
- Atomic number = 7
- Mass number = 7+8 = 15
- The number of electrons = 7
- The number of valence electrons = 5 [ K(2), L(5)]
Q24. What is the general name of the elements having 8 electrons in the outermost shell of their atoms?
Answer: Noble gases or inert gases.
Q25. The atomic number of an element X is 15.
- Write down the electronic configuration of X.
- What will be the valency of X?
- The electronic configuration of X (15) = 2, 8, 5.
- The valency of X = 8-5 = 3
Q26. How will you find the valency of Chlorine, Potassium and Sulphur?
Answer: The electronic configuration of chlorine (17) = 2, 8, 7.
So, the valency of chlorine is 8-7=1
The electronic configuration of potassium (19) = 2, 8, 8, 1
The valency of potassium is 1
The electronic configuration of Sulphur (16) = 2, 8, 6
The valency of sulphur is 8-6 = 2
Q27. Name the three subatomic particles present in an atom and compare their properties.
Answer: The three subatomic particles present in an atom are:
- protons are positively charged particles. they are present inside the nucleus along with neutrons.
- Neutrons are neutral particles.
- Electrons are negatively charged particles which revolve around the nucleus.
Q28. Why atom does not contain a neutron?
Answer: Protium (an isotope of Hydrogen) does not contain neutron.
Q29. Describe Rutherford’s model of an atom. What are its limitations?
Answer: Rutherford proposed an atomic model similar to the structure of the solar system. Just as in the solar system, the sun is at the centre (having the maximum mass) and the planets revolve around it. In an atom, the nucleus contains the main mass and the electrons revolve around it in orbits or shells.
Its limitations are:
- According to electromagnetic theory, if a charged particle (electron) revolves around the positively charged nucleus, the electron will continuously lose energy and will finally fall into the nucleus.
- This will make the atom highly unstable.
Q30. Describe Bohr’s model of the atom.
Answer: Bohr’s model of the atom states that:
- The nucleus is situated at the centre of the atom.
- An atom consists of a heavy positively charged nucleus. The whole mass of the atom is concentrated in the nucleus.
- The electrons in an atom revolve around the nucleus in a definite circular path called orbits or energy levels or shells and are numbered as 1,2,3,4…etc. (from the nucleus) or alternatively, these are designated as K, L, M, N,….shells.
- Each energy level is associated with a definite amount of energy.
- The change in energy takes place when an electron jumps from one energy level to another energy level.
- To change from one orbit to another, the electron must absorb or emit a quantity of energy exactly equal to the difference in energy between the two orbits.
Q31. Explain with examples
- Atomic number
- Mass number
- Isotopes &
Give any two uses of isotopes.
- Atomic number: The total number of protons lying in the nucleus of any atom is called the atomic number. The atomic number is denoted by ‘Z”
Example: The number of protons present in a Sodium atom is 11. So the atomic number of Sodium is 11.
- Mass Number: It is the sum of the Number of protons and neutrons present in the nucleus.
Example: The number of protons present in a sodium atom is 11 and the number of neutrons present in a sodium atom is 12. So, the mass number of atoms is 11+12 + 23.
- Isotopes: Those elements that have the same atomic number but have different mass numbers are referred to as isotopes.
Example: The isotope of hydrogen is Protium (has one proton and no neutron), deuterium (has one proton and one neutron), and tritium (has one proton and two neutrons).
- Isobars: Isobars are atoms having the same mass number but different atomic numbers.
Example: The atomic number of carbon and nitrogen is 6 and 7 respectively. Carbon-14 an isotop of carbon has a mass number of 14 which is the same as that of nitrogen. Hence carbon-14 and nitrogen are isobars.
Two uses of isotopes are
- An isotope of cobalt known as cobalt-60 is used in the treatment of cancer
- An isotope of iodine( iodine-131) finds application in the treatment of diseases like goitre.
Q32. The mass number of an element is 40. It contains 19 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
Number of protons = Number of electrons = 19
Number of neutrons = Mass number – Number of electrons = 40 – 19 = 21
The atomic number of the element = Number of electrons = 19
Q33. Write the electronic configuration of a potassium atom (Z = 19)
Answer: The electronic configuration of a potassium atom (Z=19)
= 2, 8, 8, 1.
Q33. Explain why, sodium ion (Na+), has completely filled K and L shell?
Answer: When a sodium atom loses 1 electron, it becomes sodium ion Na+ which has 10 electrons. It has 2 electrons in K-shell and 8 electrons in L-shell. K-shell(n=1) can have maximum 2 x 1x 1 =2 electrons and L-shell (n=2) can accommodate maximum 2 x 2 x 2 = 8 electrons. Therefore, Na+ has completely filled k and L shells.
O34. If both the K and L shells of an atom are full, what is the total number of electrons in the atom?
Answer: The total number of electrons in the atom is 10.
Q35. If a bromine atom is available in the form of two isotopes 7935Br (49%) and 8135Br (51%).
Calculate the average atomic mass of the bromine atom.
Answer: The average atomic mass of bromine
= (79 x 49%) + (81 x 51%)
= 79 x 49/100 + 81 x 51/100
= 79 x 0.49 + 81 x 0.51
= 38.71 + 41.31
= 80.02 u
Q36. The Composition of the nuclei of two atomic species x and y are as follows:
|Number of protons||15||15|
|Number of neutrons||15||16|
Give the mass number of X and Y. What is the relation between the two species?
The mass of X = 15 + 15 = 30
The mass of Y = 15 + 16 = 31
X and Y are isotopes.
Q37. What is the relation between the valency of an element and the number of valence electrons in its atom. Explain with examples.
Answer: The Valency of the atom of an element determines the valency of an element.
In the case of metals, the valence electrons represents the valency of the metal elements(positive valency)
|Li (lithium)||2, 1||1|
|Mg ( magnesium)||2, 8, 2||2|
|Al (aluminium)||2, 8, 3||3|
In the case of non-metal, the less number of electrons to achieve octet (8) configuration from the valence electron, gives its valency.
|C (carbon)||2, 4||8-4 = 4|
|P (phosphorous)||2, 8, 5||8-5 = 3|
|O (oxygen)||2, 6||8-6 = 2|
|F (fluorine)||2, 7||8-7 = 1|
In the case of noble gases or inert gases, the valency remains zero.
|Ne (neon)||2, 8||0|
|Ar (argon)||2, 8, 8||0|
In exception, hydrogen have both negative and positive valency i.e. 1(one) and
Boron which is a typical non-metal has valency-3
B(boron)——2, 3,——Valency 3