Inside the Atom Class 8 Science Notes Maharashtra State Board
We have seen that matter is made of molecules. Molecules are formed from atoms. Effectively an atom is the smallest unit of matter. An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes. We have learned in the earlier standard that the smallest particles of most of the substances are molecules. The molecules of a few substances contain only one atom. Molecules are formed by a chemical combination of atoms. From this, we understand that the smallest particle of an element taking part in chemical combination is an atom. The concept of the atom is more than 2500 years old. However, it was forgotten over time. In modern times, scientists based on experiments explained the nature of the atom as well as the internal structure of the atom. It started with Dalton’s atomic theory.
Indian philosopher Kanad (6th century B.C.) stated that there is a limit to dividing matter into small particles. The indivisible particles that constitute matter were named by Kanad Muni as ‘Paramanu’ (meaning the smallest particles). He also stated that ‘Paramanu’ is indistructible. The Greek philosopher Democritus (5th century B.C.) stated that matter is made of small particles and these cannot be divided. The smallest particle of matter was named by Democritus as an ‘Atom’. (In Greek language ‘Atomos’ means the one which cannot be cut.)
Dalton’s Atomic Theory:
British scientist John Dalton put forth in 1803 A.D. his celebrated ‘Atomic Theory’. According to this theory matter is made of atoms and atoms are indivisible and indestructible. All atoms of an element are alike while different elements have different atoms with different masses.
A ‘Bundi Laddu’ is found to have an internal structure. It is formed by sticking smaller particles, the ‘Bundis’ to each other. However, the solid ball, broadly speaking, does not have any internal structure. The atom, as described by Dalton, turns out to be a hard, solid sphere with no internal structure. According to Dalton’s atomic theory, the mass is distributed uniformly in an atom. The scientist J.J. Thomson demonstrated experimentally that the negatively charged particles inside an atom have a mass 1800 times less than a hydrogen atom. Later these particles were named as electrons. Common substances are usually electrically neutral. The molecules of substances and the atoms that combine chemically to form molecules are electrically neutral. How is an atom electrically neutral despite having negatively charged electrons in it? Thomson overcame this difficulty by putting forth the plum pudding model of atomic structure.
Thomson’s Plum Pudding Model of Atom
The plum pudding model of atoms put forth by Thomson in the year 1904 is the first model of atomic structure. According to this model the positive charge is distributed throughout the atom and the negatively charged electrons are embedded in it. The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral. Plum pudding or plum cake is a sweet dish prepared during Christmas. In old times, this dish was made in Western countries by adding pieces of dried fruit called plum. These days, raisins or dates are used.
Rutherford’s Nuclear Model of Atom (1911)
Rutherford studied the inside of the atom through his celebrated scattering experiment and put forth the nuclear model of the atom in the year 1911. Rutherford took a very thin gold foil (thickness: 10-4mm) and bombarded it with positively charged α-particles emitted by a radioactive element. He observed the path of α-particles using a fluorescent screen around the gold foil. It was expected that the α-particles would get reflected from the gold foil if the positively charged mass were evenly distributed inside the atoms. Unexpectedly, most of the particles went straight through the foil, a small number of α-particles were deflected from the original path through a small angle, a still smaller number of α-particles got deflected through a larger angle and surprisingly one α-particle out of 20000 bounced back in the direction opposite to the original path.
The large number of the α-particles that went straight through the foil indicates that there was no obstacle in their path. It meant that there must be mainly a space inside the atoms in the solid gold foil. The small number of α-particles that get deflected through a small or a big angle must have faced an obstacle in their path. It meant that the positively charged and heavy part causing obstruction would be in the center of the atom. From this Rutherford put forth a nuclear model for the atom as follows:
- There is a positively charged nucleus at the center of an atom.
- Almost the entire mass of the atom is concentrated in the nucleus.
- Negatively charged particles called electrons revolve around the nucleus.
- The total negative charge on all the electrons is equal to the positive charge on the nucleus. As the opposite charges are balanced the atom is electrically neutral.
- There is a space between the revolving electron and the atomic nucleus.
An established law of physics an electrically charged body is revolving in a circular orbit, its energy decreases. According to this law, the atom described in Rutherford’s model turns out to be unstable. In reality, however, all atoms, except radioactive atoms, are stable. This shortcoming of Rutherford’s atomic model was removed by the atomic model put forth by Niels Bohr in the year 1913.
Bohr’s Stable Orbit Atomic Model (1913)
In the year 1913 Danish scientist Niels Bohr explained the stability of atoms by putting forth a stable orbit atomic model. The important postulates of Bohr’s atomic model are as follows.
- The electrons revolving around the atomic nucleus lie in concentric circular orbits at a certain distance from the nucleus.
- The energy of an electron is constant while it is in a particular orbit.
- When an electron jumps from an inner orbit to an outer orbit it absorbs energy equal to the difference of its energy level and when it jumps from an outer orbit to an inner orbit it emits energy equal to the difference of its energy level.
When table salt (Sodium chloride) is thrown on an LPG gas stove flame, immediately yellow spark forms in that place. If sodium metal is put in water, it burns to give a yellow flame. On the road, a sodium vapor lamp gives a yellow color light. From all the above examples, the electron of sodium absorbs energy goes to the outermost shell, and comes back to the inner shell by emitting energy. The difference in energy level of these two shells of sodium is fixed. This difference is similar to the energy of yellow light. Therefore in the above example, the same specific yellow light is emitted. Some more atomic models were put forth after Bohr’s atomic model. The atomic structure was studied in depth with the advent of a new branch of science called quantum mechanics. With all those some well-accepted fundamental principles of atomic structure are as follows:
Atomic Structure:
An atom is formed from the nucleus and the extranuclear part. These contain three types of subatomic particles.
Nucleus:
The atomic nucleus is positively charged. Almost the entire mass of the atom is concentrated in the nucleus. The nucleus contains two types of subatomic particles together called nucleons. Protons and neutrons are the two types of nucleons.
Proton (p)
A proton is a positively charged subatomic particle in the atomic nucleus. The positive charge on the nucleus is due to the proton in it. A proton is represented by the symbol ‘p’. Each proton carries a positive charge of +1e. (1e = 1.6 × 10-19 coulomb). When the total positive charge on the nucleus is expressed in the unit ‘e’, its magnitude is equal to the number of protons in the nucleus. The number of protons in the nucleus of an atom is the atomic number of that element and is denoted by the ‘Z’ mass of one proton is approximately 1u (1 Dalton) (1u = 1.66 × 10-27 Kg) (The mass of one hydrogen atom is also approximately 1u.)
Neutron (n)
A neutron is an electrically neutral subatomic particle and is denoted by the symbol ‘n’. The number of neutrons in the nucleus is denoted by the symbol ‘N’. Atomic nuclei of all the elements except hydrogen with atomic mass 1u contain neutrons. The mass of a neutron is approximately 1u, which is almost equal to that of a proton.
Extranuclear Part
The extranuclear part of the atomic structure includes electrons revolving around the nucleus and the space in between the nucleus and the electron.
Electron (e–)
Electron is a negatively charged subatomic particle and is denoted by the symbol ‘e-’. Each electron carries one unit of negative charge (-1e). The mass of an electron is 1800 times less than that of a hydrogen atom. Therefore the mass of an electron can be treated as negligible. Electrons in the extranuclear part revolve in the discrete orbits around the nucleus. The orbits being three-dimensional, the term ‘shell’ is used instead of the term ‘orbit’. The energy of an electron is determined by the shell in which it is present. The number of electrons in the extranuclear part is equal to the number of protons in the nucleus (Z). Therefore electrical charges are balanced and the atom is electrically neutral.
The mass of an electron is negligible, therefore the mass of an atom is mainly due to the protons and neutrons in its nucleus. The total number of protons and neutrons in an atom is the atomic mass number of that element. The mass number is denoted by the symbol ‘A’. The convention to denote atomic symbol, atomic number, and mass number together is shown as follows. \({ }_{\mathrm{Z}}^{\mathrm{A}} \text { symbol }\). For example, the conventional symbol \({ }_6^{12} \mathrm{C}\) means that the atomic number, that is, the proton number of carbon is 6 and the mass number of carbon is 12. From this, it is also learned that the nucleus of carbon contains (12-6) i.e. 6 neutrons.
Distribution of Electron:
As per Bohr’s atomic model, electrons revolve in stable shells. These shells have a definite energy. The shell nearest to the nucleus is called the first shell. The next shell is called the second shell. A symbol ‘n’ is used for the ordinal number of a shell. The shells are referred to by the symbols K, L, M, N,…. corresponding to the ordinal numbers n = 1, 2, 3, 4, … The maximum number of electrons a shell can contain is obtained by the formula ‘2n2’. As the magnitude of ‘n’ increases, the energy of an electron in that shell increases.
Electronic Configuration of Elements:
We have seen that 2, 8, 18, 32…. electrons can be accommodated in the shells K, L, M, N …. respectively. This is the maximum capacity of that shell. The electrons in an atom are distributed in the shells according to their maximum capacity. The shellwise distribution of the electron in an atom of an element is called the electronic configuration of that element. Each electron has a definite energy as per the shell in which it is present. The energy of an electron in the first shell (K shell) is the lowest. The energy of electrons in the subsequent shells goes on increasing with the ordinal number of the shell. The electronic configuration of an element is such that the energy of all the electrons together is the maximum possible. Electrons get a place in the shells by the maximum capacity of the electron shell in an atom and the increasing order of energy.
The electronic configuration in the numerical form contains numbers separated by commas. Here the numbers indicate the electron number in the shells with increasing order of energy for example the electronic configuration of sodium is 2, 8, 1. It means that the total 11 electrons in sodium are distributed as 2 in shell ‘K’, 8 in shell ‘L’, and 1 in shell ‘M’. The electronic configuration of an atom can also be represented by a shell diagram as shown in the figure.
Valency and Electronic Configuration:
We have seen in the last chapter that valency means the number of chemical bonds formed by an atom. We also saw that generally, the valency of an element remains constant in its compounds.
The concepts regarding the valency of an element’s chemical bonds in compounds get clarified from the electronic configuration. Atoms form chemical bonds by using electrons in their outermost shell. The valency of an atom is determined by the configuration of its outermost shell. Therefore the outermost shell is called the valence shell. Also, the electrons in the outermost shell are called valence electrons. It can be seen that the valency of an atom is related to the number of valence electrons in that atom. Let us first look at the elements helium and neon. Atoms of both these gaseous elements do not combine with any other atom. These elements are chemically inert. It means that their valency is ‘Zero’.
Helium atom contains two electrons which are accommodated in the first shell ‘K’. Helium has only one ‘K’ shell that contains electrons and the same is also the outermost shell. The electron capacity (2n2) of the ‘K’ shell is ‘two’. This indicates that the outermost shell of helium is filled. It is said that helium has an electron duplet. The electronic configuration of the inert gas neon contains two shells ‘K’ and ‘L’. ‘L’ is the valence shell of neon. The electron capacity of the ‘L’ shell is ‘eight’ and the table shows that the valence shell of neon is filled. It is said that neon has an electron octet.
Argon is an inert gas having electrons in the shells ‘K’, ‘L’, and ‘M’. The electron capacity of the ‘M’ shell is 2 × 32 = 18. However, in argon, there are only 8 electrons in the valence shell ‘M’. It means that there are eight electrons in the valence shell of inert gases, that is an electron octet. From this, it is understood that the valency is ‘Zero’ when the electron octet (or duplet) is complete. The electronic configurations of elements other than inert gases (table 5.7) show that they do not have electron octet or their electron octet are incomplete. Regarding hydrogen, it can be said that its electron duplet is incomplete.
Atoms of all the elements except inert gases tend to combine with other atoms, meaning that they have a nonzero valency. You have seen from the formulae of the molecules formed by combination with hydrogen (for example H2, HCl) that the valency of hydrogen is ‘one’. The electronic configuration of hydrogen shows that there is ‘one’ electron less than the complete duplet state. This number ‘one’ matches with the valency of hydrogen which is also ‘one’. Moreover, it is learned that the electronic configuration of sodium (2, 8, 1) has ‘one’ electron in the valence shell and the valency of sodium is also ‘one’ as seen from the molecular formulae NaCl, NaH, etc. It means that there is some relation between the valency of an element and the number of electrons in its valence shell. The valency of an element is the same as the number of its valence electrons if this number is four or less than four. On the other hand, when an element has four or more valence electrons, the number of electrons by which the octet is short of completion is the valency of that element.
Isotopes:
The atomic number is a fundamental property of an element and its chemical identity. Some elements in nature have atoms with the same atomic number but different mass numbers. Such atoms of the same element having different mass numbers are called isotopes. For example, carbon has three isotopes, namely, C-12, C-13, C-14. The mass number of isotopes is also represented by another method as 12C , 13C, and 14C. The isotopes have the same proton number but different neutron numbers.
Uses of Isotopes:
Isotopes of some elements are radioactive. They are used in various fields such as industry, agriculture, medicine, and research field.
- Uranium-235 is used for nuclear fission and production of electricity.
- Some radioactive isotopes like Cobalt-60 are used in the medical treatment of fatal diseases like cancer.
- Iodine-131 is used in the treatment of goiter, a disease of the thyroid gland.
- The radioactive isotopes are used for the detection of cracks (leakage) in the underground pipes. eg. Sodium-24.
- Radioactive isotopes are used for food preservation from microbes.
- The radioactive isotope C-14 is used for determining the age of archeological objects.
Nuclear Reactor:
A nuclear reactor is a machine that generates electricity on a large scale by using atomic energy. In a nuclear reactor, the nuclear energy in an atom is released by bringing about nuclear reactions on the nuclear fuel. Let us understand a nuclear reaction with an example of a nuclear fuel, namely, Uranium-235. On bombardment with the slow speed of neutrons, the nucleus of the isotope Uranium-235 undergoes nuclear fission to form nuclei of two different elements Krypton-92 and Barium-141, and 2 to 3 neutrons.
On decreasing the speed these neutrons bring about fission of more U-235 nuclei. In this way a chain reaction of nuclear fission takes place. A large amount of nuclear energy is released during a chain reaction of fission. The chain reaction is kept under control to prevent the probable explosion. To control the chain reaction in the nuclear reactor it is necessary to decrease the speed and number of neutrons. For this purpose, the following provision is made in a nuclear reactor.
- Moderator: Graphite or heavy water is used as a moderator to reduce the speed of neutrons.
- Controller: To reduce the number of neutrons by absorbing them rods of boron, cadmium, beryllium, etc. are used as controllers.
- The heat produced in the fission process is taken out by using water as a coolant. Water is transformed into steam. Using this steam, turbines are driven and electricity is generated.
In India, a total of twenty-two nuclear reactors in eight places are functioning. ‘Apsara’ at Bhabha Atomic Research Centre in Mumbai is the first nuclear reactor in India which went critical on 4th August 1956. India has large reserves of the element Thorium-232. Therefore Indian scientists have developed a plan for nuclear reactors based on the production of the isotope U-233 from Th-232.
Well-maintained Maharashtra State Board Class 8 Science Notes Inside the Atom can serve as a reference for lifelong learning.