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Revision Notes
Atom
Atoms are the building blocks of matter. It is the smallest unit of matter that is composed of three sub-atomic particles: the proton, the neutron and the electron.
Cathode Ray Experiment
- Scientists J. J. Thomson and Crookes performed experiments on cathode rays to understand the flow of electrons.
- For this use of electric discharge tube was made
- An electric discharge tube is a glass tube in which two metal plates are fixed at two ends.
- One plate (A) called the cathode is connected to the negative side of an electric supply whereas another plate (B) called the anode is connected to the positive side of the electric supply.
- Air from the tube is removed through a pump (P) and a vacuum is created in the discharge tube. When high voltage electricity is given to these metal plates, cathode rays are produced from negative metal plate (A) i.e. cathode.
- Since these rays have a negative charge they move towards anode (B) with speed.
- Thomson called these cathode rays a flow of electrons.
- These rays deviated under the electromagnetic field.
- The two scientists concluded that cathode rays emitted from the cathode is the flow of electrons and -this flow of electrons possesses a negative charge.
Properties of cathode rays :
- These rays possess negatively charged electrons. Thus, cathode rays can be considered as a flow of electrons.
- These rays travel in a straight line. These rays get deviated under the effect of the electromagnetic fields.
Electrons
- Electrons are the negatively charged sub-atomic particles of an atom.
- The mass of an electron is considered to be negligible, and its charge is -1.
- The symbol for an electron is e–
- Electrons are extremely small.
- They are found outside the nucleus.
Thomson's model of the atom :
- When Thomson proposed his atomic model, only electrons and protons were known to be present in the atom. The nucleus was not yet discovered.
- Scientist J. J. Thomson was the first to propose the Positively atomic model describing the arrangement of electrons and charged protons in the atom
- As per him, the atom is like a spherical ball in which a positive electric charge is spread uniformly.
- The negatively charged electrons are embedded in this sphere just like raisins in the cookies.
Thomson's model was not accepted due to the following reasons :
- As per this model, the positively charged electron matter is equal to the negatively charged electrons. Due to such an arrangement, positive and negative charges mix with each other and thus make an atom neutral.
- Moreover, the arrangement shown by Thomson's model could not explain the chemical properties of different elements.
Radioactivity :
- The ability of isotopes to emit radiation is called radioactivity and the isotopes are called radioactive isotopes
- As per an old concept, isotopes of some elements that have a high atomic mass for example uranium U possess the property of radioactivity.
- However, this is not true because an element like lead (Pb) also has a heavy atomic mass still it is not t like lead a radioactive element.
- As per the new concept, as the atomic mass of an element increases, its number of protons increases. But at the same time if the number of neutrons also increases compared to protons then the atoms become unstable and hence are called radioactive.
- In general, if the ratio of neutrons to protons exceeds 1.6, the atoms become radioactive. Radioactive elements emit a, ß as well as y radiations.
Rutherford's experiment and its observation
- It was Rutherford who through his experiment showed that Thomson's model of the atom was not correct.
- In his experiment, Rutherford aimed a stream of the alpha rays from radioactive element plutonium (Po) at a very thin (0.004 mm) gold foil.
Rutherford observation
- If Thomson's atomic model was correct i.e. if the atom is filled uniformly electric charge, then all the alpha rays after striking the filled gold at back should deviate more or less.
- But Rutherford observed that most of the rays pass through the gold foil and only a few rays reflected back
- The proportion of rays that returned to the a-rays that passed through the foil was 1:1200. From this Rutherford determined that both, the mass and positive charge of the atom are concentrated
- in a small volume at the centre of the atom. Rutherford called this region the nucleus. Rutherford also believed that around this nucleus electrons having negligible weight and possessing negative charge must be arranged.
- Most of the rays passed through this nucleus and only a few returned back. This suggests that an atom must be hollow.
The thickness of gold foil doubled :
- When Rutherford repeated the same experiment by increasing the thickness of gold foil two times i.e. using 0.008 mm gold foil, the number of rays reflecting back also doubled. Rutherford's other conclusions :
- Rutherford's also concluded that in an atom, a proton is 1836 times heavier than an electron. The nucleus is 105 times smaller than the total area of the atom.
- The radius of the atom is 10-8 cm (10-10 m) whereas the radius of the nucleus is 10-13 cm (10-15 m).
- By these calculations, Rutherford concluded that the atom must be hollow. The centre of the atom consists of a heavy nucleus which is responsible for the mass of the atom whereas electrons that have negligible mass, revolve around the nucleus.
Bohr's model :
- Rutherford discovered that the atom consists of a nucleus and that the electrons exist around this nucleus. But, it was Niels Bohr who through his model showed how these electrons are arranged around the nucleus.
- Bohr proposed his atomic model in 1912.
- He stated that electrons move around the nucleus at atomic levels at a definite distance from the positively charged nucleus.
- This can be compared with the movement of planets around the sun.
- The atomic levels at a definite distance from the centre (nucleus) are called the energy levels or orbits.
- The electrons moving in these orbits do not lose energy and so these orbits are called stationary orbits. The orbit which is nearest to the nucleus possesses the lowest energy. As the distance of the orbits increases from the nucleus, their energy also increases.
Arrangement of electrons around the nucleus as per Bohr's Model :
- The first orbit nearest to the nucleus is called the K orbit and it can accommodate 2 electrons.
- The second orbit (L orbit) can accommodate 8 electrons.
- The third orbit (M orbit) can accommodate 18 electrons.
- The fourth orbit (N orbit) can accommodate 32 electrons.
- The second, third and fourth orbits have subsidiary orbits which are called orbitals.
- Generally, the electrons are arranged in order i.e. after filling the first orbit, the remaining electrons are arranged in the next orbit and so on.
- The number of electrons or protons in an electrically neutral atom of an element is called its atomic number. The atomic number is denoted by Z.
Valence electrons :
- The electrons present in an atom are arranged in different orbits such as N, M, L and K.
- The electrons arranged in the outermost orbit are responsible for emission spectra (i.e. producing spectra of electromagnetic radiation at high temperature. Singular-Spectrum.) and chemical properties of the elements. Hence these electrons are called valence electrons.
Valency of an atom :
- The valency of an atom of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence orbit.
Valency of metals :
- In general, for metals, valency is equal to the number of valence electrons in their atom. For example, sodium metal has atomic number 11 and an electronic configuration (2, 8, 1). Hence, the valency of sodium Na is 1.
Valency of non-metals
- Generally for non-metals,valency = 8 number of valence electrons in its atom. For example, non-metal chlorine has atomic number 17 and electronic configuration (2, 8, 7). Hence, the valency of chlorine Cl is 8 - 7 = 1.
Relation between valency and electronic structure :
- The valency of an element can be known based on its electronic structure.
- For example, the sodium Na atom has atomic number 11 and so its electronic configuration is (2, 8, 1).
- Thus, it can be seen from the structure that the valency of sodium is 1.
- Moreover, from the electronic structure, it can also be decided whether the valency is positive or negative.
- For example, since sodium has only one electron in its last orbit, its valency is +1.
- On the other hand, since chlorine has seven electrons (2, 8, 7) in its last orbit, it requires one electron to attain the octet configuration and so its valency is -1.
Atomic number :
- The number of protons in an atom of an element is known as the atomic number of that element.
- The atomic number is denoted by Z.
- Example: Sodium Na atom has 11 protons in it and so its atomic number is Z = 11.
Atomic mass :
- The total number of protons, as well as neutrons in the nucleus of the atom of an element, is known as the atomic mass of that element.
- Atomic mass is denoted by A and its unit is u.
- Thus, Atomic mass (A) = Protons (p) + Neutrons (n) Example :
- Carbon atom has 6 protons and 6 neutrons in it and so its atomic mass is A = p + n = 6 + 6 = 12 u.
- Similarly, sodium atom has 11 protons and 12 neutrons in it and so its atomic mass is A=p+ n = 11 + 12 = 23 .
- The atomic number and atomic mass of an element can be represented as A/Z * X, where x = symbol of the element, A = atomic mass and Z = atomic number. For example, 6 ^ 12 C.
Isotopes :
- By experiments done with the help of a mass spectrometer, it has been found that certain elements have a number of atoms that have the same atomic number' but different' atomic mass'.
- These atoms are known as isotopes.
- Thus, isotopes are atoms that have the same atomic number but different atomic mass i.e. more than one atomic mass.
- Even though masses of isotopes of an element differ, their properties are similar.
- Iso = same and topos = place. Thus, atoms of elements whose position in the periodic table is the same are called isotopes.
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Other Important Points
1. Experimental works carried out with a device known as a discharge tube have shown that atoms can be broken into what is known as cathode rays and anode rays.
2. Cathode rays are moving charged particles. One such particle is called an electron. It is a negatively charged particle and has a mass about 2000 times smaller than that of the hydrogen atom. The e/m ratio of Electrons is independent of the nature of the material used. An electron is represented as e-.
3. Anode rays are positively charged particles. The e/m ratio of these rays depends on the nature of the material used. The smallest value is obtained for hydrogen. The unit particle obtained from hydrogen is called a proton. A proton is represented as p+
4. J.J. Thomson proposed that an atom consists of the positively charged sphere in which the electrons are embedded.
| J.J Thomson model of Atom |
5. Alpha particle scattering experiments by Rutherford have shown that the whole mass of an atom is concentrated at a very small volume (10-⁵ times the size of an atom) in the centre of the atom. This centre is positively charged and is called the nucleus.
6. The mass of the nucleus is derived from the number of protons and neutrons it contains. A neutron is a neutral particle, a mass equal to that of a proton.
7. The number of protons and electrons in an atom are equal so as to maintain the atom as electrically neutral.
8. Protons and neutrons are of equal mass and contribute to the mass of the atom Electrons are light, about 1/1838 as heavy as protons.
9. Neils Bohr in 1912 proposed a model of the atom. Accordingly, electrons revolve around the nucleus in definite circular paths called orbits or energy levels.
10. According to Bohr and Bury model, the number of electrons that can be accommodated in different orbits or shells is fixed. The maximum occupancy of the shells is 2, 8, 18,...... respectively.
11. The outermost orbit cannot have more than 8 electrons.
12. Atomic number is the number of protons, an atom contains.
13. Mass number is the sum of protons and neutrons in the nucleus of an atom.
14. Some atoms have the same number of protons but different numbers of neutrons. These are called isotopes. Atoms with different atomic numbers but the same mass. a number are called isobars.
15. Half-life period of a radioactive substance is the time during which half of it decays by weight. This phenomenon has been used in estimating the age of substances. This is known as radioisotope dating.
16. The electrons in the last shell of the atom are called valence electrons. These govern the chemical properties of elements. The number of valence electrons that take part in chemical combinations is called the valency of the atoms.
17. When an electron from a higher energy level falls to an orbit of a lower energy level, the difference in energy is radiated in the form of electromagnetic radiation or light. This explains why different atoms on heating or under other conditions emit light of different colours.
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