notes for the chapter structure of an atom
Answers
Answer:
Explanation:
An atom consists of 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 definite circular paths called orbits or energy level. Each energy level is associated with definite amount of energy.
Introduction to Structure of an 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
J. J. Thomson discovered the existence of electrons.
He did this using a cathode ray tube, which is a vacuum-sealed tube with a cathode and anode on one end that created a beam of electrons travelling towards the other end of the tube.
The air inside the chamber is subjected to high voltage and electricity flows through the air from the negative electrode to the positive electrode.
The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.
Thomson’s model of an atom
According to Thomson
(i) An atom consists of a positively charged sphere, and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral
The first model of an atom to be put forward and taken into consideration.
He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
The red edible part of the watermelon is compared with the positive charge in the atom.
The black seeds in the watermelon are compared with the electrons which are embedded on it.
Radioactivity
Radioactivity
Radioactivity is the term for the process by which an unstable nucleus of an atom loses energy by giving out particles.
It does so by giving out particles such as alpha and beta particles.
This process is spontaneous.
An atom is unstable if the nucleus has an imbalance, meaning a difference in the protons and neutrons.
Properties of Alpha Beta and Gamma Rays
Properties of Alpha Beta and Gamma RaysRutherford Model
Rutherford’s experiment and observations
In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil. His observations were:
A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed.
The positive charge in an atom is concentrated in a very small volume.
. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.
Rutherford’s model of an atom
Rutherford concluded the model of the atom from the α-particle scattering experiment as:
(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(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.
(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.
Rutherford Model
Rutherford’s Model
Drawbacks of Rutherford’s model
He explained that the electrons in an atom
Thus, the revolving electron would lose energy and finally fall into the nucleus.
But this cannot take place as the atom would be unstable and matter would not exist in the form we know.
In the experiment, Millikan allowed charged tiny oil droplets to pass through a hole into an electric field.
By varying the strength of electric field, the charge over an oil droplet was calculated, which always came as an integral value of ‘e.’
The conclusion of this is that the charge is said to be quantized, i.e. the charge on any particle will always be an integral multiple of e which is 1.6*10-19
Neil Bohr Model
Properties of electrons, protons, and neutrons
Bohr’s Model of an atom
Bohr came up with these postulates to overcome the objections raised against Rutherford’s model:
Electrons revolve around the nucleus in stable orbits without emission of radiant energy. Each orbit has a definite energy and is called an energy shell or energy level.
An orbit or energy level is designated as K, L, M, N shells. When the electron is in the lowest energy level, it is said to be in the ground state.
An electron emits or absorbs energy when it jumps from one orbit or energy level to another.
When it jumps from a higher energy level to lower energy level, it emits energy while it absorbs energy when it jumps from lower energy level to higher energy level.
Atomic Number
The number of protons found in the nucleus of an atom is termed as the atomic number. It is denoted by the letter ‘Z’.
Mass number and representation of an atom
Protons and neutrons are present in the nucleus, so the mass number is the total of these protons and neutrons.
Mass-number-and-representation-of-an-atom