Structure of atom Flashcards
Write about cathode rays.
- The mass of an electron is considered to
be negligible and its charge is minus one. - The mass of an electron is about 1/2000 times.
- It was discovered by J.J Thomson.
- They travel in a straight line.
- They travel from cathode to anode.
What are canal rays?
- positively charged rays.
- Discovered by E. Goldstein.
- The mass of a proton is taken as one unit and its charge as plus one.
- Proton is almost 2000 times heavier than a elctron.
Describe Thomson’s model.
Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons in a sphere of positive charge.
Thomson proposed that:
(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.
Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientist cannot be explained with this.
Describe Rutherford’s gold-foil experiment.
Rutherford designed an experiment for knowing how electrodes were arranged in an atom. 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.
• α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy.
• It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms. Since the α-particles were much heavier than the protons, he did not expect to see large deflections.
Observations observed by Rutherford.
i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) one out of every 12000 particles appear to be rebound.
Conclusions drawn by Rutherford from the gold foil experiment.
(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
(iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
Describe the nuclear model of atom.
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 circular paths.
(iii) The size of the nucleus is very small as compared to the size of the atom.
Drawbacks of Rutherford’s model of the atom.
The revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.
What were the postulates of Neil Bohr.
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
Define Energy levels.
The orbits or shells are called energy levels. These orbits or shells are represented by the letters K,L,M,N,…or the numbers=1,2,3,…
Write about Neutrons.
- J. Chadwick discovered another sub atomic particle which had no charge and a mass nearly equal to that of a proton.
- Neutrons are present in the nucleus of the atom.
- Neutron is represented as ‘n’.
- The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
How are electrons distributed in different orbits?
The distribution of electrons into different orbits were suggested by Bohr and Bury.
The rules are: -
1. The maximum number of electrons present in a given shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index.
2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
3. Electrons are not accommodated in a given shell, unless the inner shells are filled. This is why shells are filled in a step-wise manner.
Define valency.
The combining capacity of atoms of elements and their tendency to react and form molecules with atoms of the same or different elements is called valency.
What is the atomic number?
It is the number of protons present in the nucleus of an atom. It is denoted by ‘Z’.
- All atoms of an element have same atomic number.
What is mass numer?
The mass number is defined as the sum of total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.
