The Structure of an Atom – Maharashtra Board Class 8 Solutions for General Science (English Medium)
Thomson’s model of the atom proposes that
- An atom is a positively charged sphere like the red part of a watermelon and negatively charged particles called electrons are embedded in it like the seeds of the watermelon.
- The positive and negative charges are equal and the atom does not have a resultant charge.
- Thomson proved the existence of negatively charged particles, called electrons, in the atom.
On the other hand, Rutherford’s theory proposes that
- The existence of a nucleus in the centre of an atom having positive charge.
- The negatively charged particles revolve around the nucleus in specific orbits.
- The size of the nucleus is very small in comparison to the size of an atom.
Hydrogen has three isotopes – protium , deuterium and tritium . Each has one proton in the nucleus. No neutron is present in the nucleus of normal hydrogen.
The capacity of an element to combine with another element is called its valency.
The sum of the number of protons and neutrons in an atom is called atomic mass number.
Electrons, protons and neutrons are the fundamental particles in an atom.
In Rutherford’s experiment, some alpha rays collide with the gold sheet and are turned back because of the very small size of the nucleus which does not allow alpha rays to pass through it.
During ion formation, an atom gains or loses electrons.
If the atom gains electrons, the number of electrons becomes greater than the number of protons, and hence, the ion carries a negative charge.
If the atom loses electrons, the number of electrons becomes less than the number of protons, and hence, the ions carry positive charge. Thus, an electric charge develops on the elements.
Ionisation takes place during a chemical reaction. Electrons revolve around the nucleus in specific orbits. The electrons in the outermost orbit take part in a chemical reaction. If the outermost orbit is incomplete, then it can gain or lose electrons and thus ions are formed.
The nucleus of an atom contains protons and neutrons. The mass of an atom depends on the number of protons and neutrons. Hence, all the mass of an atom is concentrated in the nucleus.
- If the valency of carbon is 4 and that of hydrogen is 1, then the molecular formula of methane is CH4.
- There are two electrons in the outermost orbit of the magnesium atom. Hence, the valency of magnesium is 2.
- and are isotopes of chlorine.
- Isotopes of uranium are used as fuel in atomic reactors.
a. Isotopes of cobalt
2. Treatment of cancer
b. Valency of nitrogen
c. Non – metal
4. Receives electrons
5. Loses electrons
3. Electrons in the outermost orbit