Solutions-Class 8-Science-Chapter-5-Inside the Atom-Maharashtra Board

Difference between Thomson and Rutherford Theory are as follows :
 

Valency of an element : It is the number of electrons of an atom of the element uses to combine with atoms of other elements. It is the combining power of an atom of the element.

It gives idea about the number of electrons loss or gain in order to achieve the nearest noble gas configuration.
For example: Na(11) = 2,8,1 So, its valency is 1
                      Cl(17) = 2,8,7 So, its valency is also 1

Number of valence electron : It is defined as the number of electrons present in the outermost shell of an atom.

• Helium and neon, atoms of both these gaseous element do not combine with any other atom. These elements are chemically inert, i.e. their valency is zero. Helium atom contains two electrons, indicates that the outermost shell of helium has an electron duplet.
• The valence shell of neon is completely filled, i.e. neon has an electron octet.
• Similarly argon contains eight electrons in the valence shell, i.e. argon has an electron octet. It is confirmed that the valency is zero when electron octet (or duplet) is complete.
•  Atoms of all the elements except inert gases have tendency to combine with other atoms, i.e. they have a non zero valency.
• 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.

It means that there is relationship between the valency of an element and the number of electrons in its valence shell.

Atomic Mass Number : The total number of protons and neutrons present in a nucleus of the atom is called atomic mass number.
Atomic mass number = number of protons + number of neutrons.
Atomic number = number of protons = number of electrons. It is denoted by Z.
Mass number = number of protons + number of neutrons. It is denoted by A.
For Carbon atom - Number of proton = 6,   Number of neutrons = 6,  Number of electron = 6

Atomic number of Carbon atom (Z) = number of protons = number of electrons = 6.
Atomic Mass number of Carbon atom (A) = number of proton + number of neutrons  = 6 + 6 =12.

Subatomic particles :  A particle which is a constituent of an atom hence smaller than the atom is called subatomic particle.

• An atom is formed from the nucleus and the extra nuclear part. These contain three types of subatomic particles.
• The nucleus Contains two types of subatomic particles together called nucleons.
• Protons and neutrons are the two types of subatomic particles and nucleons or electrons are subatomic particles in the extra nuclear part.

Electron :
1. Electrons are present outside the nucleus of an atom.
2. Electrons are negatively charged that is (1.6 × 10⁻¹⁹ coulomb).
3. The mass of an electron is considered to negligible. It is 1800  times less than that of a hydrogen
4. It revolves around the nucleus in the discrete orbit.

Proton :
1. Protons are present in the nucleus of an atom.
2. Protons are positively charged that is (1.6 × 10⁻¹⁹ coulomb).
3. The mass of a proton is approximately 1u(1Dalton) that is (1u = 1.66 × 10⁻²⁷ g).
4. They are closely bound in the nucleus.

Neutron :
1. Neutrons are present in the nucleus of an atom.
2. Neutrons are neutral. There is no charge.
3. The mass of a neutron is nearly equal to the mass of a proton that is 1u(1Dalton) that is (1u = 1.66 × 10⁻²⁷ g).
4. They are closely bound in the nucleus.

All the mass of an atom is concentrated in the nucleus because atom contains three subatomic particles like electron, proton and neutron. Out of which, nucleus present at the centre of an atom contains two subatomic particles that's protons and neutrons and the mass of nucleus is the sum of mass of protons and neutrons located at the centre of an atom.

Atom is electrically neutral because in an atom electrons and protons carry charges and each atom has equal numbers of protons (positively charged) and electrons (negatively charged).

Atomic mass number is a whole number because it is the sum of numbers of protons and numbers of neutrons present in an atom. Which is present in the form of integers.

Atoms are stables though negatively charged electrons are revolving within it because each atom contains equal numbers of protons and electrons. So, charge on negatively charged electrons are balanced by charge on positively charged proton. Therefore, atom is electrically neutral and stable. 

Atom: An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.
Because atoms are the smallest things in the universe and could not be divided. atoms are made up of three subatomic particles like protons, neutrons and electrons.

Isotopes are atoms of the same element that have same atomic number but different mass number. Isotopes have same numbers of protons but different numbers of neutrons.

Atomic number (Z) : The number of electrons or protons in an atom is called the atomic number. It is denoted by Z.
Atomic number determines its place in the periodic table.

Example :
Atomic number of carbon = 6
Atomic number of nitrogen = 7

Atomic mass number (A) : The total number of protons and neutrons in the nucleus of the atom is called the atomic mass number. It is denoted by A.
Atomic mass number = number of protons + number of neutrons.

Moderator in nuclear reactor: Moderator of a nuclear reactor is a substance that slows down the speed of neutrons in a fission .

Rutherford's scattering experiment :

A. Electron, proton, neutron are the types of subatomic particlesin an atom.

B. An electron carries a negativecharge.

C. The electron shell K is nearest to the nucleus.

D. The electronic configuration magnesium is 2,  8,  2. From  this it is understood that the valence shell of magnesium is M shell.

E. The valency of hydrogen is 'one as per the molecular formula H₂O . Therefore valency of 'Fe' turns out to be three as per the formula Fe₂O₃.

Match the pairs: