Science Question Answer (2 Marks)

1. Mass number of X = 8 + 8 = 16
2. Mass number of Y = 8 + 9 = 17
3. X and Y are isotopes, as they have the same number of protons but different number of neutrons.
4. X and Y represent the isotopes of oxygen.

Important information furnished about nucleus in Rutherford’s alpha-particle scattering experiment is:

• Nucleus of an atom is positively charged.
• Nucleus of an atom is very hard and dense.
• Nucleus of an atom is very small as compared to the size of an atom as a whole.

The deflection of fast moving alpha-particles through small and large angles in Rutherford’s scattering experiment is the evidence for the presence of nucleus in an atom.

Valency of X = 8 - no. of valence electrons
Valency of X = 8 - 6 = 2

1. Uranium-235: This isotope is used as a fuel in the reactors of nuclear power plants for generating electricity.
2. Cobalt-60: This is used in the treatment of cancer.

The isotopes which are unstable due to presence of extra neutrons in their nuclei and emit various types of radiations, are called radioactive isotopes or radioisotopes.
For example: Carbon - 14 , Arsenic - 74

Given,
Atomic no. = 11
Mass no. = 23
Then, electronic configuration -(2, 8, 1)
Nuclear composition is -11 protons and 12 neutrons.

When electricity at high voltage is passed through a gas at very low pressure taken in discharge tube, stream of minute particles are given out by the cathode. These stream of particles are called cathode rays.

The valency 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 shell.
Valency of metal = No. of valence electron in its atom
Valency of a non-metal = 8 - No. of valence electron in its atom
For example: Valency of sodium (metal) is 1 and that of chlorine (non-metal) is also 1.

Atoms of different elements with different atomic numbers but same mass number are known as isobars. Here, A and B are the examples of isobars, as they have same number of nucleons.

Uses of isotopes:

1. They are used in the treatment of cancer.
2. Radioactive isotopes are used as 'tracers' in medicine to detect the presence of tumors and blood clots in human body.

1. Atomic no. is the number of protons in one atom of an element.
2. Mass no. is the total number of protons and neutrons present in one atom of the element.

Example: The total no. of protons in a carbon atom is 6, so its atomic no. is 6.
Also, one atom of Na contains 11 protons and 12 neutrons, so its mass no. is 23.

Protons are positively charged particle found in the atoms of all the elements.
Relative mass of proton is 1u.
Relative charge of proton is +1C.

According to Thomson model of atom: An atom consists of a sphere of positive charge with negatively charged electrons embedded in it. The positive and negative charges in an atom are equal in magnitude. Neutron was not present in the Thomson model of atom.

Two observations which shows that atom is not indivisible are:

1. In J. J. Thomson’s experiment, the stream of cathode rays in the gas discharge tube shows the presence of negatively charged subatomic particles called electrons.
2. In Goldstein’s experiment, the faint red glow in the gas discharge tube shows the presence of positively charged subatomic particles called protons.

The arrangement of electrons in various shells of an atom of the element is known as electronic configuration of the element. Electronic configuration of oxygen (atomic no. = 8) is (2, 6).

Valency shown by A(atomic no. 2) – 0
Valency shown by B(atomic no. 4) – 2
Valency shown by C(atomic no. 8) – 2
Valency shown by D(atomic no. 10) – 0
Valency shown by E(atomic no. 13) – 3

1. Atomic no. = 5
2. Mass no. = 6 + 5 = 11
3. No. of electrons = 5
4. No. of valence electrons, per atom = 3

Most of the alpha-particles passed straight through the gold foil without any deflection in Rutherford’s alpha- particle scattering experiment, this shows that most of the space in an atom is empty.

Atomic No. = 7
E.C = 2, 5
Valency of given element = 3
Given element is NITROGEN.

The capacity of an atom of an element to form chemical bonds is known as its valency. Valency of an atom with atomic no. 14 is 4.

Given,
Mass no. = 23
No. of electrons = 11
Then, no. of protons = 11
No. of neutrons = 23 - 11 = 12
Atomic no. = 11

There are equal no. of positive and negative particles in an atom, so it is neutral as a whole.

1. Due to identical electronic configuration containing the same no. of valence electrons, these isotopes have almost identical chemical properties.
2. All of them have 1 electron and 1 proton, so, they are electrically neutral.

The stream of particles coming out from cathode (negative electrode) are called cathode rays. Cathode rays are negatively charged.

When mass no. is 18 and no. of electrons is 7 then:

• No. of protons = 7
• No. of neutrons = 18 - 7 = 11
• Atomic no. = 7

Protons and neutrons are collectively present in the nucleus at the center while electrons revolve rapidly round the nucleus in fixed circular orbits called energy levels.

Difference between proton and neutron:

• Proton is positively charged while electron is negatively charged.
• Proton is much heavier than electron.