Structure Of The Atom

Size of the nucleus of an atom is very small as compared to the size of the atom.

According to Rutherford gold foil experiment nucleus is very small in size as compared to the size of the atom as a whole. Nucleus is very hard, dense and positively charged which consists of protons and neutrons.

James Chadwick in 1932 discovered the neutrons.

Neutron is a neutral particle found in the nucleus of an atom and is represented by symbol n.

The number of valence electrons in the chlorine atom is 7.

The atomic number of chlorine is 17. So its electronic configuration is 2, 8, 7. So there are 7 valence electrons.

¹⁶₈X, ¹⁷₈X are a pair of isotopes as they have a same atomic number but a different mass number.

Here 8 is the atomic number and 16, 17 are a mass number which is different so these are isotopes.

Atomic number of the element is 15 as the number of protons in one atom of the element represents the atomic number of that element.

Mass number of element = number of protons + number of neutrons

= 15 + 16

= 31

a. According to Thomson’s model of the atom, the atom is a sphere of positive charge with negatively charged electrons embedded in it. It is compared with a watermelon because the sphere of positive charge represents the red, edible part of watermelon whereas electrons embedded in a positively charged sphere represent the black seeds.

b. Isotopes of an element differ in the number of neutrons in the nuclei so they have a different mass number as the mass number is the sum of a number of protons and neutrons in the nucleus.

The valency of chlorine is 1 because 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 8 electrons in the valence shell. Since chlorine is a non-metal so valency of nonmetal = 8 – the number of valence electrons in its atom which is 8-7 = 1.

Isobars are the atoms of different elements having different atomic numbers but a same mass number. One pair of isobar is ⁴⁰Ar₁₈ and ⁴⁰Ca₂₀.

Isobars do not have the same number of protons or electrons so the chemical properties are not similar.

a. Elements D and E are isobars as they have a same mass number but a different atomic number.

b. The atomic number of element B is 3. So its electronic configuration is 2, 1. Hence its valency is 1 as it has only 1 valence electron.

a. Two postulates of Bohr’s model of an atom are-

1. Electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells.

2. Each shell can hold electrons up to some limits and each shell is associated with a fixed amount of energy.

b. Bohr’s model

c. The three sub-atomic particles are electrons, protons, and neutrons. Electrons are negatively charged, protons are positively charged, neutrons are neutral.

a. Canal rays are anode rays which are a beam of positive ions given by anode in an experiment conducted by Goldstein when he passed electricity at high voltage through a gas at very low pressure in a discharge tube. They consist of positively charged particles.

b. E. Goldstein discovered these canal rays.

The capacity of an atom of an element to form chemical bonds is known as valency of an element.

An atomic number of chlorine is 17 so its electronic configuration is 2, 8, 7.

Valency of non-metal chlorine = 8 – the number of valence electrons in its atom

= 8 – 7

= 1

Atomic number of magnesium is 12 so its electronic configuration is 2, 8, 2

Valency of magnesium = number of valence electrons in its atom = 2.

Chlorine atomic number is 17 so protons = electrons = 17; since number of protons/electrons in an atom determine the atomic number. Mass number will be sum of protons and neutrons = 17 + 18 = 35.

Since the atomic number is 17 for chlorine so electronic configuration is 2, 8, 7. Valency is 1.

Silicon atomic number is 14 = number of electrons/protons. Since the mass number is 28 so the number of neutrons is a mass number – the number of protons = 28-14 = 14.

The electronic configuration will be 2, 8, 4 and valency is 4.

Fluorine atomic number is 9 = number of protons/electrons. Mass number is 9 + 10 = 19. Electronic configuration is 2, 7 and valency 1.

Chlorine atomic number is 17 so its number of protons = number of electrons = 17 so chlorine atom is electrically neutral. But when chlorine atom is changed to chloride ion it has gained one electron so the number of electrons is more than a number of protons hence it is negatively charged.

a. James Chadwick discovered neutrons in 1932. Neutrons are neutral particles and they are found in the nucleus of an atom.

b. The three steps of Rutherford’s experiment for his model of an atom are-

i. He allowed fast moving alpha particles to strike a very thin gold foil in a vacuum, he found most of the alpha particles pass straight through the gold foil without any deflection from the original path.

ii. Few alpha particles deflected through small angles and few through large angles.

iii. Few alpha particles bounce back on hitting the gold foil.

c.Isobars are the atoms of different elements having a different atomic number but a same mass number. Example- ₁₈Ar⁴⁰, ₂₀Ca⁴⁰

d.Rutherford through his gold foil experiment concluded that the size of the nucleus is very small as compared to the size of an atom.

a. Octet means the presence of 8 electrons in the outermost shell of an atom which means atom is stable and has the electron arrangement of inert gas. Every atom wants to attain stability so they combine with one another and by gaining or losing or mutually sharing electrons they reach the octet state.

b. Structure of magnesium whose atomic number is 12- 2, 8, 2

Structure of phosphorus whose atomic number is 15- 2, 8, 5

a. Isobars are the atoms of different elements having different atomic number but same mass number. Example- ₁₈Ar⁴⁰, ₂₀Ca⁴⁰

b. Differences between isobar and isotopes-

c.Two uses of isotopes are-

i. Radioactive isotopes are used as tracers in medicine to detect the presence of tumours, blood clots in the human body.

ii. They are used in industry to detect leakage in underground oil, gas or water pipelines.