Chapter - 4 Structure of the Atom

Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Gold stein in 1886.

An electron is a negatively charged particle, whereas a proton is a positively charged particle.
The magnitude of their charges is equal. Therefore, an atom containing one electron and one proton will not carry any charge. Thus, it will be a neutral atom.

According to Thomson’s model of the atom, an atom consists of both negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere. These negative and positive charges are equal in magnitude. Thus, by counterbalancing each other’s effect, they make an atom neutral.

On the basis of Rutherford’s model of an atom, protons (positively-charged particles) are present in the nucleus of an atom.

If the α-scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. In the α-scattering experiment, a gold foil was taken because gold is malleable and a thin foil of gold can be easily made. It is difficult to make such foils from other metals.

The three sub-atomic particles of an atom are:
(i) Protons
(ii) Electrons, and
(iii) Neutrons

Helium atom has two neutrons. The mass of an atom is the sum of the masses of protons and
neutrons present in its nucleus.
Since helium atom has two protons, mass contributed by the two protons is (2 × 1) u = 2 u.
Then, the remaining mass (4 − 2) u = 2 u is contributed by 2u /1u = 2 neutrons.

The total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom
is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 4 electrons
Or,
we can write the distribution of electrons in a carbon atom as 2, 4.
The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom
is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 8 electrons

Third orbit or M-shell = 1 electron
Or,
we can write distribution of electrons in a sodium atom as 2, 8, 1.

The maximum number of electrons that can occupy K and L-shells of an atom are 2 and 8 respectively. Therefore, if K and L-shells of an atom are full, then the total number of electrons in the atom would be (2 + 8) = 10 electrons.

If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. 
On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.
The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7; 2, 8, 6 and 2, 8, 2 respectively. Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively.
Thus,
The valency of chlorine = 8 −7 = 1
The valency of sulphur = 8 − 6 = 2
The valency of magnesium = 2

(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.

(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.

Mass number of oxygen = Number of protons + Number of neutrons
= 8 + 8
= 16
Mass number of sulphur = Number of protons + Number of neutrons
= 16 +16
= 32

Two isotopes of carbon are ¹²₆C and ¹⁴₆C
The electronic configuration of  ¹²₆C is 2, 4.
The electronic configuration of  ¹⁴₆C is 2, 4.
[Isotopes have same electronic configuration]

⁴⁰₂₉Ca and ⁴⁰₁₈Ar are a pair of isobars
The electronic configuration of  ⁴⁰₂₉Ca is 2, 8, 8, 2
The electronic configuration of  ⁴⁰₁₈Ar  is 2, 8, 8.
[Isobars have different electronic configuration]

According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found that the positively charged particles reside at the center of the atom called the nucleus, and the electrons revolve around the nucleus.

According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it will experience acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the nucleus. In such a case, the atom would be highly unstable and collapse. 

Bohr’s model of the atom Niels Bohr proposed the following postulates regarding the model of the atom.

(i) Only certain orbits known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in these discrete orbits, the electrons do not radiate energy. These discrete orbits or shells are shown in the following diagram.

The first orbit (i.e., for n = 1) is represented by letter K. Similarly, for n = 2, it is L − shell, for n = 3, it is M − shell and for n = 4, it is N − shell. These orbits or shells are also called energy levels.

The rules for writing of the distribution of electrons in various shells for the first eighteen elements
are given below:

(i) The maximum number of electrons that a shell can accommodate is given by the formula '2n²',

where 'n' is the orbit number or energy level index (n = 1, 2, 3...).
The maximum number of electrons present in an orbit of n = 1 given by
2n² = 2 × 1² = 2
Similarly, for second orbit, it is 2n² = 2 × 2² = 8

For third orbit, it is 2n² = 2 × 3² = 18

And so on .....

(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons.
(iii) Shells are filled with electrons in a step wise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.

The valency of an element is the combining capacity of that element. The valency of an element is determined by the number of valence electrons present in the atom of that element. If the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons.

Thus, the valency of silicon is four. On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. 

Thus, the valency of oxygen is (8 − 6) i.e., two.

(i) Atomic number
The atomic number of an element is the total number of protons present in the atom of that element.
For example, nitrogen has 7 protons in its atom. Thus, the atomic number of nitrogen is 7.

(ii) Mass number
The mass number of an element is the sum of the number of protons and neutrons present in the atom of that element. For example, the atom of boron has 5 protons and 6 neutrons. So, the mass number of boron is 5 + 6 = 11.

(iii) Isotopes
They are atoms of the same element and have same atomic number but different mass number/atomic mass. For example:

Carbon: ¹²₆C and ¹⁴₆C 

(iv) Isobars
They are atoms of different elements having same mass number but different atomic number.
For example calcium, atomic number 20 and argon, atomic number 18.
The number of electrons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of neutrons is the same in the atoms of this pair of elements.

Two uses of isotopes are as follows:
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.

Na has atomic number 11, so its electronic configuration is = 2, 8, 1

When it gives away its outermost shell single electron it changes to Na+ =10= 2, 8

The above configuration indicates completely filled K, L shells.

The average atomic mass of bromine
= (79 x 49.7)+ (81 x 50.3)/100
= (3926.3 + 4074.3)/100
= 8000.6/100
= 80 u

Since average atomic mass

= 16 x X + 18 x (100 – X )/100

16.2 = 16X + 1800 – 18X/100

1620 = – 2X + 1800

2X = 1800 – 1620

X = 180/2 = 90

When 90% is the X-16 sample so for X-18 sample % = 100-90=10%

By Z = 3, we mean that the atomic number of the element is 3. Its electronic configuration is 2, 1.

Hence, the valency of the element is 1 (since the outermost shell has only one electron).

Therefore, the element with Z = 3 is lithium.

Mass number of X    = Number of protons + Number of neutrons

                                 = 6 + 6
                                 = 12

Mass number of Y    = Number of protons + Number of neutrons

                                 = 6 + 8
                                 = 14

These two atomic species X and Y have the same atomic number, but different mass numbers.
Hence, they are isotopes.

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)
(c) The mass of an electron is about 1/2000 times that of proton. (T)
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a)    Atomic nucleus   (√ )
(b)    Electron              ( x )
(c)    Proton                 ( x )
(d)    Neutron              ( x )

Isotopes of an element have
(a) the same physical properties   ( x )
(b) different chemical properties  ( x )
(c) different number of neutrons  (√ )
(d) different atomic numbers       ( x )

Number of valence electrons in Cl⁻ion are:

(a)  16           ( x )
(b)  8             (√ )
(c)  17            ( x )
(d)  18           ( x )

(d) The correct electronic configuration of sodium is 2, 8, 1.