Structure Of The Atom

The radiations that are present in a gas discharge are canal rays. These are the positively charged radiations that ultimately lead to the discovery of protons.

The three subatomic particles in an atom are:

i. Neutrons

ii. Protons

iii. Electrons

There are three subatomic particles in an atom which are the electrons, protons and the neutrons. Neutron is electrically neutral and proton is positively charged and an electron is negatively charged. In an atom, the number of protons is equal to the number of electrons and hence, the overall charge in an atom is zero and it is a neutral particle.

The atomic number of sodium and chlorine is : 11 and 17.
The distribution of electrons in sodium and chlorine atoms is as follows:-

Sodium- K shell: 2 electrons

L shell: 8 electrons

M shell:1 electron

Chlorine- K shell: 2 electrons

L shell: 8 electrons

M shell: 7 electron

K shell can accommodate a maximum of 2 electrons. L shell can accommodate a maximum of 8 electrons and M shell can accommodate a maximum of 8 electrons. Hence, If K, L and M shell of an atom are full, then the total number of electrons in the atom would be: 2+8+8=18 electrons.

The combining capacity of an atom is called its valency.

Now, sodium has only one electron in its outermost shell which it can lose very easily to gain an octet configuration. Hence, the valency of sodium is 1.

Chlorine has seven electrons in its outermost shell and it could complete its octet by gaining one electron. Hence, the valency of Chlorine is 1.

Argon has fully filled outermost hell with no vacancy available for an extra electron. Hence, it’s valency is 0.

Ernest Rutherford selected a gold foil for his experiments because he wanted as thin a layer as possible. The gold foil that the had used was only 1000 atoms thick.

According to Thomson’s atomic model electrons were embedded in a sphere of positive charge. This implies that according to Rutherford’s gold foil experiment in which the alpha particles which are positively charged must be reflected or deflected in large numbers but after performing the experiment it was being found out that most of the alpha particles pass straight through. This observation clearly meant that most of the space inside an atom is empty which was contrary to that predicted in Thompson’s model. Hence, Ernest Rutherford got unexpected results from his experiment.

An atoms chemical reactivity is mostly based upon its desire to have a fully filled outermost shell become stable. If an atom is having a completely filled outermost shell it is already stable and hence shows less chemical activity.

According to Thomson’s model of the atom electrons were embedded in a sphere of positive charge and negative and positive charges are equal in magnitude. But after performing Rutherford’s gold foil experiment it was concluded that most of the space inside an atom is empty because most of the alpha particles pass straight through without getting deflected which was in contrary to what has been predicted by Thomson’s model. Hence, Thomson’s model of an atom was rejected.

The drawback of Rutherford’s model of an atom are:-

The orbital revolution of electrons is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus the revolving electrons would lose energy and finally fall into the nucleus. If this were so the atom would be highly unstable and hence matter would not have existed but as we know the atoms are quite stable.

The conclusions drawn by Rutherford from alpha particle scattering experiment are:-

i. There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides inside the nucleus.

ii. The electrons revolve around the nucleus in well-defined orbits.

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

The drawback of Rutherford’s model of an atom are:-

The orbital revolution of electrons is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus the revolving electrons would lose energy and finally fall into the nucleus. If this were so the atom would be highly unstable and hence matter would not have existed but as we know the atoms are quite stable.

Thomson’s model of an atom: -

Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons in a sphere of positive charge, where like currants (dry fruits) in a spherical Christmas pudding.

Thomson proposed that:-

i. An atom consists of a sphere of positive charge with negatively charged electrons embedded in it.

ii. The positive and the negative charges in an atom are equal in magnitude due to which an atom as a whole is electrically neutral.

The diagram is shown below:

Bohr’s model of an atom:-

i. Only certain special orbits known as discrete orbits of electrons are allowed inside an atom.

ii. While revolving in discrete orbits the electrons do not radiate energy. These orbits are known as energy levels.

The diagram is shown below:

Rutherford’s nuclear model of an atom: -

i. There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides inside the nucleus.

ii. The electrons revolve around the nucleus in well-defined orbits.

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

Bohr-Bury scheme –

i. The maximum number of electrons that could be present in a shell is given by the rule 2n², where n is the orbit number or energy level.

Example, in first shell, n = 1, then number of electrons = 2

Similarly for other shells, the diagram is shown below:

ii. The maximum number of electrons that can be accommodated in the outermost orbit is 8.

iii. Electrons are not accommodated in a given shell unless all the inner shells are completely filled.

Atoms of the same element having a same atomic number but different mass numbers are known as isotopes.

Applications of isotopes:-

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.

iii. An isotope of iodine is used in the treatment of goiter.

a. Atomic number:

The number of protons present in an atom is called its atomic number. It is represented by the symbol: Z.

b. Atomic mass:

Atomic mass is the sum of the total number of protons and neutrons present in the nucleus of an atom.

c. Isobars:

Isobars are the atoms of different elements having a different atomic number but the same mass number.

a.

Sketch of Bohr’s model of an atom

b.

Chlorine atom

Chloride ion

Neon atom

Helium atom

Gold is one of the most malleable metal we have and we could extract thin foils from it but other metals are not that malleable. So if we use a metal other than gold in the alpha-particle scattering experiment we would not being able to get a thin foil. A comparatively thick foil means that the number of atoms per unit volume would be large and hence the number of alpha particles passing through the foil without being deflected would decrease and those getting deflected would increase in comparison with the gold foil.

On the basis of Thomson’s model of an atom, an atom consists of an electrically positively charged sphere in which negative charges are embedded. The magnitude of the positive and negative charges are equal and hence, the atom as a whole is electrically neutral.

On the basis of Rutherford’s model of an atom all the positive charge and mass of an atom resides inside the nucleus. More particularly we could say that on the basis of Rutherford’s model of an atom proton is present in the nucleus.

Distribution of electrons in chloride ion are:-

Chlorine ion - K shell: 2 electrons

L shell: 8 electrons

M shell: 8 electron

Hence, the number of valence electrons in chloride ions are 8 as its outermost orbit contains 8 electrons.

We know that,

Atomic number = Number of protons = 9.

Hence, 9p

Mass number = Number of protons + Number of neutrons

⇒ 19 = 9 + Number of neutrons

⇒ Number of neutrons = 19-9 =10

Hence, 10n

Now, Number of electrons = Number of protons

⇒ Number of electrons =9

Hence, 9e

Therefore the correct option is B

An alpha particle is obtained by removing 2 electrons from a helium atom. So, an alpha particle is doubly-charged helium ion.

Hence, the correct option is D.

Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons in a sphere of positive charge, where like currants (dry fruits) in a spherical Christmas pudding.

Thomson proposed that:-

i. An atom consists of a sphere of positive charge with negatively charged electrons embedded in it.

ii. The positive and the negative charges in an atom are equal in magnitude due to which an atom as a whole is electrically neutral.

Hence, the statements i, ii, iii are correct and the correct option is A.

The conclusions drawn by Rutherford from alpha particle scattering experiment are:-

i. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides inside the nucleus.

ii. The electrons revolve around the nucleus in well-defined orbits.

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

Hence, statements ii and iv are correct and the correct option is B.

Atoms of the same element having same atomic number but different mass numbers are known as isotopes.

As the charge is zero implies that there is no change in the number of electrons and also the number of protons in the two oxygen atom is the same as they have the same atomic number. Different number of neutrons implies that they are isotopes.

Hence, the correct option is C.

The maximum number of electrons that can be accommodated in the first shell is 2 and that in the 2^nd shell is 8.

In fig ii, the number of electrons in the first shell is 4 and in fig iv the number of electrons in the second shell is 9 both of which are above the allowed capacity. So, they do not represent Bohr’s model of an atom correctly.

Hence, the correct option is C.

An atom always have equal number of electrons and protons to maintain the electrical neutrality.

Hence, the correct option is A.

The charge on an electron is equal to 1.6 x 10^-19 C of –ve charge which is a calculated value which always stands to be true whatever the atom may be.

Hence, the correct option is A.

I – 131 and Co – 60 are respectively the isotope of iodine and cobalt used in the treatment of cancer and goiter.

Hence, the correct option is C.

U – 235 and C – 14 are respectively the isotope of uranium used as a fuel in nuclear reactors and carbon used in radiocarbon dating.

Hence, the correct option is D.

The elements with valency +1 are always metals present in the left most corner of the periodic table in group I. They have only one electron in their outermost shell which they could lose easily to get an outermost shell with the maximum number of allowed electrons. But a -1 valency could be shown by the group 17 nonmetals also which have 7 electrons in their outermost shell and require only one electron to complete their octet.

Hence, the correct option is C.

J.J.Thomson proposed the first model of the atom to be similar to that of a Christmas pudding. On the basis of Thomson’s model of an atom, an atom consists of an electrically positively charged sphere in which negative charges are embedded. The magnitude of the positive and negative charges are equal and hence, the atom as a whole is electrically neutral.

An atom with 3 protons would have 2 electrons in its innermost shell and 1 proton in its outermost shell. The outermost proton could be easily given away by the atom to attain a stable two electron inner shell configuration. So, the valency would be 1.

Hence the correct option is C.

The atomic number of aluminum is 13.

Hence, the electron distribution in an aluminum atom would be 2, 8, 3.

So, A is the correct option.

According to Bohr-Bury scheme, the maximum number of electrons which can be accommodated in a given shell is given by the formula 2n².

Hence, the correct option is A.

The nucleus of hydrogen contains only one proton and hence it is known as a proton.

Hence, the correct option is A.

Rutherford’s atomic model has the following features:-

i. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides inside the nucleus.

ii. The electrons revolve around the nucleus in well-defined orbits which can be compared to solar system.

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

Hence, statement I and iii are correct t.

So, the correct option is A.

Mass number is always equal to the number of protons plus the number of neutrons and Atomic number is always equal to the number of protons which is equal to the number of electrons to maintain the neutrality of the atom.

Hence, option D is correct.

Thomson proposed the first atomic model followed by Rutherford and Bohr.

Hence, option C is the correct option.

The presence of protons was suggested by J.Chadwick and the presence of

neutrons at the centre of the atom was suggested by J.J.Thomson.

Hence, option A is correct.

The presence of electrons outside the nucleus was suggested by J.J.Thomson and the presence of nucleus at the centre of the atom was suggested by E.Rutherford.

Hence, the correct option is C.

Any element is represented symbolically as:

Here mass number = 15 +16 =31

Atomic number = number of protons =15

Hence, the correct representation of the element with 15 electrons and 16 neutrons is .

Hence, the correct option is – A

Postulates of Dalton’s atomic theory.

● The matter is made up of indivisible particles known as atoms.

● The properties of all the atoms of a given element are same including mass. This can also be stated as all the atoms of an element have identical mass while the atoms of different elements have different masses.

● Atoms of different elements combine in fixed ratios to form compounds.

● Atoms are neither created nor destroyed. This implies that during chemical reactions, no atoms are created nor destroyed.

● The formation of new products (compounds) results from the rearrangement of existing atoms (reactants).

● Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size and many other chemical or physical properties.

Hence, the correct option is D.

Given that the Mass number of the atom is 27.

⇒ Number of protons + number of neutrons =27

⇒ Number of protons + 14 =27

⇒ Number of protons = 13

Therefore number of electrons in a neutral atom = 13

The ion of the element has 3 positive charges.

⇒ Number of electrons = 13-3=10

Hence, option B is correct.

True

The above statement is true as 1m = 10⁹nm.

False

Atoms of a given element may be identical in mass or may not as isotopes are also the atoms of the same element but have different masses, but all the atoms have same chemical properties.

Hence, the above statement is false.

True

According to Dalton’s theory atoms combine in the ratio of small whole numbers to form compounds.

Hence, the above statement is true.

True

According to the postulates of Dalton’s atomic theory atoms of different elements combine in fixed ratios to form compounds. So, in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever is the source of water.

Hence, the above statement is true.

False

According to the postulates of Dalton’s atomic theory atoms of different elements combine in fixed ratios by mass to form compounds. So, in chemical substance, the elements are always present in fixed proportions by mass.

Hence, the above statement is false.

True

One atomic mass unit is a mass unit which is equal to the mass of exactly one-twelfth (1/12^th) of the mass of one atom of carbon-12 isotope.

Hence, the above statement is true.

False

The relative atomic masses of all elements have been found with respect to the mass of exactly one-twelfth (1/12^th) of the mass of one atom of carbon-12 isotope.

Hence, the above statement is false.

False

According to the postulates of Dalton’s atomic theory the relative number and kinds of atoms are fixed in a given compound.

Hence, the above statement is false.

False

Each isotope of an element is a pure substance.

Hence, the above statement is false.

True

The combining capacity of an atom is called its valency. Thus, atoms of each element has a definite combining capacity which is determined by its valency.

Hence, the above statement is true.