Chemistry - Chapter 4 - Structure of an atom

What was the first indication that atoms were indivisible?

One of the first indications that atoms are not indivisible, comes from studying static electricity and the condition under which electricity is conducted by different substances.

How could you prove atoms were indivisible?

From these activities, can we conclude that on rubbing two objects together, they become electrically charged? Where does this charge come from? This question can be answered by knowing that an atom is divisible and consists of charged particles.

Name the scientists who contributed in revealing the presence of charged particles in an atom. Also, name which particle they discovered respectively.

1. It was known by 1900 that the atom was indivisible particle but contained at least one sub-atomic particle – the electron identified by J.J. Thomson.

2. E.Golstein discovered the proton

3. In 1932, J. Chadwick discovered another sub-atomic particle which had no charge and a mass nearly equal to that of a proton. It was eventually named as neutron

How was the proton discovered?

Even before the electron was identified, E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.

These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.

This sub-atomic particle had a charge, equal in magnitude but opposite in sign to that of the electron.

Its mass was approximately 2000 times as that of the electron. It was given the name of proton. I

What is the mass and charge of a proton and an electron?

In general, an electron is represented as ‘e– ’ and a proton as ‘p+ ’.

The mass of a proton is taken as one unit and its charge as plus one.

The mass of an electron is considered to be negligible and its charge is minus one.

Who was the first one to propose a model for the structure of an atom ?

J.J. Thomson was the first one to propose a model for the structure of an atom.

Explain J.J.Thomson’s model of an atom with a diagram included.

Thomson proposed the model of an atom to be similar to that of a Christmas pudding.

The electrons, in a sphere of positive charge, were like currants (dry fruits) in a spherical Christmas pudding.

We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon,

while the electrons are studded in the positively charged sphere, like the seeds in the watermelon (Fig. 4.1).

What were the two things Thomson proposed?

Thomson proposed that:

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

What were the drawbakcs of Thomson’s model of an atom?

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model, as we will see below.

What was Ernest Rutherford’s model of an atom? Explain with a diagram included.

Ernest Rutherford was interested in knowing how the electrons are arranged within an atom.

Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil.

• He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick.

• α-particles are doubly-charged helium ions. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy.

It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms.

Since the α-particles were much heavier than the protons, he did not expect to see large deflections.

What observations were made in Rutherford’s experiment?

The following observations were made:

(i) Most of the fast moving α-particles passed straight through the gold foil.

(ii) Some of the α-particles were deflected by the foil by small angles.

(iii) Surprisingly one out of every 12000 particles appeared to rebound.

In the words of Rutherford,

“This result was almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you”.

What did Rutherford conclude from the particle scattering experiment ?

Rutherford concluded from the α-particle scattering experiment that—

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.

(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

(iii) A very small fraction of α-particles were deflected by 180⁰ ,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

From the data he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom

What were the features of Rutherford’s model of an atom?

Rutherford put forward the nuclear model of an atom, which had the following features:

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

(ii) The electrons revolve around the nucleus in circular paths.

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

What were the drawbacks of Rutherford’s model of an atom?

The revolution of the electron in a circular orbit 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 electron would lose energy and finally fall into the nucleus.

If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.

What was Bohr’s model of an atom?

In order to overcome the objections raised against Rutherford’s model of the atom, Neils Bohr put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

Describe Bohr’s model of an atom. Draw a diagram of the same.

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

(iii)These orbits or shells are called energy levels

(iv)These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral. Hence,Thomson’s model explained that atoms are electrically neutral,

On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

The proton.

What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

(i) Most of the fast moving α-particles passed straight through the gold foil.

(ii) Some of the α-particles were deflected by the foil by small angles.

(iii) Surprisingly one out of every 12000 particles appeared to rebound.

Why is the mass of an atom given by the sum of masses of protons and neutrons present in the nucleus.

atomic particle which had no charge and a mass nearly equal to that of a proton.The mass of a proton is taken as one unit and The mass of an electron is considered to be negligible.

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

2 neutrons.

By who was the distribution of electrons into different orbits of an atom suggested by?

The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.

What are the rules for writing the number of electrons in different energy levels or shell?

The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) The maximum number of electrons present in a shell is given by the formula 2n² , where ‘n’ is the orbit number or energy level index, 1,2,3,…. Hence the maximum number of electrons in different shells are as follows: first orbit or K-shell will be = 2 × 1²= 2, second orbit or L-shell will be = 2 × 2²= 8, third orbit or M-shell will be = 2 × 3²= 18, fourth orbit or N-shell will be = 2 × 4²= 32, and so on.

(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 the inner shells are filled. That is, the shells are filled in a step-wise manner

What are valence electrons?

The electrons present in the outermost shell of an atom are known as the valence electrons.

What is an atom’s valency?

Therefore, an atom of each element has a definite combining capacity, called its valency.

How will you find the valency of chlorine, sulphur and magnesium?

Atomic number / number of electrons in the atom

Then the number of electrons it needs to lose or gain in order to posses an octet.

Cl → 17, 1-

Mg→ 12, 2+

S → 16, 2-

What is the atomic number of an atom?

It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number,

How is the atomic number of an atom used to define which element an atom is .

. In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, Z = 6.

What are protons and neutrons also known as and why?

mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons

What is the mass number of the atom?

The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.

What are isotopes ? Give examples.

isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.

Therefore, we can say that there are three isotopes of hydrogen atom, namely protium, deuterium and tritium.

The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.

Other such examples are (i) carbon, 12 6 C and 14 6 C, (ii) chlorine, 35 17 Cl and 37 17 Cl, etc.

what should we take as the mass of chlorine atom?

The average atomic mass of chlorine atom, on the basis of above data, will be:

The mass of an atom of any natural element is taken as the average mass of all the naturally occuring atoms of that element

if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated.

This does not mean that any one atom of chlorine has a fractional mass of 35.5 u. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5 u.

What are some applications of isotopes?

Since the chemical properties of all the isotopes of an element are the same, normally we are not concerned about taking a mixture. But some isotopes have special properties which find them useful in various fields.

Some of them are :

(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 goitre.

What are isobars? Give an example of them.

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. The number of protons in these atoms is different, but the mass number of both these elements is 40. That is, the total number of nucleons is the same in the atoms of this pair of elements.