Chapter 4 - STRUCTURE OF ATOM

CLASS - 9 [CHEMISTRY]

Matter: Any which has mass and occupy space is called matter.

Atoms and molecules are the building blocks of the matter.

Atoms are made up of three subatomic particles like proton, electrons and neutrons.

Electron has negative charge

Proton has positive charge

Neutron has no charge (neutral).

Proton and neutrons are present in a small nucleus at the centre of the atom (entire mass of an atom).

Mass of electron is very very small as compared to the proton and neutron (negligible).

Electrons revolve around the nucleus of an atom in a fixed circular path called shells.

Total charge on an atom is neutral due to the presence of the equal no.s of protons and electrons.

Exception: Hydrogen atom is made of only one electron and one proton. It does not contain any neutron.

Discovery of Electron

Electron is discovered by the scientist William Crookes in 1979 & J.J. Thomson.

He passed the electricity at high voltage (10000 - 20000 V) through a gas at very low pressure (0.0001 mm of Hg) taken in a discharge tube. The electrical energy pushes out some of the electron from the atoms of the gas. At the end of the tube glow was seen on the walls of the glass tube . The rays emitted by the cathode travels towards the anode reveals that the rays carries negative charge.

PROPERTIES OF CATHODE RAYS

* When any opaque object is placed in the path of the cathode rays these rays cast shadow of the object on the wall opposite to the cathode, this shows that cathode travels in straight line.

* When wheel was placed in the path of cathode rays the wheel will rotate, this shows that rays consists of some particles having mass and kinetic energy.

* In the presence of an electric field the rays are deflected towards the positive plate, this confirms that cathode rays consists of negatively charged particles.

* Cathode rays deflected by a magnetic field:

Streams of minute particles were given out by the cathode(negative electrode).

These streams of particles are called cathode rays.

The mass and charge of the cathode ray particles does not depend on the nature of the gas taken in discharge tube.

Electrons: Cathode rays consist of small negatively charged particles.

Symbol of electron : e-

Mass of an electron:

Relative mass: 1/1840 u ( mass compared with mass of hydrogen atom i.e. 1u)

Absolute mass of an electron: 9 × 10 -28 g

Charge of an electron:

Relative charge: -1

Absolute charge: 1.6 × 10 -19 coulomb.

Discovery of proton

Protons are discovered by the scientist by E. Goldstein.

When he passed the electricity at high voltage through a hydrogen gas at very low pressure taken in a discharge tube. The electrical energy removes the electron from the hydrogen atoms. After the removal of electrons from hydrogen atom, a positively charged particles called protons are formed. These fast moving protons forms the anode rays.

The mass and charge of the anode ray particles depend on the nature of the gas taken in discharge tube.

Protons: Anode rays consist of small positively charged particles.

Symbol of protons : P+

Mass of proton:

Mass of proton is equal to the mass of hydrogen atom.

Relative mass: 1 u

Absolute mass: 1.6 × 10 -24 g

Charge of a proton:

Relative charge: +1

Absolute charge: 1.6 × 10 -19 coulomb.

Discovery of Neutrons

After the discovery of proton and electrons , it was noticed that all the mass of an atom cannot be accounted on the basis of only protons and electrons present in it.

For example: Carbon - Atomic no. 6

Protons - 6

Electrons - 6

The mass of electron is so small that it can be ignored.

So. the atomic mass of carbon should be only equal to the mass of 6 protons i.e. 6 u.

But the actual atomic mass of the carbon atom is 12 u.

This problem was solved by the scientist James Chadwick in 1932.

He discovered the another subatomic particle of the atom called neutron.

The neutron is a neutral particle found in the nucleus of an atom.

Exception: Atoms of hydrogen element do not contain neutron.

Symbol of neutrons : n

Mass of neutron:

Relative mass: 1 u

Absolute mass: 1.6 × 10 -24 g

Charge of a neutron:

Relative/ Absolute charge: no charge( electrically neutral)

Atomic Mass: Mass of protons + Mass of neutrons

Atomic number : No. of protons or the no. of electrons

Mass no. : No. of protons + No. of neutrons

Structure of Atom

Thomson's Model Of The Atom:

J.J. Thomson proposed his model in 1903, then only electrons and protons were known to be present in the atom.

According to the Thomson's model of atom:

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

2. The positive and negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral.

Drawback of Thomson's model of atom:

It could not explain the results of various experiments carried out by other scientist such as Rutherford's experiments.

RUTHERFORD'S MODEL OF AN ATOM

Alpha particles: Alpha particle is a positively charged particle having 2 units of positive charges and 4 units of mass.

He - Helium

Atomic no. - 2

No. of protons - 2

No. of electrons - 2

When helium looses 2 electron , then it develops the +2 charge.

He 2+ ion is called the alpha particle.

Alpha particles are emitted by radioactive elements like radium and polonium.

Rutherford's Experiment - Discovery of Nucleus

When fast moving alpha particles are allowed to strike a very thin gold foil in vacuum.

Why he has taken only gold foil?

Because he needs very thin sheet of substance to conduct this experiment, Gold is the most malleable metal which can be beaten into thin sheet easily.

When fast moving alpha particles are allowed to strike a very thin gold foil in vacuum.It is found that,

1. Most of the alpha particles pass straight through the gold foil without any deflection from original path.

2. A few alpha particles are deflected through small angles and a few are deflected through large angles.

3. A very few alpha particles completely rebound on hitting the gold foil and turn back on their original path.

Rutherford explained these observation in the following ways.

1. Most of the alpha particles pass straight through the gold foil without any deflection from original path, it shows that there is a lot of empty space in the atom.

2. A few alpha particles are deflected through small angles and a few are deflected through large angles, it shows that there is a 'centre of positive charge' in the atom which repels the positively charged alpha particles and deflects them from their path.

The centre of positive charge in the atom is known as nucleus.

3. A very few alpha particles completely rebound on hitting the gold foil and turn back on their original path, it shows that nucleus is very dense and hard which does not allow the alpha particles to pass through it.

4. No. of alpha particles which are deflected is very small, so the size of the nucleus must be very small as compared to the size of the atom.

On the basis of the experiment, Rutherford put forward the nuclear model of atom:

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

2. The electrons revolve around the nucleus in circular paths.

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

Drawback of Rutherford's model of the atom.

a) In Rutherford's model of the atom, the negatively charged electrons are revolving around the positively charged nucleus in the circular path.

If an object moves in a circular path, then its motion is said to be accelerated ( change in velocity).

According to the electromagnetic theory of physics, if a charged particles undergoes accelerated motion, then it must radiate energy continuously. Thus, the energy of revolving electrons will decrease gradually and their speed will go on decreasing.

The electron will then be attracted more strongly by the oppositely charged nucleus due to which they will come more closer to the nucleus and ultimately electrons fall into the nucleus and atom collapse.

b) Rutherford model could not solve the problem of atomic mass of atom as it proposed only the existence of protons in the nucleus.

Introduction to the Bohr Model

Bohr model of the atom was proposed by Neil Bohr in 1915. It came into existence with the modification of Rutherford’s model of an atom. Rutherford’s model introduced the nuclear model of an atom, in which he explained that a nucleus (positively charged) is surrounded by negatively charged electrons.

Bohr modified this atomic structure model by explaining that electrons move in fixed orbitals (shells) and not anywhere in between and he also explained that each orbit (shell) has a fixed energy level. Rutherford basically explained the nucleus of an atom and Bohr modified that model into electrons and their energy levels.

Bohr’s model consists of a small nucleus (positively charged) surrounded by negative electrons moving around the nucleus in orbits. Bohr found that an electron located away from the nucleus has more energy, and electrons close to the nucleus have less energy.

Postulates of Bohr’s Model of an Atom

• In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a definite circular path called orbits or shells.

• Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.

• The energy levels are represented by an integer (n=1, 2, 3…) known as the quantum number. This range of quantum number starts from nucleus side with n=1 having the lowest energy level. The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the lowest energy level, it is said to be in the ground state.

• The electrons in an atom move from a lower energy level to a higher energy level by gaining the required energy and an electron moves from a higher energy level to lower energy level by losing energy.

Limitations of Bohr’s Model of an Atom

• Bohr’s model of an atom failed to explain the Zeeman Effect (effect of magnetic field on the spectra of atoms).

• It also failed to explain the Stark effect (effect of electric field on the spectra of atoms).

• It violates the Heisenberg Uncertainty Principle.

• It could not explain the spectra obtained from larger atoms.

Atomic number (Z)

Atomic number of an element is equal to the number of protons present in the nucleus of an atom.

Since an atom is electrically neutral, thus number of protons and number of electrons are equal, therefore:

Atomic number (z) = number of protons = number of electrons.

Mass Number (A) The total number of the protons and neutrons present in the nucleus of an atom is called mass number. It is denoted by A.

The protons and neutrons together are called nucleon.

The number of neutrons present in the nucleus of an atom is represented by N.

The symbol of the element with atomic number and mass number -

For example - Nitrogen is written as 147N, where 14 is the mass number and 7 is the atomic number.

Atomic no. - Subscript

Mass no. - Superscript

Electronic Configuration:The arrangement of electrons in various shells (energy levels) of an atom of the element is known as Electronic configuration.

The Maximum number of electrons that could be put in a particular shell, i.e., energy levels, was given by Bohr and Bury.

According to Bohr-Bury Scheme:

• The maximum number of electrons that can be accommodated in any energy level is given by 2n2where n = 1, 2, 3, 4, …. (for K, L, M, N…..)

• The maximum number of electrons in the outermost orbit will be 8 electrons even if it has capacity to accommodate more electrons.

• Electrons, are not accommodated in a given shell. Unless earlier shells are filled, that is stepwise filling of shells is followed.

Q1. Write the distribution of electrons in Carbon and Sodium atoms.

Q2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Q3. If the number of electrons in an atom is 8 and number of protons is also 8, then

(i) What is the atomic number of the atom? and

(ii) What is the charge on the atom?

Q4. Na+ has completely filled K and L shells. Explain.

Q5. Composition of the nuclei of two atomic species X and Y are given as under

X Y

Protons = 6 6

Neutrons = 6 8

Give the mass numbers of X and Y. What is the relation between the two species?

Valency

• Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.

• The atoms which have completely filled the outermost shell are not very active chemically.

• The valency of an atom or the combining capacity of an atom is given by the number of electrons lose, gain or share by an atom.

• For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.

• What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?

Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.

Isotopes

• The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.

• Isotopes are pure substances.

• Isotopes have a similar chemical nature.

• Isotopes have distinct physical characteristics.

For example -

1. Hydrogen - It exists in three different isotopic forms:

2. Carbon - It exists in three different isotopic forms:

Where can we use Isotopes? 1. The fuel of Nuclear Reactor – Isotope of Uranium 2. Treatment of Cancer – Isotope of Cobalt 3. Treatment of Goiter – Isotope of Iodine

Fractional Mass number of elements: If an element is a mixture of isotopes, it may have fractional mass number.

For example, chlorine exists in two different isotopic forms i.e. 3517Cl and 3717Cl in the ratio of 3 : 1 i.e. 75 % and 25 %. The varying mass numbers are 35 and 37 respectively. For calculating Mass number of chlorine, percentage of the isotopic form is used to calculate the average mass.

Isobars The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars.

Questions

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

Q 2. Number of electrons and neutrons in an atom are 3 and 4 respectively. Find its:

(a) Atomic number

(b) Mass number?

Q 3. Name the particles which determine the mass of an atom.

Q 4. What is octet rule?

Q 5. Why does Helium has Zero valency?

Q 6. The isotopes of an atom are chemically same. Why?