9th Class Science Structure of the Atom

Structure of the Atom

Chapter Overview

• Introduction
• Thomson’s Model of Atom
• Rutherford’s Model of Atom
• Bohr’s Model of Atom
• Discovery of Neutron
• Atomic Number and Mass Number
• Electronic configuration (Bohr-Bury Sheme)
• Concept of Valency
• Isotopes
• Isobars

1. Introduction

On the basis of experimental observations, different models have been proposed for the structure of an atom.

2. Thomson’s model of Atom

According to Thomson's model, atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it. This model was called as plum pudding model. The electrons present the plums in the pudding made of negative charge. This model is similar to of watermelon in which the pulp represents the positive charge and positive charge the seeds denote the electrons,  

Fig. 2.1. Thomson?s plum-pudding model

1. Rutherford’s Model of Atom

(Alpha Particle Scattering Experiment)

Rutherford in 1911 performed an experiment which led to the downfall of Thomson's model According to Rutherford's model:

1. An atom contains a dense and positively charged region located at its centre, it was called as nucleus.
2. All the positive charge of an atom and most of its mass was contained in the nucleus.
3. The rest of an atom must be empty space which contains the much smaller and negatively charged electrons.
4. Total negative charge on the electrons is equal to the total positive charge on the nucleus, so that atom on the whole is electrically neutral.

On the basis of the proposed model, the experimental observations in the scattering experiment. The a-particles passing through the atom in the region of the electrons would pass straight without any deflection. Only those particles that come in close vicinity of the positively charged nucleus get deviated from their path. Very few \[\alpha \]-particles, those that collide with the nucleus, would face a rebound.      

2. Bohr’s Model of Atom

In 1913 Niels Bohr, a student of Rutherford proposed a model to account for the shortcomings of Rutherford's model. Bohr's model can be understood in terms of two postulates proposed by him. The postulates are:                                                                 

Postulate 1. The electrons move in definite circular paths of fixed energy around a central nucleus.      

Fig. 4.1. Illustration showing different orbits or the energy levels of fixed energy in an atom according to Bohr?s model

Postulate 2. The electron can change its shells or energy level by absorbing or releasing energy. An electron at a lower state of energy \[{{E}_{i}}\] can go to a final higher state of energy \[{{E}_{f}}\]by absorbing a single photon of energy given by.

\[E=hv={{E}_{f}}-{{E}_{i}}\]

Fig. 4.2. The electrons in an atom can change their energy level by absorbing suitable amounts of energy or by emitting energy.

1. Discovery of Neutron

It was suggested that there must be one more type of subatomic particle present in the nucleus which may be neutral but have mass.

Such a particle was discovered by James Chadwick in 1932. This is found to be electrically neutral and was named neutron. Neutrons are present in the nucleus of all atoms, except hydrogen. A neutron is represented as 'n' and is found to have a mass slightly higher than that of a proton. Thus, if the helium atom contained 2 protons and 2 neutrons in the nucleus, the mass ratio of helium to hydrogen (4 : 1) could be explained. The characteristics of the three fundamental particles constituting the atom are given in Table 4.1.

Table 5.1 Characteristics of the Fundamental Subatomic Particles 

1. Atomic Number and Mass Number

Nucleus of atom contains positively charged particles called protons and neutral particles called neutrons. The numbers of protons in an atom is called the atomic number and is denoted by the symbol "Z'.

The number of nucleons in the nucleus of an atom is called its mass number. It is denoted by 'A' and is equal to the total number of protons and neutrons present in the nucleus of an element. Thus,

Atomic number and mass number are represented on the symbol of an element. An element X with an atomic number, Z and the mass number, A is denoted as follows:

\[_{Z}^{A}X\]

For example, \[_{6}^{12}C\] means that the carbon has an atomic number of 6 and the mass number of 12. This can be used to compute the number of different fundamental particles in the atom.

As the atomic number is 6

\[\Rightarrow \]Number of protons = number of electrons = 6

As         mass number = number of protons + number of neutrons

\[\Rightarrow \]   12 = 6 + number of neutrons

\[\Rightarrow \]   number of neutrons =12 – 6=6

\[\Rightarrow \] An atom of    has 6 protons, 6 electrons and 6 neutrons.

1. Electronic configuration: Distribution of electrons in different orbits (Bohr bury scheme)

The arrangement of electrons in various shells of an atom of an element in known as electronic configuration of the element. We should know the following rules for writing down the electronic configuration of an element:

1. Number of electrons present in one atom of the element.
2. Maximum number of electrons which can be accommodated in different energy shells (electron orbits) of the atom.

The distribution of electrons in different energy shells of an atom is governed by a schema known as Bohr-Bury Scheme. According to this scheme:

(I) These orbits or shells in an atom are represented by the letters K, L, M, N,... or the positive integral numbers, n = 1, 2, 3, 4..........

(II) The orbits are arranged in the order of increasing energy. The energy of M-shell is more than that of the L-shell which in turn is more than that of the K-shell.

(III) The maximum number of electrons present in a shell is given by the formula 2n², where 'n' is the number of the orbit or the shell.

(IV) The shells are occupied in the increasing order of their energies.

(V) Electrons are not accommodated in a given shell, unless the inner shells are completely filled.

(VI) The outermost shell of an atom cannot accommodate more than eight electrons, even if it has the capacity to accommodate more electrons.

Table 6.1: Electronic configurations of First Twenty Elements

1. Concept of Valence

Valency of an element may also be defined as:

The number of hydrogen atoms or chlorine atoms or twice the number of oxygen atoms which combine with one atom of an element is called its valency.

For example, in forming water, one atom of oxygen combines with two atoms of hydrogen,

Hence, the valency of oxygen (O) in water \[({{H}_{2}}O)\] is two. The valency of aluminium in its oxide \[A{{l}_{2}}{{O}_{3}}\]is found as follows-

Number of oxygen atoms combining with two atoms of Al = 3.

Number of oxygen atoms combining with one atom of \[Al=\frac{3}{2}\]

Hence, the valency of Al (as per definition it is twice the number of oxygen atoms)

\[=2\,\times =3\]

In general, the valency of an element is equal to the number of valence electrons or is equal to eight minus the number of valence electrons.

2. Isotopes

Basically, isotope is a Greek term where isos = same and topos = place. That is, all the isotopes of an element belong to the same group and same period in the periodic table to which that element belongs.

Atoms of the some element which have same number of protons but different number of neutrons inside their nuclei are called isotopes.

Properties-1. Since the chemical properties of an element are dependent on its number of electrons, and the isotopes have same number of electrons because of same atomic number the isotopes have similar chemical properties.

2. Since the mass numbers of isotopes are different because of the presence of different number of neutrons, the physical properties like density, melting point and boiling point etc.

when depend on the atomic mass, show variation.

Example: Isotopes of Hydrogen-There are three isotopes of hydrogen: protium, deuterium and tritium. They are represented as

Fig. 9.1

Characteristics of Isotopes

1. The isotopes of an element have the same number of protons and electrons but different number of neutrons.
2. The isotopes of an element have different mass numbers and therefore, different masses.
3. The isotopes of an element possess the same electronic configuration and the same number of valence electrons and exhibit the same chemical properties.
4. The isotopes of an element exhibit different physical properties such as density, melting point, boiling point etc.

Application of Isotopes

1. Use in medicine - Different isotopes are used for the treatment of various diseases.
2. In Radio carbon Dating - It is the method of estimating the age of dead objects containing carbon such as fossils and pieces of wood by measuring the amount of the isotope in the dead object relative to that of the isotope in a living object.
3. In Tracer Technique - some isotopes are used as tracers to detect the presence of tumours
4. Industrial Use - Isotope\[_{15}^{33}\] is used in the manufacturing of steel from cast iron in order to find out the complete removal of phosphorous from steel.
5. Industrial Use - \[_{15}^{32}\]is used in fertilizer industry and radiations from radioisotopes are used to develop disease free, high yielding seeds.
6. In electricity generation - Isotopes \[_{92}^{235}U\]and \[_{94}^{239}Pu\]are used as fuel in nuclear reactors for generation of cheaper electricity.

11. Isobars

The atoms of different elements with different number of protons (i.e., different atomic numbers) but equal sum of the number of protons and neutrons (i.e., same mass number) are called isobars. (Greek meaning isos = equal, boros = weight), Isobars have different physical and chemical properties.

Examples:

Chapter at a Glance

• According to Dalton's atomic theory, the atom is considered to be the smallest indivisible constituent of all matter. This theory could explain the law of conservation of mass, law of constant composition and law of multiple proportions.
• Sir J.J. Thomson discovered that when very high voltage was passed across the electrodes in the cathode ray tube, the cathode produced rays that travel from cathode to anode and were called cathode rays.
• Eugene Goldstein discovered anode rays by using a perforated cathode (a cathode having holes in it) in the discharge tube filled with air at a very low pressure.
• According to Thomson's plum pudding model, atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it.
• The \[\alpha \text{-}\]ray scattering experiment performed by Geiger and Marsden led to the failure of
• Thomson's model of atom. In this experiment, a stream of a-particles from radioactive source was directed on a thin piece of gold foil. Most of the a-particles passed straight through the gold foil, some a-particles were deflected by small angles, a few particles by large angles and very few experienced a rebound.
• The results of a-ray scattering experiment were explained in terms of Rutherford's model.
• The Rutherford's model however, failed as it could not explain the stability of the atom, the distribution of electrons and the relationship between the atomic mass and atomic number (the number of protons).
• The problem of the stability of the atom and the distribution of electrons in the atom was solved by Neils Bohr in terms of Bohr's model of the atom.
• In 1932 James Chadwick discovered an electrically neutral particle in atom and named it as neutron.
• The number of protons in an atom is called the atomic number and is denoted as 'Z'. On the other hand, the number of nucleons (protons + neutrons) in the nucleus of an atom is called its mass number and is denoted as "A'.
• The electrons are distributed in different shells in the order of increasing energy. The distribution is called electronic configuration. The maximum number of electrons present in a shell is given by the formula 2n², where 'n¹ is the number of the orbit or the shell.
• The valence is the number of chemical bonds that an atom can form with univalent atoms. If the number of valence electron is four or less, then the valency is equal to the number of the valence electrons. On the other hand, if the number of valence electrons is more than four, then generally, the valency is equal to 8 minus the number of valence electrons.
• The atoms of an element which have the same number of protons but different number of neutrons are called isotopes. Isotopes have the same atomic number but different mass numbers.
• The atoms of different elements which have the same mass number but different atomic number are called isobars.