### ABSTRACT

This blog lays down the important concepts associated with Atoms and gives the students insights of its application for problem solving.

### TABLE OF CONTENTS

- Introduction
- Alpha Particle
- Scattering and Rutherford’s Nuclear Model of Atom
- Atomic Spectra
- Bohr Model of the Hydrogen Atom
- The Line Spectra of the Hydrogen Atom
- De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation
- Applications
- Summary

### 1. INTRODUCTION

Atoms are the most fundamental unit of matter. Molecules are combinations of atoms of different atoms in different proportions. Atoms play an important role in determining the nature of a substance. Atoms are composed of electrons, protons and neutrons, of which protons and neutrons reside inside the nucleus and electrons revolve around the nucleus in orbits.

### 2. ALPHA PARTICLE

An alpha particle is identical to a helium atom that has been stripped of its two electrons; thus, an alpha particle contains two protons and two neutrons. Because an alpha particle has no electrons to balance the positive charge of the two protons, it has a charge of +2 and can be represented as He2+. If a particle has a charge, whether negative or positive, it can be shown as a superscript. Thus He2+ means a helium atom that has lost two electrons and has a +2 charge. The symbol O2- means an oxygen atom that has added two electrons and thus has a charge of -2. Atoms that have acquired a charge by losing or gaining electrons are called ions.

### 3. SCATTERING AND RUTHERFORD’S NUCLEAR MODEL OF ATOM

Rutherford performed an experiment in which he bombarded a gold foil with alpha particles. It was observed that many alpha particles passed through the gold foil without any deviation and some were deviated and very few didn’t pass the gold foil and it seemed as if they collided with a heavy mass. From this Rutherford concluded that most part of the atom is empty and nearly all the mass of the atom is concentrated in the central part of the atom known as nucleus.

### 4. ATOMIC SPECTRA

When electrons in an atom are excited by energy then they jump to higher energy levels where they are unstable, then to attain stability they jump back to their original energy level and in the process, they emit spectral lines of different wavelength. There are five spectral series observed in Hydrogen and Hydrogen like atoms, namely Balmer, Lyman, Paschen, Brackett and Pfund series. The wavelength of the Series is given by:

$\frac{1}{\lambda}=R\left(\frac{1}{n_{0}^{2}}-\frac{1}{n^{2}}\right)$,

Where n0 = 2, 3, 4, 5, 1 for Balmer, Paschen, Brackett, Pfund and Lyman series.

### 5. BOHR MODEL OF HYDROGEN ATOM

According to Bohr’s model of Hydrogen atom:

An electron revolves around the nucleus in orbits.

The angular momentum of the electron revolving in an orbit is $\mathrm{L}=\frac{n h}{2}$.

When an electron transits from a higher energy to a lower energy orbit then the extra energy gets radiated in the form of photon $h v=E_{i}-E_{f}$

### 6.THE LINE SPECTRA OF HYDROGEN ATOM

The concept of atomic spectra applies to hydrogen atom as well, there are five spectral series observed in Hydrogen and Hydrogen like atoms, namely Balmer, Lyman, Paschen, Brackett and Pfund series. The wavelength of the Series is given by:

$\frac{1}{\lambda}=R\left(\frac{1}{n_{0}^{2}}-\frac{1}{n^{2}}\right)$,

Where n0 = 2, 3, 4, 5, 1 for Balmer, Paschen, Brackett, Pfund and Lyman series.

### 7. DE BROGLIE’S EXPLANATION OF BOHR’S SECOND POSTULATE OF QUANTISATION

De-Broglie assumed that the wavelength associated with an electron must be an integral multiple of the circumference of the circular orbit, i.e. $2 \square r=r$. Now De-Broglie wavelength is given by $=\frac{h}{p}$. Therefore $2 \square r=n \frac{h}{p}$ , we know that p = mv, then on rearranging the above equation we get $\mathrm{mvr}=\frac{n h}{2 \pi}$.

### 8. APPLICATIONS

- Radioactive elements are used to produce energy for our daily uses.
- Radioactive radiation helps in killing microorganisms.

### CHEAT SHEET:

Wavelength of spectral lines $\frac{1}{\lambda}=R\left(\frac{1}{n_{0}^{2}}-\frac{1}{n^{2}}\right)$,

Angular momentum of an electron in the nth orbit $\mathrm{L}=\frac{n h}{2}$.

Change in energy when an electron jumps from one energy level to another energy level,$h v=E_{i}-E_{f}$

### 9. SUMMARY

In this blog we have seen what is an atom and its various constituents and how are ions formed from atoms. We have now studied the Rutherford’s experiment and its result, about the atomic spectra and the bohr model of hydrogen atom and de broglie's explanation of postulate of quantisation.