- An atom is the smallest particle of an element that can participate in a chemical reaction.
- The radius of an atom is measured in nanometers.
- The hydrogen atom is the tiniest of all atoms.
Structure of an Atom:
- By 1900, it had been discovered that the atom is not a simple, indivisible particle, but rather a collection of sub-atomic particles.
- J.J. Thomson discovered the 'electron,' a subatomic particle.
- J.J. Thomson was the first to propose a model for atom structure.
- E. Goldstein discovered new radiations in a gas discharge in 1886 and named them canal rays.
- Another positively charged subatomic particle was discovered using canal ray experiments and named proton.
Constituent of Atom:
- Several phenomena, such as radioactivity, demonstrate that atoms are divisible and consist of three fundamental particles: electron, proton, and neutron.
- J.J. Thomson discovered the electron in .
- When a high-voltage electric charge is passed through a low-pressure gas, a stream of rays is emitted from the cathode surface. These are referred to as cathode rays.
- The magnitude of charge on electron is .
- The mass of electron is .
Properties of Cathode Rays:
- Cathode rays always travel away from the cathode in a straight line, casting shadows on metallic objects in their path.
- Cathode rays cause mechanical motion in the path of a spin wheel. As a result, they have kinetic energy and must be material particles.
- Because the cathode ray beam is deflected towards the positively charged plate, the particles in the cathode rays have a negative charge.
- Cathode rays can be deflected by both an electric and a magnetic field.
- Cathode rays have the ability to penetrate through thin layers of matter.
- Cathode rays emit X-rays when they strike a metallic target with a high melting point, such as tungsten.
- Cathode rays glow when they strike glass or certain other materials.
The nature of cathode rays is unaffected by :
- The cathode's nature and
- 2. The gas in the discharge tube.
Charge/Mass Ratio (e/m)
- J.J. Thomson determined the charge-to-mass ratio of the electron by measuring the deflection under the influence of both electric and magnetic fields at the same time.
- For electrons, the value of the ratio was found to .
- R.A. Millikan measured the charge on the electron.
- E. Goldstein demonstrated the existence of positively charged particles in an atom.
- When a high voltage is applied across the electrodes, a new type of ray is produced that passes through the perforated cathode and causes a glow on the opposite wall to the anode.
- Anode rays and canal rays are other names for these rays.
- The magnitude of charge on proton is .
Properties of Anode Rays
- Anode rays travel in a straight line and cast shadows.
- Anode rays are bent in the opposite direction as cathode rays by the magnetic and electric fields.
- The anode rays can also rotate the wheel in their path and cause heating.
- The charge to mass ratio of positive rays is less than of electrons and depends on the nature of the gas used in the tube.
- When electrons are removed from neutral atoms or molecules of gas, positively charged ions are formed. Positively charged ions move towards the perforated cathode, forming a beam of positive rays.
- The neutrons were discovered by James Chadwick.
- These particles are neutral having charge equals zero.
- Their mass is approximate same as that of a hydrogen atom, i.e. .
-Various atomic models were proposed to show the arrangement and distribution of particles (electrons, protons and neutrons) within an atom.
1) Thomson’s Atomic Model :
- The first simple model of an atom was proposed by J.J. Thomson.
- According to Thomson, an atom is a positively charged uniform sphere of radius in which electrons are embedded in such a way that negative charge equals to positive charge.
- This model is also called plum-pudding model and watermelon model.
- This model was unable to explain the stability of an atom.
2) Rutherford’s Atomic Model :
- The particle scattering experiment served as the foundation for this model.
- Rutherford's particle scattering experiments are concerned with the discovery of the nucleus.
- These experiments resulted in the following conclusions:
- The atom has a very small rigid, positively charged body called the nucleus, and the particles are repelled from the metal foil due to this positively charged nucleus.
- The entire mass of the atom is concentrated in the nucleus, so it is very heavy and rigid.
The following are the model's main postulates:
- An atom is mostly made up of empty space. At its core, each atom has a heavy positively charged body known as the nucleus.
- Planetary electrons are negatively charged electrons that revolve around the nucleus.
- The nucleus is very small in comparison to the atom.
- A strong interaction force holds all protons and neutrons together in the nucleus.
- Rutherford's atomic model failed because he was unable to explain the atom's electronic structure. In other words, it does not provide information about the distribution of electrons around the nucleus.
Note: During particle scattering experiment, particles were bombarded by a thin sheet of heavy metals such as gold and platinum because their nuclei are large, resulting in good results.
- These radiations have particle like and wave like properties.
- These radiations do not require a medium to move and can exist in a vacuum.
- The following is a list of the different types of electromagnetic radiations in increasing wavelength order.
- The small portion around frequency is known as visible light.
- The relation between frequency, wavelength and velocity of light is
- Wave number:
- Its unit is .
Note: X-rays are effectively produced when electrons strike the dense metal anode and have a very high penetrating power through matter, which is why these rays are used to study the interiors of objects.
- By increasing the potential difference between the anode and cathode, the penetration power of X-rays can be increased.
- Atomic spectrum of hydrogen atom:
where, is the Rydberg constant and its value is
and have integral values as follows:
- Lyman Series - U.V region -
- Balmer Series - Visible region -
- Pascher Series - IR -
- Brackett Series - IR -
- Pfund Series - IR -
- When a metal surface is illuminated with light of a sufficient frequency, electrons are ejected.
- The threshold frequency is the minimum frequency required for electron ejection .
- The frequency of radiation is directly related to the energy of the ejected electrons.
- The number of electrons ejected each second is determined by the radiation intensity.
- It can be expressed as:
Plank’s Quantum Theory:
- Max Planck proposed this theory.
- The following are the main postulates of this theory:
- The energy emitted or absorbed by atoms and molecules will be in the form of discrete packets of energy called quanta.
- The energy of quanta is directly proportional to its frequency shown below:
Here, is the Planck’s constant .
- The energy of quanta is quantized or fixed, thus,
Bohr’s Atomic Model:
- This atomic model, proposed by Neils Bohr, is based on Planck's quantum theory of radiations.
- Electrons in an atom only revolve around the nucleus in specific circular paths known as orbits. Each orbit contains a fixed amount of energy.
- Only those orbits are permitted in which the electron's angular momentum is a whole number multiple of (is a Planck's constant, i.e. where )
- When an electron jumps from a higher energy level to a lower energy level, energy is emitted, and energy is absorbed when an electron jumps from a lower energy level to a higher energy level.
- When a transition between two different energy levels or states occurs, the frequency of radiation absorbed or emitted is given by where, are the energies of lower and higher energy states.
- The energy of an electron in a specific energy level is given by:
- The radius of nth orbit is shown below:
- The velocity of electron in nth orbit is
Failures of Bohr’s Atomic Theory:
- He was unable to explain the splitting of spectral lines in the presence of a magnetic field (Zeeman effect) and an electric field (Stark effect).
- He was unable to explain the line spectra of multi-electron atoms, which contain more than one electron.
- He was unable to explain the atom's three-dimensional existence.
- Because of the dual nature of matter and the uncertainty principle, Bohr's theory failed.
Atomic Number (Z):
- Moseley introduced the concept of atomic number.
- Atomic number denotes the number of protons in the nucleus.
- The number of protons in a neutral atom equals the number of electrons.
Mass Number (A) :
- The mass number is the sum of the number of protons and neutrons in the nucleus.
- An atom with an atomic number and a mass number is denoted by .
- Protons and neutrons are referred to collectively as nucleons.
Different Atomic Species:
There are different types of atomic species.
-Isotopes share the same atomic number but have different mass numbers.
-Isotopes share the same chemical properties but have different physical properties.
-For example: (protium), (deuterium), (tritium).
-Isobars are atoms that share the same mass number but have different atomic numbers.
-Isobars are atoms of different elements with distinct physical and chemical properties.
-For example: and ,
-Isotones are atoms of different elements that have the same number of neutrons but different mass numbers.
-Isoelectronic species contain the same number of electrons.
-For example all contain 10 number of electrons.
Quantum Mechanical Model of the Atom
The formulation of a quantum mechanical model of an atom is based on two key concepts.
1) Dual Nature of Material Objects (de-Broglie Concept):
- In 1924 de-Broglie proposed that matter such as radiation acts as both particle and wave.
- According to de-Broglie, the wavelength associated with a particle of mass, moving with velocity is given by
where, = Planck’s constant, momentum
- This equation is known as de-Broglie equation.
2) Heisenberg’s Uncertainty Principle:
- Heisenberg pointed out that measuring the position and momentum (or velocity) of a microscopic particle at the same time is impossible with absolute precision.
Here,= uncertainty in position
= uncertainty in momentum
On putting the value of in the above expression,
-Four sets of quantum numbers completely describe the position and nature of an electron.
1. Principal Quantum Number : (Bohr)
- It determines the size of an electron's orbits and its energy.
- It can be represented by n, where (only positive integers).
- The various shells are denoted as
- The maximum numbers of electrons in any orbit can be calculated as where n is principal quantum number.
2. Azimuthal Quantum Number : (Sommerfeld)
- Also referred to as the angular momentum quantum number or the secondary quantum number.
- It determines the shape of an electron's orbit and its orbital angular momentum.
- It can be represented by . It has the values from zero to . is equal to for orbitals respectively.
3. Magnetic Quantum Number : (Lande) :
- Because it gives the orientation or distribution of the electron cloud, it determines the direction of orientation of electrons in suborbit (subshell).
- It has all values from through zero to , with total of values.
- It describes the splitting of spectral lines caused by a magnetic field (Zeeman effect).
4. Spin Quantum Number : (Uhlenbech and Goldschmidt)
- It determines the orientation of the electron's spin.
- It has two values for electron spinning about its own axis.
- The spin quantum number either equal to (anti-clockwise) and (clockwise) totally depending on the spin of electron.
- For example: Assume the electron of an atom
Electronic Configuration of Elements
- The electronic configuration of an element is the arrangement of electrons in various shells, subshells, or orbitals of an atom.
- The following rule governs the filling of electrons in different orbitals:
Aufbau’s Principle :
-"Sub-shells are filled with electrons in increasing order of their energies," according to this principle.
-This implies that lower energy sub-shells will be filled first, followed by higher energy sub-shells.
- The lower the value of for an orbital, the lower is the energy. This is in according with rule. For e.g. between and , the 4s will be filled before .
- If two orbitals have the same value, the orbital with the lower n value will be filled first. For example: between will be filled first than .
- The order of increasing energies can be written as :
Note: Only Chromium and Copper do not obey this principle.
Their configurations are shown below.
- Because completely filled and completely half-filled subshells have less energy, they are more stable than any other arrangement. Therefore, arrangements are more stable than that of respectively.
Hund’s Rule of Maximum Multiplicity:
- "Pairing of electrons in a sub-shell begins after all available atomic orbitals or the sub-shell are singly filled (half-filled)," according to this rule.
Pauli’s Exclusion Principle :
- It says that, “No two electrons would have the same number of all the four quantum numbers.”