Atomic Structure — Long Questions

Hydrogen atom is the simplest and most abundant element in the universe. Hydrogen atom consists of just one proton in its nucleus and one electron orbiting around the nucleus. This electron is typically found in the first energy level or shell of the atom that is K-shell.

Structure
The structure of hydrogen atom can be visualized as tiny nucleus at the center surrounded by a cloud of probability where the electron can be found. The nucleus of the hydrogen may or may not have neutrons in it. There are three isotopes of hydrogen represented as ₁H, ₂H and ₃H. All these isotopes have one proton in nucleus and one electron around nucleus but number of neutrons are different. ₁H (Protium) has no neutron in it, ₂H (Deutrium) has one neutron in it while ₃H (Tritium) has two neutrons in it.
The electrons doesn't follow a fixed path like planets around the sun but is more accurately described by a probability distribution indicating the finding of electron at a particular location around the nucleus.

Transition of electrons The electron in a hydrogen atom occupies the lowest energy level known as the ground state. When energy is added to the atom, the electron can jump to higher energy level but it tends to return to the ground state by emitting energy in the form of light.

When a radioactive isotope undergoes decay, it emits radiation in the form of alpha, beta or gamma particles. These emitted particles can interact with atoms leading to the ionization of atoms.

Theory of atomic structure
According to this theory, atoms are composed of a nucleus containing protons and neutrons surrounded by electrons in various energy levels or shell.

Ionization of atoms by a radioactive source
Radiation emitted from a radioactive source causes atoms to ionize. For example, radiation emitted by a radioactive element Radium-226 can remove electrons from the atom. However, this ionizing radiation should have enough energy to remove the tightly bound electron from the orbit of an atom. Electron can be lost because an ionizing radiation collides with the atom and forces the electron to move out of the atom. If an atom of sodium is hit by an ionizing radiation, it may lose an electron. This process converts the atom into a positively charged ion (cation)

Na ——Energy→ Na⁺ + e⁻
Cation

However, the electron will be lost only when there is present another atom which can accept it.

Cl + e⁻ ——→ Cl⁻ + Energy
Anion

Radioactivity The isotopes of the same element do not have the same physical properties. Several isotopes of the same elements exist whose nuclei are unstable. They emit excess energy in the form of radiation. This process of emission is called radioactivity and the isotope which emits energy is called radioactive isotope. Every element has one or more radioactive isotope.

Radioactive Decay Tritium (₃H) is a radioactive isotope and the other two are stable and do not emit any radiation. When a radioactive element emits radiation, it is transformed into another element. This process is called radioactive decay. This new element may be stable or may be radioactive so that it also emits radiation.

Examples ₉₂²³⁸U ———→ ₉₀²³⁴Th + energy
(Uranium) (Thorium)

₉₀²³⁴Thorium is unstable and further disintegrates to give ₉₁²³⁴Pa (Protactinium)

₉₀²³⁴Th ———→ ₉₁²³⁴Pa + Energy
(Thorium) (Protactinium)

₈₃²¹⁰Bi ———→ ₈₁²⁰⁶Tl + Energy
(Bismuth) (Thallium)

Application of Radioactive Isotope

i. Medical purpose
Radioactive isotopes are useful in medical imaging. Doctors use them to diagnose the disease by injecting the patient with a small amount of radioactive fluid. Technetium-99 is used for diagnostic imaging across human organs like brain, lungs, etc. Doctors use a special camera to watch how the radioactive fluid moves.

ii. Archeological purpose
Radiocarbon dating is a method for finding out the age of an historical object containing organic material with the help of radioactive isotope of carbon ¹⁴C. The method involves measuring the proportion of ¹⁴C in sample from a dead plant or animal like a piece of wood or a bone which provides information that can be used to calculate when an animal or plant died. The older the sample is, the less ¹⁴C is to be detected.

iii. Construction and power generation purpose
Radioactive isotopes are used to test the strength of metals and concrete mixture. They are used to generate cheap nuclear power and to find oil fields.

Isotope Relative Abundance
¹⁹⁶Hg = 0.0146%
¹⁹⁸Hg = 10.02%
²⁰¹Hg = 13.22%
²⁰⁴Hg = 6.85%

Isotope Relative Abundance
¹⁹⁹Hg = 16.34%
²⁰⁰Hg = 23.13%
²⁰²Hg = 29.80%

Ans Relative atomic mass of mercury

= 0.0146x196+10.02x198+13.22x201+6.85x204+16.34x199+23.13x200+29.80x202
100

= 199.387

Scientists can synthesize elements in the laboratory through processes like nuclear fission, nuclear fusion and particle bombardment. These methods involve manipulating atomic nuclei to create new elements that do not exist naturally or are not abundant in nature.

Nuclear Fusion
In nuclear fusion, scientists combine atomic nuclei into heavier elements. This process is commonly used in nuclear reactors to generate energy.

Nuclear Fission
It involves splitting of heavy atomic nuclei into lighter elements. This process is commonly used in nuclear reactors to generate energy.

Particle Bombardment
Particle bombardment is another method where high-energy particles are directed at target atoms to induce nuclear reactions and create new elements.

The idea that an atom is similar to a mini solar system is helpful to explain atomic structure.

In an atom, there is a nucleus at the center, much like the sun in our solar system and electrons revolving around the nucleus like planets orbiting around the sun. This model helps visualize how electrons are arranged around the nucleus. Although the concept of mini solar system is helpful to describe the atomic structure, it also has following contradictions with atomic structure in following aspects:

i. In solar system planets follow predictable orbits around the sun but electrons around the nucleus do not follow fixed paths but follow probability patterns.

ii. The solar system is governed by gravitational forces but atomic structure are maintained by electromagnetic and nuclear forces.

iii. Electrons in an atom can jump between discrete energy levels, emitting and absorbing energy in a process but this jumping does not exist in our solar system.

Experiment A discharge tube is a hard glass tube provided with two metallic electrodes and a vacuum pump to evacuate the gas present in it. When a very high voltage is applied to a gas at a very low pressure present in a glass tube the glass surface behind the positive electrode started to glow, due to the rays emitted from the cathode. These rays were named as cathode rays.

Characteristics of cathode Rays.
In 1897, British physicist Joseph John Thomson studied the properties of cathode rays by passing them through the oppositely charged electric plates.

i. Cathode rays bent towards the positively charged plate showing that they carry negative charge.

ii. Thomson used the findings of his experiments to calculate the mass to charge ratio of cathode rays which finally proved that cathode rays are in fact, negatively charged material particles.

iii. These particles were later named as electrons. It was also shown that electrons are the subatomic particles of all elements

Discovery The presence of positively charged particles in an atom had been first observed by E. Goldstein in 1886. It was based on the concept that atoms are electrically neutral having same number of positive and negative charges.

Experiment He performed a series of experiments with a gas-discharge tube having a perforated cathode. A new type of rays were produced from the anode which moved towards the cathode. He called these new rays as canal rays or anode rays.

Properties i. The properties of these rays seemed to very depending on the gas used in the discharge tube. In fact what he discovered was gas ions and this also included hydrogen ions (H⁺). Goldstein at that time knew nothing about its significance.

ii. In 1917, Rutherford performed experiments which proved that the hydrogen nucleus is present in other nuclei. Rutherford thought that a hydrogen nucleus or a proton must be the fundamental building block of all nuclei and also possibly a new fundamental particle as well

All the atoms of an element must necessarily have the same atomic number, but their mass number may vary depending upon the number of neutrons present in the nucleus.

Definition Atoms of the same element having different number of neutrons in their nuclei are called isotopes.

Properties of isotopes Chemical properties of the elements are determined by the number of electrons, all isotopes will show almost the same chemical behaviour, although their physical properties may be different.

Isotopes of carbon Element carbon has three isotopes as its atoms have six, seven and eight neutrons in their nuclei. These isotopes are represented as ₆¹²C, ₆¹³C, ₆¹⁴C.

Isotopes of Hydrogen Hydrogen exists as three isotopes, Hydrogen, Deuterium and Tritium represented by ₁H, ₂H and ₃H. Hydrogen (₁H) is the only atom which does not have a neutron. ₂H has twice the mass of ₁H while the mass of ₃H is thrice the mass of ₁H similarly, the masses of three different isotopes of carbon are different.