Atoms, Molecules and Nuclear Chemistry
Category : UPSC
ATOMS, MOLECULES AND NUCLEAR CHEMISTRY
ATOMS AND MOLECULES
The combination of different elements to form compounds is governed by some basic rules. These rules, collectively called ‘laws of chemical combination’.
LAW OF CHEMICAL COMBINATIONS
Law of Conservation Mass:
Lavoisier, who is widely regarded as the father of modem chemistry gave the law of conservation of mass. This law states that in any chemical reaction, the mass of the substances that react equals the mass of the products that are formed.
Law of Definite Proportions:
This Jaw was given by Joseph Proust, a French chemist, in 1799. Proust’s law of definite proportions states that different samples of the same compound always contain its constituent elements in the same proportion by mass.
Law of Multiple Proportions:
In 1803 Dalton gave this law. As per this law if two elements combine to form more compounds, the masses of one element combine with a fixed mass of the other element, are in the ratio of small whole numbers.
The Law of Gaseous Volume:
When gases react, the volumes consumed and produced, measured at the same temperature and pressure, are in ratios of small whole numbers. This is also known as Gay-Lussac’s Law.
Dalton’s Atomic Theory
The hypotheses about the nature of matter on which Dalton’s atomic theory is based can be summarized as:
Laws of Chemical Combination and Dalton’s Theory
ATOMS
Atoms are building blocks of all matter. On the basis of Dalton’s atomic theory, we can define an atom as the basic unit of an element that can enter into chemical combination. The size of an atom is extremely small and not visible to eye. The comparative idea regarding the size of atom can be had from the following:
Relative sizes |
|
Radius (in meter) |
Example |
\[{{10}^{-10}}\] |
Atoms of hydrogen |
\[{{10}^{-4}}\] |
Grain of sand |
\[{{10}^{-1}}\] |
Water melon |
\[0.2\times {{10}^{-1}}\] |
Cricket ball |
Atomic Symbols
It was Jon Jacob Berzelius who devised the modern convenient system of using letters of the alphabet to represent elements. The systems of naming the elements are enumerated below:
Atomic Number, Mass Number and Isotopes
The subatomic particles present in atom arc-neutron, proton and electron. All atoms can be identified by the number of protons and neutrons they contain.
Atomic number
The number of protons in the nucleus of an atom decides which element it is. This very important number is called the atomic number (Z). In a neutral atom the number of protons is equal to the number of electrons, so the atomic number also indicates the number of electrons present in the atom. The chemical identity of an atom can be determined solely by its atomic number.
Mass number
The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element.
Mass number = number of protons + number of neutrons
= atomic number + number of neutrons
The number of neutrons in an atom is equal to the difference between the mass number and the atomic number, or (A - Z).
ISOTOPES
Atoms that have the same atomic number but different mass numbers are called isotopes.
The first isotope of uranium is used in nuclear reactors and atomic bombs, whereas the second isotope lacks the properties necessary for these applications.
Handy Facts
The chemical properties of an element are determined primarily by the protons and electrons in its atoms; neutrons do not take part in chemical changes under normal conditions. Therefore, isotopes of the same element have similar chemistry, forming the same types of compounds and displaying similar relativities.
Isobars
Thus, elements atoms of different elements having same mass number (A) but different atomic number (z) are termed as isobars. Examples: \[_{7}^{14}N\]and \[_{6}^{14}C\]
\[_{11}^{24}Na\] and \[_{12}^{24}Mg\]
Isotones
The atoms of an element which have atomic numbers and mass number both different but the number of neutrons in atomic nuclei are same called isotones.
Atomic Mass
A property closely related to an atom's mass number is its atomic mass. The mass of an atom depends on the number of electrons, protons, and neutrons it contains.
Carbon-12 is the carbon isotope that has six protons and six neutrons. Setting the atomic mass of carbon-12 at 12 amu provides the standard for measuring the atomic mass of the other elements.
MOLECULE
Amedeo Avogadro, an Italian chemist, first coined the term molecule in 1801 in order to explain the Gay-Lussac’s law.
Molecule may be defined as a combination of two or more than two atoms of the same or different elements in a definite arrangement. These atoms are held together by chemical forces or chemical bonds.
Difference between Atoms and Molecules
Representing a Molecule Chemically
The chemical composition of a molecule can be expressed with the help of symbols of elements and formulae.
Handy Facts
Buckminsterfullerene is a soccer ball-shaped molecule.
Molecular Formula
Formulae are combinations of symbols that represent a compound. A formula indicates:
Steps in Formula Writing
In writing formulae for compounds, there are four steps that should be followed:
For example, the formula for calcium chloride may be written as follows:
\[C{{I}^{-1}}\]ions (\[-1\times 2=-2\]). Thus, the formula would be \[CaC{{I}_{2}}\].
Empirical Formula
The empirical formula of a compound is the simplest formula which expresses its percentage composition. It is the ratio of the different elements present in a chemical compound. Empirical formula does not show the exact number of elements present. For example, molecular formula of Benzene is\[{{C}_{6}}{{H}_{6}}\].
Structural Formula
Structural formula of a molecule represents the structure of the molecule. Structural formula shows how the atoms are bonded to each other.
Molecular Mass
Molecular formula of a compound is normally used for determining the molecular mass of that compound.
For example:
The molecular mass of \[C{{O}_{2}}\] is obtained as:
\[C=1\times 12.0\,\,u=12.0\,\,u\]
For two \[O=2\times 16.0\,\,u=32.0\,\,u\]
Mass of \[C{{O}_{2}}=44.0\,\,u\]
Hence, we write molecular mass of\[C{{O}_{2}}=44.0\,\,u\].
Equivalent Mass
The formula to calculate the equivalent mass of an element is given by:
Equivalent mass =\[\frac{Atomic\,\,mass}{Valency}\]
IONS
An ion is an atom or a group of atoms that has a net positive or negative charge.
AVOGADRO’S LAW (AVOGADRO'S THEORY; AVOGADRO’S HYPOTHESIS):
This law states that equal volumes of gases at the same temperature and pressure contain the same number of molecules regard-less of their chemical nature and physical properties. Avogadro’s number is\[6.022\times {{10}^{23}}\]. It is the number of molecules of any gas present in a volume of 22.4 L and is the same for the lightest gas (hydrogen) as for a heavy gas such as carbon dioxide or bromine. Avogadro’s law provides a method to determine molecular weights of gaseous element.
Avogadro’s number and Molar mass of an element
Chemists measure atoms and molecules (or any particle like ions, radicals, etc.) in moles. Mole is chemist's counting unit and is central to all of quantitative chemistry.
In the SI system the mole (mol.) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 g (or 0.012 kg) of the carbon-12 isotope. The actual number of atoms in 12 g of carbon-12 is determined experimentally. This number is called Avogadro’s number (\[{{N}_{A}}\]), in honor of Amedeo Avogadro.
The currently accepted value is, \[{{N}_{A}}=6.0221415\times {{10}^{23}}\]Generally, Avogadro’s number is rounded to\[6.022\times {{10}^{23}}\]. This mass of carbon-12 is its molar mass (M), defined as the mass (in grams or kilograms) of 1 mole of units (such as atoms or molecules) of a substance.
ATOMIC STRUCTURE AND NUCLEAR CHEMISTRY
Matter is made up of atoms, and therefore an understanding of the structure of atom is very important.
FUNDAMENTAL PARTICLES OF ATOM
Electrons, Protons and neutrons are called fundamental particles. Characteristics of the fundamental particles are given below:
|
Subatomic particles |
|
||
\[\downarrow \] |
\[\downarrow \] |
\[\downarrow \] |
||
Electrons (Symbol: e) |
Proton (Symbol: p) |
Neutron (symbol: n) |
||
· Discovered by J.J. Thomson in 1897. |
· Discovered by Ernest Rutherford in 1911 |
· Discovered by James Chadwick in 1932 |
||
· Negatively charged. |
· Positively charged |
· Mass= \[1.674\times {{10}^{-27}}kg\] |
||
· Mass = \[9.109389\times {{10}^{-31}}kg\] |
· Mass = \[1.672\times {{10}^{-27}}kg\] |
· Charge = 0 |
||
· Charge = \[-1.602\times {{10}^{-19}}\]coulomb |
· Charge = \[1.602\times {{10}^{-19}}\] coulomb |
· Relative charge = 0 |
||
· Relative charge = -1 |
· Relative charge = +1 |
|
||
|
· 1840 times heavier then electron |
|
||
The discovery of the sub-atomic particles led to the enunciation of different models of the atoms which tried to explain the internal structure of the atom.
MODELS OF ATOM
Thomson Model
Rutherford’s Model
In 1909, Rutherford discovered proton in his famous gold foil experiment. In this experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide (ZnS) screen.
Results
Conclusion
Based on his observations, Rutherford proposed the following structural feature of an atom:
Bohr’s Model
The assumptions of Bohr’s Theory are as follows:
Modern Atomic Model
The present accepted model of atom, called quantum mechanical or wave-mechanical concept of atom, is basically mathematical in nature. This was proposed by Erwin Schrodinger- an Austrian physicist in 1926.
Handy Facts
Heisenberg’s Uncertainty Principle
An important consequence of the wave-particle duality of matter and radiation was discovered by Werner Heisenberg in 1927 and is called the Uncertainty Principle. According to this principle, it is not possible to simultaneously measure both the position and momentum (or velocity) of an electron accurately.
The characteristics of each of the quantum numbers are given below:
Quantum numbers |
|||
\[\downarrow \] |
\[\downarrow \] |
\[\downarrow \] |
\[\downarrow \] |
Principal quantum number (denoted by n)
|
Azimuthal quantum number (denoted by l)
|
Magnetic quantum number (denoted by \[{{m}_{l}}\])
|
The Spin quantum number (denoted by \[{{m}_{s}}\]) describes the spin of the electron, i.e. whether it is clock- wise or anticlockwise. This quantum number was introduced later, it’s not an outcome of the solution of Schrodinger Equation. |
• Specifies the energy level (or principal shell) of the electron within the atom and size of the orbital.
|
• Specifies the shape of an orbital with a particular principal quantum number.
|
• Describes the direction or orientation of the orbital in space.
|
|
• Can take only positive non-zero integral values i.e. 1,2,3,4 etc.
|
• Divides the shells into smaller groups of orbitals called subshells (sublevels).
|
• Takes-up any integral value from \[-l\] to \[+l\] (For example, for \[l\]= 1;\[{{m}_{l}}\] can have the values as -1,0 and 1. That means the p-orbital can have three orientation i.e. there are three p orbitals- \[{{p}_{x}},{{p}_{y}}\] and\[{{p}_{z}}\].) |
|
• The shells or energy levels are designated as K, L, M, N etc. depending on the values of n i.e. 1,2,3,4 etc. respectively.
|
• \[l\] may be zero or a positive integer-less than or equal to (n-1) (n is the principal quantum number), i.e. =0, 1, 2, 3... (n - 1).
|
|
|
• The number of electrons that can be accommodated in one shell is \[2{{n}^{2}}\]. |
• Different values correspond to different types of subshells and each subshell contains orbitals of a given shape as shown below: 1 = 0 (s orbital): Spherical 1=1 (p-orbital): Dumb-bell 1= 4 (d-orbital): cloverleaf |
|
|
ARRANGEMENT OF ELECTRONS IN AN ATOM
Each electron in an atom is described by four different quantum numbers. The first three (\[n,l,{{m}_{l}}\]) specify the particular orbital of interest, and the fourth (\[{{m}_{s}}\]) specifies how many electrons can occupy that orbital.
Table of Allowed Quantum Numbers
n |
\[l\] |
\[{{m}_{l}}\]\[l\] |
Number of orbitals |
Orbital Name |
Number of electrons |
1 |
0 |
0 |
1 |
1 s |
2 |
2 |
0 |
0 |
1 |
2 s |
2 |
|
1 |
-1, 0, + 1 |
3 |
2 p |
6 |
3 |
0 |
0 |
1 |
3 s |
2 |
|
1 |
-1, 0, + 1 |
3 |
3 P |
6 |
|
2 |
-2, -1, 0, +1,+2 |
5 |
3 d |
10 |
4 |
0 |
0 |
1 |
4 s |
2 |
|
1 |
-1,0,+1 |
3 |
4 p |
6 |
|
2 |
-2, -1, 0, +1,+2 |
5 |
4 d |
10 |
|
3 |
-3, -2, -1, 0,+1,+2, |
7 |
4 f |
14 |
1s \[\to \]2s\[\to \] 2p\[\to \]3s\[\to \]3p\[\to \]4s\[\to \]3d\[\to \] 4p \[\to \] 5s \[\to \] 6s \[\to \] 4f\[\to \] 5d, 6p \[\to \] 7s \[\to \] 5s
Handy Facts
Because an electron spins, it creates a magnetic field, which can be oriented in one of two directions. For two electrons in the same orbital, the spins must be opposite to each other; the spins are said to be paired. These substances are not attracted to magnets and are said to be diamagnetic. Atoms with more electrons that spin in one direction than another contain unpaired electrons. These substances are weakly attracted to magnets and are said to be paramagnetic.
NUCLEAR CHEMISTRY
In 1896, a French physicist named Henri Baequeral discovered that uranium-containing crystals emitted rays that could expose and fog photographic plates. He called these rays-uranic rays. Marie curie, later discovered two other elements-polonium and radium emitting uranic rays. She renamed uranic rays as radioactivity (or radioactive decay).
Radioactivity may be defined as disintegration or decay of unstable atoms accompanied by emission of radiation
Radioactivity can be of two types-Natural and artificial or induced radioactivity.
Nature of Radiations
The invisible radioactive radiations are of three types:
Comparison of the Properties of Alpha, Beta, and Gamma Rays
Property |
\[\alpha \] ray |
\[\beta \] ray |
\[\gamma \] ray |
Nature |
Helium nuclei, \[_{2}^{4}\]He |
Fast electrons |
Electro-magnetic radiation |
Velocity |
One-tenth of the velocity of light |
Velocity of light |
Velocity of light |
Penetrating power |
Low |
moderate |
high |
Stopped by |
Paper of 0.01 mm thick |
1 cm of aluminum |
Several cm thick lead/concrete layer |
Nuclear Reactions
A nuclear reaction is that which proceeds with a change in the composition of the nucleus resulting in the formation of an atom of a new element.
Therefore, the process in which the artificial transmutation of a stable nuclide leads to the formation of radioactive isotope is called artificial radioactivity or induced radioactivity.
DIFFERENCE BETWEEN NUCLEAR REACTIONS AND CHEMICAL REACTIONS
Following are some of the important points, given in table, which differentiate a nuclear reaction from ordinary chemical reaction.
Nuclear Fusion
Nuclear fusion refers to a nuclear reaction in which two light nuclei fuse together to form heavy nucleus with release of large amount of energy.
Nuclear Fission
Nuclear fission is a nuclear reaction in which a heavy atomic nucleus (such as that of uranium) disintegrates into two nearly equal fragments with release of large amount of energy.
Application of Nuclear Fusion For The Benefit Of Mankind (Nuclear Reactor)
It has been possible control fission of U-235 so that energy is released slowly at a usable rate. Controlled fission is carried out in a specially designed plant called a nuclear power reactor or simply nuclear reactor. The chief components of a nuclear reactor are:
Nuclear power is a major source of energy for electrical generation worldwide.
Hydrogen Bomb or H-Bomb
This destructive device makes use of the nuclear fusion of the isotopes of hydrogen. It consists of a small plutonium fission bomb with a container of isotopes of hydrogen.
\[^{1}{{H}_{2}}{{+}^{1}}{{H}_{3}}{{\xrightarrow[{}]{{}}}^{2}}H{{e}_{4}}{{+}^{0}}{{n}_{1}}+Energy\]
Uses for Radioactive Substances and Radiation
Radioactive substance and radiation have been used for the benefit of people also.
Medicine
Radionuclides are used to directly treat illnesses. For example radioactive iodine is used, which is taken up almost exclusively by the thyroid, to treat cancer or hyperthyroidism. Radioactive tracers and dyes are also used to accurately map a specific area or system, such as in a cardiac stress test, which may use a radioactive isotope like Technetium-99 to identify areas of the heart and surrounding arteries with diminished blood flow. Cobalt-60 is also used to treat cancer patients. In Positron emission tomography (PET), a computer imaging diagnostic technique, radioactivity of some substances is utilized.
Smoke Detectors
Some smoke detectors also use radioactive elements as part of their detection mechanism, usually americium-241. The ionizing radiation of the alpha particles is used to cause and then measure changes in the ionization of the air immediately around the detector. A change due to smoke in the air will cause the alarm to sound.
Radiography
Essentially high-powered versions of the types of X-Ray machines used in medicine, industrial radiography cameras use X-rays or even gamma sources (such as Iridium-192, Cobalt-60 or Cesium-137) to examine hard to reach or hard to see places This is frequently used to examine welds for defects or irregularities, or examining other materials to locate structural anomalies or internal components.
Food Safety
Food irradiation is the process of using radioactive sources to sterilize foodstuffs. The radiation works by killing bacteria and viruses, or eliminating their ability to reproduce by severely damaging their DNA or RNA.
Archaeology
One important contribution that nuclear science has made in this area is the ability to determine the age of ancient artifacts. There are several techniques for doing this, but the most common process for dating objects of up to about 50,000 years is called radiocarbon dating.
Tracer
Unstable nuclei have also been used as radioactive tracers in scientific research. A tracer is a radioactive element whose pathway through a chemical reaction can be followed. For example, scientists have used carbon-14 to study many aspects of photosynthesis. Likewise, phosphorus-32 atoms can be used to trace phosphorus-containing chemicals as they move from the soil into plants.
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