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Chemistry is the study of the composition,
structure, properties and change of matter [matter is defined as anything that
has rest mass and volume (it takes up space) and is made up of particles].
Chemistry is chiefly concerned with atoms (a basic unit in chemistry) and their
interactions with other atoms.
Atoms
The atom is a basic unit of matter that
consists of a central nucleus surrounded by negatively charged electrons.
Nucleus contains protons and neutrons. Electrons revolve around the nucleus in different
orbits.
Subatomic
particles
The constituent particles of an atom are
called sub atomic particles. They mainly include protons, electrons and neutrons.
The electron is the least massive of these particles at 9.11×10−31 kg with a
negative charge. Protons have a positive charge and Neutrons have no electrical
charge.
Discoveries:
Proton by Ernest Rutherford in 1918.
Electron by J.J. Thomson in 1897.
Neutron by James Chadwick in 1932.
Nucleus
The central part of an atom is called nucleus.
Particles present inside the nucleus are called nucleons and they include
mainly protons and neutrons. Due to the presence of protons nucleus has
positive charge.
Molecules
Molecules are made from atoms of one or
more elements. Some molecules are made only by one type of atoms (two oxygen
atoms bond together to form O2 molecule) while molecules like protein are made
up of atoms from different elements.
Elements
A chemical element is a pure chemical substance
consisting of one type of atom distinguished by its atomic number. Carbon,
Oxygen, Silicon, Arsenic, Aluminum, Iron, Copper, Gold, Mercury etc. are all examples
of elements.
Note :
*
Hydrogen and Helium are the most abundant elements in the universe.
*
Iron is the most abundant element (by mass) in earth.
*
Oxygen is the most common element in the earth’s crust.
*
The 8 most abundant elements in Earth’s crust (by mass) are the
following :
1.
46.6% Oxygen (O)
2.
27.7% Silicon (Si)
3.
8.1% Aluminum (Al)
4.
5.0% Iron (Fe)
5.
3.6% Calcium (Ca)
6.
2.8% Sodium (Na)
7.
2.6% Potassium (K)
8.
2.1% Magnesium (Mg)
Periodic
Table
Dmitri Ivanovich Mendeleev also he is the
father of periodic table. The first detailed form of periodic table was develop
by Mendeleev (based on mass number) but laterHenry Gwyn Jeffrey’s Moseley made
a new periodic table based on atomic number. Mosley is called the father of
modern periodic table.
Natural
elements
The elements which occur in naturally on
earth are called natural elements.
Eg: Oxygen, Nitrogen, Carbon etc.
Synthetic
or man-made elements
Synthetic element is chemical element that
does not occur naturally
on the earth. These are prepared by artificially
and are unstable. The first synthetic element made was Technetium.
Compounds
Compounds contain more than one kind of
atoms (more on atoms, later). It cannot be separated into constituent atoms by
simple methods.
Eg: common salt (NaCl), Sodium carbonate
(Na2CO3).
Mixtures
Mixtures are combination of two or more substances,
which when combined, each substance retains its own chemical identity. Examples
of Mixtures include:
*
sand and water.
*
salt and water.
*sugar
and salt.
Atomic
number (Z)
Atomic number is the number of protons or
electrons present in an atom (for every atom, the number of proton and electron
are same).
Eg: Nitrogen (N) = 7, Calcium (Ca) = 20,
Oxygen (O) = 8.
Mass
number (A)
Mass number is the sum of protons and
neutrons present in an atom (or it is the sum of electron and neutron present
in an atom.)
Eg: Nitrogen (N) = 14, Calcium (Ca) = 40, Oxygen
(O) = 16
Isotopes
Elements having same atomic number but different
mass number are called isotopes.
Eg: Protium, Deuterium, Tritium.
Isobars
Elements having same mass number but different
atomic numbers are called isobars.
Eg:
40S, 40Cl, 40Ar, 40K, and 40Ca.
Allotropes
Different forms of a single element are
called allotropes.
Eg: Diamond and graphite are two allotropes
of carbon; ie. pure forms of the same element that differ in crystalline
structure.
Radioactivity
Unstable atomic nuclei will spontaneously
decompose to form nuclei with a higher stability. The decomposition process is
called radioactivity.
Energy and particles released during the
decomposition process are called radiation. There are three major types of
natural radioactivity : alpha, beta and gamma radiation.
Alpha
Radiation
23892U → 42He + 23490Th.
The
helium nucleus is the alpha particle.
Beta
Radiation
23490 → 0-1e + 23491Pa.
The electron is the Beta particle.
Gamma
Radiation
Gamma rays are high-energy photons with a
very short wavelength. Gamma emission changes neither the atomic number nor the
atomic mass.
Nuclear
reactions
Nuclear reactions are mainly two types :
1.
Nuclear fission.
2.
Nuclear fusion.
Nuclear Fission
Nuclear fission takes place when an atom’s
nucleus splits into two or
more smaller nuclei. These smaller nuclei
are called fission products. Particles (e.g., neutrons, photons, alpha
particles) may also be released along fission.
Example:
23592U + 10n → 9038Sr + 14354Xe + 310n.
Nuclear
Fusion
Nuclear fusion is a process in which atomic
nuclei are fused together to form heavier nuclei. Large amounts of energy are
released when fusion
occurs.The reactions which take place
inside the sun is an example of nuclear fusion.
Examples:
11H + 21H → 32He.
32He + 32He → 42He + 211H.
11H + 11H → 21H + 0+1β.
