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GENERAL KNOWLEDGE BOOSTER : Chemistry Basics
Monday, 09 February 2015 05:55

 

 

GENERAL KNOWLEDGE BOOSTER

Chemistry Basics

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.