User talk:Saksham K Verma

Welcome to Wikimedia Commons, Saksham K Verma!

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-- Wikimedia Commons Welcome (talk) 06:55, 15 July 2016 (UTC)

(1) STRUCTURE OF AN ATOM

When two objects are rubbed together they get electrically charged. The origin of this charge can be explained, if it is assumed that the constituent particles of matter, the "atoms" are divisible and consist of charged particles.

Sub atomic particles: Atom is defined as the smallest particle of the element that can exist independently and retain all its chemical properties.

According modern atomic theory atoms are composed of particles. The three main sub-atomic particles are proton, neutron and electron.

Cathode rays- Discovery of electrons: In 1878 William Crooks carried out discharge tube experiments and discovered new radiations and called them cathode rays. Since these rays travel from the cathode towards anode. Later J.J Thomson studied on characteristics of cathode rays and concluded that cathode rays are negatively charged particles, now called electrons. The name electron was given by Johnson Stoney.

Discharge tube experiment

Canal rays Or Anode rays: In 1886, E. Goldstein carried out discharge tube experiments and discovered new radiations and called them canal rays. These rays were made up of positively charged particles and led to the discovery of proton.

Properties of electron, proton and neutron:

  Parameters	           Electron	       Proton  	        Neutron

Position Present outside the nucleus and revolves in the orbits Present inside the nucleus Present inside the nucleus Mass 9.108X10-28 g 1.67X10-24g 1.67X10-24 g Charge -1.602 X10-19 coulombs 1.602 X10-19 coulombs Zero Representation e- p+ n (2)

Atomic models: Atomic models proposed by scientists show the arrangement of the various sub-atomic particles in an atom.

Thomson’s atomic model: J.J. Thomson was the first to put forward a model to explain the structure of an atom.

Thomson's atomic model is also called water melon model or Christmas pudding model. He compared the electrons with the raisins in the Spherical Christmas pudding and to seeds in a watermelon.

Postulates of Thomson’s Model:

   •  An atom consists of a positively charged sphere, with electrons set within the sphere.
   •  An atom is electrically neutral as the positive and negative charges within it are equal. 

Draw backs of Thomson’s Model

   •  It could not explain the stability of an atom, i.e how a positive charge in the atom holds the negatively charged electrons.
   •  It could not explain the position of nucleus in an atom.
   •  It could not explain the scattering of alpha particles

Rutherford’s Experiment: Thomson’s student, Ernest Rutherford conducted an experiment using gold foil which disproved Thomson’s model.

To study the structure of atom, Rutherford performed a thin gold foil scattering experiment. For his experiments Rutherford used a gold foil. He made a narrow beam of alpha particles to fall on the gold foil.

Observations made from the alpha ray scattering experiment

   •  Most of the alpha particles passed straight through the gold foil without getting deflected.
   •  A small fraction of the alpha particles were deflected through small angles.
   •  A few alpha particles bounced back.

(3)

Based on his observations, Rutherford proposed the nuclear model of an atom.

Postulates of Rutherford nuclear model:

   •  Positive charge is concentrated in the center of the atom, called nucleus.
   •  Electrons revolve around the nucleus in circular paths called orbits.
   •  The nucleus is much smaller in size than the atom.

Drawbacks of Rutherford’s Model

   •  The orbital revolution of the electron is not expected to be stable.
   •  According to Rutherford’s model, the electrons, while moving in their orbits, would give up energy.

This would make them slow down, gradually and move towards the nucleus. The electrons will follow a spiral path and then fall into the nucleus. Ultimately, the atom would collapse. But in reality the atom is stable.

Bohr’s Model: Keeping the shortcomings of the Rutherford’s model in mind Niels Bohr formed his postulates about the structure of an atom as the following. Postulates of Bohr’s Model:

   •  Electrons revolve in discrete orbits called shells.
   •  Electrons revolve in their orbits without radiating energy. Within a particular orbit, the energy of an electron is constant. This is why orbits are called stationary orbits or stationary shells.
   •  Orbits or shells are also known as energy levels.
   •  These orbits or shells are represented by the letters K, L, M, N,… or the numbers n=1, 2, 3, 4,….

(4)

Drawbacks of Bohr’s Model:

   •  Bohr’s model did not apply to elements like helium and lithium and the higher elements containing more than one electron.
   •  The model was also unable to explain the structure of chemical bonds.

The discovery of neutrons: Consider the element helium (He24). It was found that helium has two protons and two electrons however its mass was found to be four times that of Hydrogen. Similarly, the masses of some other elements were also found to be double or more than double the number of protons.

This problem was solved on the discovery of another particle ‘neutron’ by James Chadwick In 1932 by bombarding beryllium with alpha particles.

Be49 + He24 → C612 + n01 ‘Neutron’ is a neutral particle with a mass equal to that of a proton and is present in the nucleus along with a proton.

Atomic number

Atomic number (z) is the number of protons in an atom; it is also equal to the number of electrons in the atom. Atomic number = Number of protons Example: The atomic number of an element is 12 then its atom contains 12 protons and 12 electrons.

Mass Number The mass number (A) is defined as the sum of the number of protons and neutrons present in the nucleus of an atom. Example: Mass number of Nitrogen atom is 14 then it contains 7 protons and 7 neutrons. Mass number = Protons + Neutrons

Notation of element

(5) Atomic number Element Mass number  ; Z XA Example: Nitrogen atom has notation as 7 N 14 . Question: An atom X has mass number 40 and atomic number 18.Find out the number of protons, number of electrons and number of neutrons present in the atom X? Solution: Number of protons: Number of protons = Atomic number

                              = 18

Number of electrons: In an atom number of electrons = Number of protons

                                                   = 18

Number of neutrons = Mass number – Atomic number

                                = 40-18 = 22

Electronic configuration Electrons are placed in fixed energy levels called shells. These shells are also called orbits. These orbits or shells are represented by the letters K, L, M, N,… or the numbers n=1, 2, 3, 4,…. The arrangement of electrons in the shells is known as electronic configuration.

Rules for accommodating electrons in various shells (Bohr-bury scheme) The maximum number of electrons that can be accommodated in any energy level of the atom is given by a formula 2n2. Where n is the number of that energy level. According to Bohr’s model of an atom, the first energy level K can have 2 electrons, the maximum number of electrons which can be accommodated in shell L is 2X22 = 8. Similarly, for M and N shells, maximum number of electrons that can be accommodated are 2X32= 18 and 2X42= 32 respectively. Electrons in an atom do not occupy a new shell unless all inner shells like K, L, M, N….. are completely filled. Hence, filling of electrons in an atom follows a stepwise approach. The maximum number of electrons that can be filled in a valence shell is eight even if it has capacity to accommodate more than eight.

After a series of experiments and a detailed study by scientists like louis de broglie, schrodinger, somerfeld and others proved that shells or energy levels have sub shells within them. Electrons are distributed in these sub-shells.

The various sub shells are s, p, d and f. Every sub-shell can accommodate a fixed number of electrons .(6)

"s" sub shell can hold a maximum of 2 electrons, "p" sub shell can hold a maximum of 6 electrons, "d" sub shell can hold a maximum of 10 electrons, "f" sub shell can hold a maximum of 14 electrons.

Structure of Sodium atom Sodium has atomic number 11 and mass number 23. The nucleus of sodium has 11protons and 12 neutrons and it is surrounded by 11 electrons. The electronic configuration of sodium: K shell or I shell = 2 electrons L- shell or II shell = 8 electrons M-Shell or III shell = 1 electron ∴ Electronic configuration of sodium atom = 2, 8, 1

Valence electrons The electrons, which are present in the outermost shell of an atom are called valence electrons. Example: Sodium. Atomic number of sodium is 11 Electronic configuration of sodium is 2 8 1 In sodium 3rd shell is the outermost shell (valence shell) .In this shell it has 1 electron. Hence the number of valence electrons present in sodium is 1.

The chemical properties of an atom are dependent on these valence electrons. Since they are ones that are participate in a chemical reaction. Elements having valence electrons 1, 2 or 3 are called metals. Exception: Hydrogen has 1 valence electron but it is not consider as a metal. These elements lose electrons easily and form a positively charged ion called cation. (7)

Example: Na - e- → Na+ Elements having valence electrons 4, 5, 6 or 7 are called as Non-metals These elements gain electrons and forms a negatively charged ion called anion. Example: F + e- → F- Elements with same number of valence electrons will have similar chemical properties. Whereas the elements with different valence electrons will have different chemical properties.

Octet configuration An element with 8 electrons in an outermost-shell is said to possess an octet. Elements with octet configuration are stable. Example: Except helium all other noble gases possess octet configuration. — Preceding unsigned comment added by Saksham K Verma (talk • contribs) 07:06, 15 July 2016 (UTC)