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Sunday, 29 March 2020

Facts about Atoms and Molecules

Facts about Atoms and Molecules


The term 'atom' was introduced by Ostwald.

Atom was derived from a greek word ‘atomos' which means indivisible.

Atoms are the smallest units of matter that have all the characteristics of an element. Atoms are the building blocks of everything in the universe.

Atoms cannot exist free. They exist in combination with the atoms of the same element or another element.

Atoms of noble gases like Helium, Neon, Argon and Krypton are unreactive and exist in the free state.

Atoms of metals are also exist free in nature. (Iron, Copper, Gold,Silver....)

An atom can take part in chemical combination but does not occur free in nature.

A molecule is the smallest particle of an element or compound that can have a stable and independent existence.

All substances are made of tiny particles called molecules.

The term 'molecule' was introduced by Avogadro.

Facts about an Atomic Structure

On the basis of discharge tube experiments it was stated that atom is comprised of three fundamental particles called protons, neutrons and electrons.

The central part of atom is called nucleus. Protons and neutrons are present in nucleus and electrons are revolving around the nucleus in definite path called orbits.

Nucleus was discovered by Rutherford. It has positive charge

Protons and neutrons are collectively known as nucleons.

The radius of nucleus of an atom varies in the order 1.5 x 10-13 cm to 6.5 x 10-13 cm

Atom - Atom theory (John Dalton) - Structure (Niels Bohr) -  Plum pudding model (J.J. Thomson) - Wave mechanical model (Max Planck )

The smallest atom: Helium (He)

The simplest atom: Hydrogen (H)

Biggest known atom: Francium (Fr)

Fundamental Particles

1. Electron

Electron was discovered by Cathode ray experiment in 1897.

The name 'electron' was proposed by Stoney.

The charge of an electron(-1.602x 10-'9C) was determined by Millikan in his oil drop experiment.

The actual mass of an electron (9.1x10-31 kg) was calculated by J.J. Thomson

The dual nature of electron was proposed by Victer de-Broglie.

The dual nature of electrons was experimentally proved by I.H.Germer.

Electron is the lightest fundamental particle of an atom.

Electron is the significant particle of atom/ matter/nature .

2. Proton

Presence of proton was observed by Goldstein in 1896 in a discharge tube with a perforated cathode.

Experiments carried out by Lord Rutherford showed that positive particles produced in a discharge tube containing hydrogen gas are same as protons.

Proton is called the identity card of an element.

3. Neutron

Neutron was discovered by James Chadwick in 1932

The reason for the late discovery of neutron was its neutral nature.

Neutron is the heaviest fundamental particle of an atom.

Of all the elementary particles present in an atom, neutron is the least stable particle.

Neutrons are bound very tightly in the atomic nucleus with the protons.

When a neurtron splits an electron and a proton are obtained, hence it is chargeless.

Fundamental Particles

Fundamental Particles – Proton, Neutron, Electron

1. Proton – Positive – Rutherford - Identity Card of an
element
2. Neutron - Neutral - James Chadwick - Heaviest particle
3. Electron - Negative - J.J.Thomson - Lightest Particle

Sub Atomic Particles

Positrons: Discovered by Anderson. these are positive counterpart of electrons. Highly unstable and combine with electrons producing γ- rays.

Neutrinos and Anti-neutrinos: Postulated by Fermi, these are particles of small mass and zero charge.

Pi- mesons (pions) and mu-mesons (muones): Positively and negatively charged mesons were postulated by Yukawa, these are particles having a mass intermediate between that of the electron and the proton. Neutral Mesons were postulated by Kemmer to account for the binding forces between the nucleons (protons + neutrons).

What do you mean by Green Chemists?

The term green chemistry is defined as the programme of developing new chemical products and chemical processes or making improvements. The Green chemists Yves Chauvin, Robert H. Grubbs and Richard.R.Schrock won the 2005 Nobel prize in Chemistry for work that reduces hazardous waste in creating new chemicals.

Characteristics of Atoms

Valency is the combining capacity of one atom to another.

Noble gases have zero valency.

Hydrogen shows univalency,

Sodium and Potassium show univalency.

The common valency of carbon is four.

Atomic Mass Unit (amu): One twelvth of a mass of a Carbon-12 atom: a unit used for stating atomic and formula weights

Atomic Orbitals: are regions of space around the nucleus of an atom where an electron is likely to be found.

Atomic Radius: Radius of an atom.

Atomic Weight: The average weight of an atom of an element, based on the 1/12th mass of C-12 atom.

The atomic size is about 105 times larger than the nuclear size.

Avogadro's Number: The number (6.022 x 1023) of atoms, molecules or particles found in 1 mole of any compound.

Difference between Mass number and Atomic number

Ans: Mass Number is the sum of number of protons and neutrons present in the nucleus of an atom. It is denoted by the letter A. Atomic Number is derived from nuclear charge i,f atom, hence it will be the number of protons. It is generally denoted by the letter Z.

Different types of Atomic Species

Isotopes are the different types of atoms of the same element having same atomic number and different mass numbers.

The term `isotope' was introduced by Soddy.

Isotopes differ in the no. of neutron.

Protium, Deuterium and Tritium are the isotopes of hydrogen.

Protium has no neutron (1H1).

Protium is the simplest isotope.

Deuterium has one neutron. It is called heavy hydrogen (1H2).

Tritium has two neutrons. It is the radioactive Isotope of hydrogen (1H3).

Isobars are atoms of different elements having same mass number and different atomic numbers
eg: Argon -40 (18Ar40) and Calcium -40 (20Ca40)

Isotones are atoms of different elements having different atomic numbers and mass numbers, but they have same number of neutrons.
eg: Tritium (1H3) and Helium(2He4) both contain two neutrons.

Nuclear Isomers: These are the different types of atoms of the same element having different radioactivity constants and nuclear energies.

Allotropes are different forms of the same element with different physical appearances.

eg: Red phosphorus and white phosphorus, oxygen and ozone.

Carbon exists in crystalline as well as amorphous allotropes.

Atomic Species – Isotopes, Isobars, Isotones

Isotopes - Same atomic number and different Mass number – 6C126C14

Isobars - Same mass number and different atomic number – 18Ar4020Ca40

Isotones – Different atomic number and mass number but have same number of neutrons – 1H3, 2He4

The father of periodic table is Mendeleev.

The father of modern periodic table is Moseley

Isotopes of Hydrogen – Protium, Deuterium, Tritium

1. Protium - Simplest isotope - No neutron
2. Deuterium – Heavy hydrogen - One neutron
3. Tritium - Radioactive isotope - Two neutrons

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