Step Up Chemistry

An  atom  is the smallest particle of an element that can take part in a chemical reaction. It may or may not exist independently. A  molecule  is the smallest particle of an element or a compound that can normally exist independently. A substance that cannot be decomposed into simpler substances by chemical means and is made up of only one kind of atoms is called an  element . The smallest particle of an element is an atom. A substance formed by the chemical combination of two or more elements in fixed proportions is called a  compound . The smallest particle of a compound is a molecule. When two or more elements combine together not chemically but physically we get a  mixture .   Q. Why are the properties of different compounds so different from each other? A. The properties of different compounds are different from each other because the molecules of the compounds are different from each other. Q. Why are most elements not found in the free state in nature? A. Many elements

It is observed that in lanthanide series, there is a progressive decrease in the atomic and ionic radii with increasing atomic number. This regular decrease with increase in atomic number is called lanthanide contraction. This is due to the weak shielding of f orbitals. These f orbitals are unable to counter balance the effect of increasing nuclear charge because of which the size keeps on decreasing with increase in atomic number.  CAUSE OF LANTHANIDE CONTRACTION: As we move along the period from left to right in lanthanide series, the atomic number increases that is number of protons keeps on increasing. For every proton added in the nucleus the extra electron goes to the same 4f orbital.  The 4f orbital shows poor shielding effect because of which there is a gradual increase in effective nuclear charge experienced by the outer electrons. Thus, the attraction of the nucleus for the electrons in the outermost shell increases with atomic number.