Atoms and Molecules
• Matter is made of tiny indivisible particles.
• In any chemical reaction, the total mass of the substance before and after the reaction is the same although its matter undergoes a physical change.
• Molecule is the combined state of atoms.
• Number of atoms constituting a molecule is atomicity.
• Cation is a positively charged particle in a molecule.
• Negatively charged particle in a molecule is known as anion.
• Combining capacity of an element is known as valency.
• Formula is a short hand representation of a molecule.
• For writing a formula a set of rules are followed.
Structure of an Atom
• An atomic Number is the number of protons in an atom.
• The arrangement of electrons in various energy levels of an atom is known as electronic configuration.
• The maximum number of electrons in any energy level of the atom is given by 2n2.
• The outermost shell of an atom cannot to form molecules either with the same or different elements is known as valency.
• Combining capacity of the atoms to form molecules either with the same or different elements is known as valency.
• Neutron is a neutral particle present in the nucleus.
• The total number of protons and neutrons present in one atom of an element is referred as atomic mass.
• Isotopes are atoms with same atomic number but different atomic mass numbers.
• Isotopes of certain elements are used in a nuclear reactor, and in the treatment of cancer and goitre.
• Atoms of different elements with different atomic numbers that have the same mass number are called isobars.
Chemical Reactions and Equations
• A chemical reaction involves a change in the physical and chemical properties, the composition and the physical state of a substance.
• A chemical equation is a symbolic representation of the reactants and the products in terms of their chemical formulae.,
• The law of conservation of mass states that the number of atoms of each element should be equal on both sides of a chemical equation.
• The steps in a balancing an equation are:
* Determine the reactants and products.
* Count the number of atoms of each on both sides.
* Select the element that occurs at the minimum number of places.
* Change the coefficient of molecules of the reactants or the products.
* Continue balancing the atoms till all the elements have been balanced in the equation.
