ATOMS & MOLECULES

#Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.#“In a chemical substance the elements are always present in definite proportions by mass”.#Dalton atomic theoryThe postulates of this theory may be stated as follows: (i) All matter is made of very tiny particles called atoms.(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (iii) Atoms of a given element are identical in mass and chemical properties. (iv) Atoms of different elements have different masses and chemical properties. (v) Atoms combine in the ratio of small whole numbers to form compounds.(vi) The relative number and kinds of atoms are constant in a given compound. #IUPAC (International Union of Pure and Applied Chemistry) # The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).EX:hydrogen : HCarbon-DI-oxide: CO2cobalt: Co#Aluminium Al Copper Cu Nitrogen N Argon Ar Fluorine F Oxygen O Barium Ba Gold Au Potassium K Boron B Hydrogen H Silicon Si Bromine BrIodine I Silver Ag Calcium Ca Iron Fe Sodium Na Carbon C Lead Pb Sulphur S Chlorine Cl Magnesium Mg Uranium U Cobalt Co Neon Ne Zinc Zn#in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12#Hydrogen 1 Carbon 12 Nitrogen 14 Oxygen 16 Sodium 23 Magnesium 24 Sulphur 32 Chlorine 35.5 Calcium 40 #A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.#. Atoms of the same element or of different elements can join together to form molecules# The number of atoms constituting a molecule is known as its atomicity.#The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged.# A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. #A group of atoms carrying a charge is known as a poly atomic ion.#The combining power (or capacity) of an element is known as its valency.# The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.# It is therefore the relative mass of a molecule expressed in atomic mass units (u)#Example 3.1 (a) Calculate the relative molecular mass of water (H 2O). (b) Calculate the molecular mass of HNO3 . Solution: (a) Atomic mass of hydrogen = 1u, oxygen = 16 u So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is = 2 × 1+ 1×16 = 18 u (b) The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O = 1 + 14 + 48 = 63 u#The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.#The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023#