Atom and Molecules

Atom and Molecules Laws of Chemical Combination Antoine L. Lavoisier, known as ‘Father of Modern Chemistry’ laid the foundation of chemical science. Law of conservation of mass It states that ‘ mass can neither be created nor destroyed in a chemical reaction’. Total mass of reactants must be equal to total mass of products i.e., why all chemical reactions are balanced. Example: C + O2 → CO2 12 + 32 4 4u = 12 + 2 x 16 = 44 u Mass reactants = Mass of products Law of constant proportion or Law of Definite proportion It states ‘In a pure chemical substance (compound) , the elements are always present in definite proportions of mass’. Compound prepared by any method contains the same elements in the fixed ratio by mass, e.g., H2O, the ratio of mass of hydrogen to mass of oxygen is always 1:8, whatever may be the source. 2H2O → 2H2 + O2 If we decompose 9g of H2O , we shall get 1 g of hydrogen and 8g of oxygen. 2H2 2x2 + O2 + 2 x 16 =4 = 32 → 2H2O 4g of H2 reacts with 32 g of Oxygen; 1g of H2 will react with 8 g of O2. If we react 5g of hydrogen with 8 g of oxygen, 1 g of hydrogen will react with 8 g of oxygen to form 9 g of water and 4 g of hydrogen will remain unreacted. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained. All matter whether element, compound or mixture is composed of small particles called ‘atoms’. Postulates of Dalton’s atomic Theory: • • • • • • All matter is made up of tiny particles called atoms. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. Atoms of a given element are identical in mass and chemical properties. Atoms of different elements have different masses and chemical properties. Atoms combine in the ratio of small whole numbers to form compounds. The relative number and kinds of atoms are constant in a given compound. Advantages of Dalton’s Atomic theory • • • This theory could explain the existing laws at that time. It could predict the law of multiple proportion. It proposed concept of atomic mass. Drawbacks Today, we know atoms are not indivisible, i.e., they themselves are made up of protons, electrons and neutrons. Accordinig to Dalton, all atoms of an element have exactly the same mass. It is, however, now known that atoms of the same element can have slightly different masses.eg . According to Dalton, the atoms of different elements have different masses. It is, however, not known that even atoms of different elements can have the same masses. Atoms Atom is smallest particle which takes part in physical and chemical processes. It may or may not exist freely. Atoms form molecules and ions. Atomic radius 1= 109nm is measured in nanometers. 1 / 109 m= 1nm Dalton was the first scientist to use the symbols for elements in a very specific sense. The symbol also meant a definite quantity of that element ,i.e., one atom of that element. Earlier the names were derived from the places where they were found , specific colors , etc. Other symbols have been taken from the names of elements in Latin. Significance of symbol: • • • • It represents name of the symbol. It represents an atom of the element. It represents one mole of element. It represents atomic mass of element. Atomic mass Each element has a characteristic atomic mass. Since determining the mass of an individual atom was a difficult task, relative atomic masses were determined using laws of chemical combinations and the compounds formed. Atomic mass unit: Earlier it was (amu). Scientists took 1 /16 of the mass of an atom of naturally occurring oxygen as the unit because• • Oxygen reacted with large no. of elements and formed compounds. This atomic mass gave masses of most of the elements as whole numbers At present In 1961 Universally accepted atomic mass unit, carbon – 12 isotope was chosen as the standard reference for measuring atomic masses. Atomic mass unit (u) Atomic mass unit (u) is mass units equal to exactly 1/12th the mass of one atom of carbon-12. Relative atomic mass : It is the average mass of the atom as compared to 1/12th the mass of one carbon-12 atom.( Hydrogen is the lightest element as its Atomic mass is 1) Molecule It is the smallest particle of an element or a compound that is capable of 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. Molecules of elements The molecules of an element are constituted by the same type of atoms. Molecules of Argon(Ar) , helium (He) , etc. are made up of only one atom of that element. But it is not so in case of non-metals. Eample. A molecule of oxygen consists of two atoms of oxygen; so it is called diatomic molecule , O2. If 3 atoms of oxygen form one molecule , we get ozone. Atomicity The number of atoms constituting a molecule . Molecules of compounds Atoms of different elements join together in definite proportions to form molecules of compounds. Ions Compounds composed of metals and non-metals contain charged species known as ions. It can be negatively (anion) or positively ( cation) charged. Polyatomic ions A group of atoms carrying a charge is called polyatomic ions. Writing Chemical Formulae The chemical formula of a compound is a symbolic representation of its composition. For writing a chemical formula we use symbols and the combining capacity (valency) of an element. Rules for writing a chemical formula • • • The valencies or charges on the ions must balance. When a compound consists of a metal and a non-metal , the name or symbol of metal is written first. ( CaO , FeS, etc.) In compounds with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. If no. of polyatomic ion is one , bracket is not needed ( NaOH) Valency • • The charge of an ion or radical which has either lost or gained electrons Note that metals lose electrons easily to become positive ions. This is why most metals are good conductors of electricity. Molecular Mass It is the sum of the atomic masses of all atoms in a molecule of the substance. Formula unit mass The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound. It is used for those substances whose constituent particles are ions. Example: NaCl The formula unit mass of NaCl = 1 x 23 + 1 x 35.5 = 58.5 µ Mole Concept • • • • The word mole was introduced by Wilhelm Ostwald around 1896 and it means heap or pile. Mole is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles present in 1 mole of any substance is fixed, with a value of 6.022 x 1023 . This is called the Avocado Constant or Avocado Number (represented by No)1 mole = 6.022 x 1023 in number Mass of one mole is also fixed. It is equal to its relative atomic or molecular mass in grams. Molar mass of atoms is also known as grain atomic mass. For this ‘µ’ is changed to ‘g’. Atomic mass of hydrogen is 1 µ and its gram atomic mass is 1 g. Chemists need the no. of atoms and molecules while carrying out reactions so, they need mass in grams to the number. Thus, a mole is the chemist’s counting unit.