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S CHAPTER 1: ATOMS AND MOLECULE THE ATOMIC THEORY THE ATOMIC THEO a «sii The atomic theory is the scientific theory that deser ott atomic theory ‘eo matter composing of atoms. John Dalton can be summarized as follows; © Elements are made up of in atoms “i Atoms can neither be created nor destroye e All atoms of same element are alike . When atoms combine, they do so in simple ratio Chemical reactions occur from the combination of divisible particles called atoms THE STRUCTURE OF THE ATOM The structure of the atom was devised as a result of the various scientific discoveries: ¢ Sir J J Thompson discovered the electron and the charge to mass ratio of the electron. His atomic model was called the "plum-pudddling model". e R.A Millikan discovered the charge of the electron in his oil-drop experiment e Sir Ernest Rutherford discovered the proton as well as the nucleus of the atom. His atomic model was called the "planetary model". e James Chadwick discovered the neutron e Niels Bohr gave a theoretical explanation of the emission spectrum of Hydrogen atom. In summary, the structure of the atom can thus be described as follows; e Anatom has a nucleus and an electron. The mass of the electron is negligible. e The nucleus of an atom comprises the protons and neutrons. e¢ The protons are responsible for the atomic number of atoms while the neutrons are responsible for isotopy. e Protons and neutrons are collectively called nucleons. e The number of nucleons is the mass number. ¢ Protons are positively charged, electrons are negatively charged and neutrons have no charge. e The nucleus of the atom is responsible for nuclear reactions while the electrons are responsible for chemical reactions. ¢ An atom is electrically neutral; proton number = electron number. MASS RELATIONSHIPS OF ATOMS ACTUAL ATOMIC MASS The mass of an atom simply refers to the mass of the nucleons. It refers to the number of nucleons multiplied by their masses. It can be given as; mass number Actual atomic mass = x amu relative atomic mass where: a.m.u = atomic mass unit = 1.66 x 10 —7*g The actual mass of an atom can also be given as; . mass of eler Actual atomic mass = f element Where; avogadro's number = 6.02 x 107 avogadro's number ( I 3 é 1 M. 1§ Ss ‘ sa A nile aw nit f that element having avogadro's number of an . | sails i.e 6.02% 102, The molar mass of an atoms/mole ch element/compound is the mass Of | mol ol a : | element/compound. Eg; | mol of oxygen gas Nas a Molar /mol. oe " Oe rite of moles is related to the molar mass by the given formula; ‘ n= = M Where; m = mass of element (g) M = molar mass of element (g/mol) n = number of ad SMP. | FORMUL os ne to take when calculating empirical formula from given percentage composition/relative abundances; — Step 1: Obtain the mass of each element present in grams Mass of element present (m) = percentage composition Eg; 65% of Oxygen = 69g of Oxygen in 100g of the compound. Step 2: Determine the number of moles of each type of atom present mass(m)--atomic mass = Molar amount (M) N.B: Atomic masses are used in empirical formula and not molar masses Step 3: Divide the number of moles of each element by the smallest number of moles M least M value Step 4: Convert numbers to whole numbers. This set of whole numbers is the subscripts in the empirical formula. N.B: When the decimal is more than 0.1 from the whole number, multiply with a number until you get a whole number or a very close decimal. MOLECULAR FORMULA The molecular formula is related with the empirical formula with the equation given below: (A)x =B Where; A = empirical formula B= molecular formula (when value of x is known) B= molecular mass (when value of x is unknown) x = constant ISOTOPY This is a phenomenon that describes atoms having the same atomic number but different mass and neutron numbers. The following points should be noted: ° Isotopes have the same atomic number. ° Isotones have the same neutron number. ° Isobars have the same mass number. PAST QUESTION REVIEW Consider the species of °Co, “26Fe and °:sCu. These species have? A. the same mass number B. the same nuclear charge C. the same number of neutrons [D. the same number of electrons E. the same number of protons CALCULATIONS IN RELATIVE ATOMIC MASS AND PERCENTAGE COMPOSITION The percentage composition of an element in a compound can be calculated with the formula below: = atomic ratio 
