Make The Grade: As And A2 Chemistry: Chemistry Revision Guide: Edexcel As A2 Modular (nelson Advanced Science)

1 Structur� bo tUUl �jYOUfP clteuti£try Ato m ic structure � Introduction Much of chemistry depends upon Coulomb's Law which states that the electrostatic force of attraction, F, is given by: F ex: (q+.q_) + r2 where q+ and q_ are the charges on the objects (e.g. the nucleus, an 2 electron, ions etc. ) and r is the square of the distance between their centres. This means that the bigger the charge, the bigger the force, and the further the centres are apart, the weaker the force. � Things to learn Atomic number (Z) of an element is the number of protons in the nucleus of its atom . Mass number of an isotope is the number of protons plus the number of neutrons in the nucleus. r r The relative molecular mass is also called the relative formu la mass especially for ionic su bstances. r Isotopes are atoms of the same element which have the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. Relative atomic mass (Ar) of an element is the average mass (taking into account the abundance of each isotope) of the atoms of that element relative to l/ 1 2th the mass of a carbon - 1 2 atom. Relative isotopic mass is the mass of one atom of an isotope relative . to l / 1 2th the mass of a carbon- 1 2 atom. Relative molecular mass (Mr) of a substance is the sum of all the relative atomic masses of its constituent atoms. Molar mass is the mass of one mole of the substance. Its units are grams per mole (g mor1 ) , and it is numerically equal to the relative molecular mass. Ionisation energies are always endotherm ic and relate to the formation of a positive ion. 1 st ionisation energy is the amount of energy required per mole to remove one electron from each gaseous atom to form a singly positive ion 2 nd ionisation energy is the energy change per mole for the removal of an electron from a singly positive gaseous ion to form a doubly positive ion 2 S T R U CT U R E , B O N D I N G A N D M A I N G RO U P C H E M I ST R Y 1 st electron affinity i s the energy change per mole for the addition of one electron to a gaseous atom to form a singly negative ion E(g) + e- � r(g) 2 nd electron affinity is the energy change per mole for the addition of an electron to a singly negative gaseous ion to form a doubly negative ion E-(g) + e- � E2-(g) s block elements are those in which the h ighest occupied energy level is an s orbital . They are in Groups 1 and 2. Similar definitions apply to p block (Groups 3 to 7 and 0) and d block (Sc to Zn) elements. :RJ Things to understand Mass spectra An element is first vapourised and then bombarded by high-energy electrons that remove an electron from the element and form a positive ion . This ion is then accelerated through an electric potential, deflected accordi ng to its mass and finally detected. Metals and the noble gases form singly positively charged ions i n the ratio of the abundance of their isotopes. Non-metals also give molecular ions. For example Br21 which has two 7 isotopes 9Br (SO%) and 8 1 Br (50%), will give three l ines at m/e values of 7 1 58, 1 60 and 1 62 in the ratio 1 :2: 1 . These are caused by (9Br- 9 Br) + , 9 ( Br-81 Br) + and ( 8 1 Br-8 1 Br) + . A commo