OCR Chemistry A-Level - Modules 1-4 with 100% correct answers

what is water of crystallisation? water molecules that make up part of the crystal structure of a solid define relative isotopic mass the ass of an atom of an isotope of an element relative to one twelfth the mass of an atom of C-12 00:03 01:39 why is relative atomic mass written as a decimal? it is a weighted average mass define molecular formula the actual number of atoms of each element in a compound define what an orbital is a region within an atom that can hold up to two electrons of opposite spins what shape are S orbitals? spherical what shape are orbitals? figure of 8 how can you make out the electron structure of an element using the periodic table? period = energy level columns = electrons group = subshell show the oxidation of sodium and the reduction of chlorine Na - Na+ + e- Cl + e- - Cl- ions always go on the RHS whether oxidation or reduction define ionic bond the electrostatic force of attraction between positive and negative ions ionic bonding occurs between metals and non-metals but which ion is the exception? NH4+ define covalent bond the strong electrostatic force of attraction between the shared pair of electrons and the nuclei of the bonded atoms when non-metals covalently bond, what is their aim? the octet rule: - to reach a noble gas configuration (8 electrons in the outer shell) what are there exceptions to the octet rule? - NO (odd number of electrons) - BF3 (boron has less than 8 outershell electrons) - SF6 (sulfur has more than 8 outershell electrons) why can sulfur have more than 8 electrons in its outershell? it has an empty 3d subshell define a dative covalent bond a shared pair of electrons which has been provided by only one of the bonding atoms give some common examples of ions with a dative bond NH4+ H3O+ what is a base? proton (H+) acceptors what is the shape of an atom determined by? - the number of electron pairs - electron pairs repel to become as far apart as possible - lone pairs of electrons repel more strongly than bonding pairs define electronegativity the ability of an atom to attract the bonding electrons in a covalent bond what creates a polar bond? - lone pairs offset symmetry by 102.5 degrees each - asymmetrical molecule - dipoles do not cancel out - bonded atoms with different electronegativities what are London forces? - uneven distribution of electrons causes a temporary dipole on one molecule - this induces an opposite dipole on the next molecule which type of molecules are London forces common to? - all molecules - however London forces are most obvious in nonpolar molecules