Chem Notes

Reverse sublimation

Elements and atomic structure   ·      An element is a pure substance made up of only one type of atom. ·      An atom is the smallest particle of an element that can exist. ·      An atom contains the sub-atomic particles protons, neutrons and electrons   sub-atomic particle relative charge relative mass location proton +1 1 nucleus neutron 0 1 nucleus electron -1 1/2000 shells around the nucleus               ·      Atomic (proton) number = number of protons (this is also equal to the number of electrons for atoms) ·      Mass number = number of protons + number of neutrons ·      Therefore the number of neutrons = mass number – atomic number   ·      Isotopes are atoms with the same number of protons but a different number of neutrons   e.g. chlorine has atoms with mass numbers 35 and 37 in a ratio of 3:1 (75% 35Cl and only 25% 37Cl)   The relative atomic mass of chlorine is taken as the average of these two types of atom BUT the average must be weighted to take into account the fact that the isotopes are present in different proportions i.e. there are more 35Cl atoms than 37Cl.   To calculate the weighted average mass:                    (75 x 35)  +  (25 x 37)            =           35.5                                                                                                       100

·      Atomic (proton) number = number of protons (this is also equal to the number of electrons for atoms) ·      Mass number = number of protons + number of neutrons ·      Therefore the number of neutrons = mass number – atomic number   ·      Isotopes are atoms with the same number of protons but a different number of neutrons   e.g. chlorine has atoms with mass numbers 35 and 37 in a ratio of 3:1 (75% 35Cl and only 25% 37Cl)   The relative atomic mass of chlorine is taken as the average of these two types of atom BUT the average must be weighted to take into account the fact that the isotopes are present in different proportions i.e. there are more 35Cl atoms than 37Cl.                                                                                               

To calculate the weighted average mass:                    (75 x 35)  +  (25 x 37)            =           35.5              100

·      An ionic bond is formed when a metal reacts with a non-metal ·      Ionic substances are always compounds ·      The metal atom gives up electrons from its outer shell and becomes a positively charged ion (CATION); the non-metal atom gains these electrons into its outer shell and becomes a negatively charged ion (ANION) ·      An ion is a charged particle formed by the loss or gain of electrons from an atom. ·      Ions are charged because they either have more electrons than protons (this makes them negatively charged) or fewer electrons than protons (this makes them positively charged) ·      An ionic bond is the electrostatic attraction between oppositely charged ions ·      Ionic bonding can be shown using dot and cross diagrams:

·      Covalent bonding occurs between two non-metals ·      Both atoms achieve a full outer shell by SHARING PAIRS of electrons ·      Each shared pair of electrons is a single covalent bond ·      This type of bonding does not involve the transfer of electrons and so ions are NOT formed. ·      The electrostatic attraction is between the nuclei (of both atoms) and the shared pairs of electrons ·      A covalent substance could either be an element e.g. H2 or a compound e.g. H2O ·      Many covalent substances are simple molecules ·      Dot and cross diagrams can be used to show covalent bonding

Particle Theory

Elements and atomic structure

Ionic Bonding

Covalent Bonding