lecture 1: atoms and atomic structures

chemistry

the study of the nature, properties and transformations of matter

matter

anything that has mass and occupies space, that is, things you can see, touch, taste, or smell

mass number (A)

total number of protons and neutrons in the atom

atomic number (Z)

the number of protons in the nucleus

where do electrons reside?

they are confined to regions of space called principle energy levels (shells), in which each shell can hold 2n² electrons, n=1,2,3

division of shells

each shell is divided into subshells, which are designated by the letters s,p,d,f… and are further divided into orbitals; each shell only gets occupied at increasing energy levels

valence electrons

the electrons in the outermost shell of an atom that are involved in chemical bonding; the GROUPS in the periodic table all have the same number of valence electrons and therefore have similar chemical properties

covalent bond

a type of chemical bond formed when two atoms share one or more pairs of electrons, allowing them to achieve stable electron configurations

ionic bond

a type of chemical bond formed through the electrostatic attraction between positively and negatively charged ions, typically resulting from the transfer of electrons from one atom to another

weak interactions

hydrogen bonding and dipole-dipole bonding; affects the properties of the molecule; by themselves they are weak, but added up together they become strong interactions

electronegativity

describes the ability of an atom to attract an electron from another atom; fluorine is the most electronegative element

electronegativity differences

0.0-0.4: pure (nonpolar), covalent bond

0.4-2.0: polar covalent bond

2.0-4.0: ionic bond

covalent bonds

forms when a portion of an atomic orbital of one atom overlaps a portion of an atomic orbital of another atom

lewis structures

• diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist

• must fulfill octet rule

• divide bonding electrons equally

formal charge

formal charge = valence electrons - non-bonding electrons -(bonding electrons/2)

resonance

electron pair delocalisation

resonance structures

more than one possible lewis structure for structures with alternate single-double bond arrangements

• electronically neutral

• resonance hybrids can also be created (dotted lines represent delocalised electrons)

linear

• 2 electron domains

• domains are 180 degrees from each other

• predicted bond angle = 180 degrees

trigonal planar

• 3 electron domains

• domains are 120 degrees from each other

• predicted bond angle = 120 degrees

tetrahedral

• 4 electron domains

• electron domains are 109.5 degrees from each other

• predicted bond angle = 109.5 degrees

VSEPR

Valence Shell Electron Pair Repulsion

• predicts the shape of molecules

• each group of valence electrons around a central atom is located as far away from others as possible to minimise repulsions