Fundamentals of Chemistry

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The Atom

All substances, living and non-living, are composed of matter; and the fundamental unit of matter is an atom

Atoms consist of a central space called a nucleus, which contains both positively charged particles (protons) and neutral particles (neutrons), while negatively charged electrons circle the nucleus in regions known as orbitals


The Periodic Table of Elements

Atoms differ in the number of protons they possess, and different atoms have different chemical properties (i.e. they react differently with different things)

Atoms can be grouped together according to key properties as organised into a chart known as the periodic table of elements


The Periodic Table of Elements


  • Atoms in the same group (columns) share similar chemical properties as they have the same number of electrons in their outer valence shell
  • Atoms in the same period (rows) have the same number of electron shells but do not share any consistent chemical properties
  • Atoms always have the same number of protons and electrons - when they gain or lose electrons they become charged ions
  • An atom may have a different number of neutrons - these different forms of the atom are called isotopes and can cause radioactive decay


Types of Bonding

  • Atoms always try to have a full outer shell of electrons - in order to achieve this they will bond with other atoms
  • When atoms bond together they form molecules - those made of one type of atom are elements while those made of many types are compounds
  • Atoms may join together by either gaining and losing electrons (ionic bonding) or by sharing electrons (covalent bonding) 


Ionic Bonding

  • Ionic bonding occurs between a metal and a non-metal
  • The metal has a nearly empty outer shell and so loses electrons to form a positively charged cation
  • The non-metal has a nearly full outer shell and so gains electrons to form a negatively charged anion
  • The resulting charge of these two ions creates a strong electrostatic attraction between them - an ionic bond


Ionic Bonding


Covalent Bonding

  • Covalent bonding occurs between two non-metals
  • Because both atoms have a large number of electrons in their outer shell, it is not feasible to lose or gain so many and so they share
  • The number of covalent bonds able to be formed reflects the number of missing electrons from the outer shell (e.g. carbon needs four electrons and so can form four covalent bonds)
  • Because there is no ionic charge, covalent molecules are not as strongly attracted to each other as ionic molecules (covalent bonds are weaker)


Covalent Bonding


Metallic Bonding

  • Metallic bonding occurs between two metals
  • The atoms lose their outer shell electrons to become positively charged cations, and the electrons circulate as a delocalised sea
  • This is why metals are good conductors of electricity and usually highly malleable
  • Metallic bonding is not as integral to the structure and function of living organisms as ionic and covalent bonding


Intermolecular Bonding

  • Intermolecular bonding occurs between covalently bonded molecules and is significantly weaker than intramolecular bonding
  • It results from the weak attraction between electrons and protons of different covalently bonded molecules


Polarity

  • While covalent bonding represents sharing of electrons between atoms, the sharing may not always be equal and will depend on:
  • The number of protons in an atom (more protons equals a greater attraction for electrons)
  • The number of electron shells in an atom (electrons in higher shell numbers are a further distance from the nucleus = less attraction to nucleus)
  • Atoms that have a stronger affinity for electrons are said to have a higher electronegativity
  • Covalently shared electrons will orbit closer to atoms with a higher electronegativity, resulting in a slight charge difference between atoms
  • These molecules are said to be polar and can form weak electrostatic associations, while molecules that do not have polarity are non-polar


Polarity


Hydrogen Bonds

  • A polar association that occurs between the hydrogen atom of a polar molecule and a fluorine, oxygen or nitrogen atom (F, O, N) of another polar molecule is called a hydrogen bond
  • This is because fluorine, oxygen and nitrogen have the highest electronegativities, while hydrogen has the lowest - this results in the greatest polar associations between these atoms


Dispersion Forces

  • Non-polar substances cannot form permanent polar associations but will still be attracted to each other by temporary attractions
  • Because electrons are moving within an orbit, at any particular instant it will closer to the nucleus or further away, creating momentary associations due to uneven electron distributions called dispersion forces (or Van der Waals forces)