Chapter 5 - Atoms and the periodic table

• Atoms are particles that make up all substances in the universe
• Atoms are made up of 3 subatomic particles:

  1 - Protons (positive charge)

  2 - Neutrons (neutral charge)

  3 - Electrons (negative charge)
• The nucleus is the centre of the atom. 
• The nucleus contains both protons and neutrons. 
• The nucleus makes up most of the atoms mass; this is because both protons and neutrons are around 1800 times larger than electrons. 
• The positive charge from the protons means the nucleus is also positively charged. 
• Atoms are the smallest unit of an element.
• All atoms within an element are identical and contain the same number of protons in their nuclei.
• There are 118 different elements.
• Mass number = number of protons + neutrons
• Atomic number = number of protons
• Electron number = same as the atomic number (number of protons), unless the element is an ion.
• To find the number of neutrons in an element, subtract the atomic number from the mass number.
• Isotopes have the same chemical properties but differing physical properties due to differences in mass.
• Isotopes are atoms of the same element that have a different number of neutrons.

  • This means the atomic number is the same but the mass number is different.
• Relative atomic mass (Ar) is the average mass of an elements atoms.

  • Relative atomic mass = the sum of 2 or more isotopes (relative mass x relative abundance) divided by 100

  
  

  This is the mass number that appears for each element in the periodic table.
• Compound = a substance made of two or more elements chemically bonded together.
• A compound can be:

  • A molecule - e.g., H2O
  • A lattice - e.g., NaCl
• Group 1 = Alkali metals

  Group 2 = Alkaline Earth metals 

  Group 17 = Halogens 

  Group 18 = Noble gasses
• Periods:

  • Horizontal rows (1-7)
  • The number of the period tells you how many electron shells an atom has (an element in period 3 has 3 electron shells)
• Groups:

  • Vertical rows (1-18)
  • The group number (for main groups 1,2, 12-18) tells you how many valance electrons there are in the atom.
  • Valance electrons= electrons in the outer shell
• Group 18 - Noble gases


  • Full outer shell, and are therefore unreactive/inert
  • They are monatomic (single atom)
• Group 17 - Halogens


  • Form ions with -1 charge (gain an electron)
  • Not found in nature in their pure form because they are highly reactive
  • As you move down the group they get larger and become less reactive
  • All form molecules which contain two atoms e.g. Cl2
  • Have coloured and poisonous vapour
• Bonding in Halogens

  Halogens exist in diatomic form (two atoms). They form a covalent bond by sharing an electron with another atom to become stable.

  • Form ions with -1 charge (gain an electron)
  • Become less reactive as you go down the group
• Group 1 - Alkali metals

  • Extremely reactive
  • Rarely found in their elemental form
  • Form ions with 1+ charge (lose 1 electron)
  • As you move down the group the atoms get larger
  • As the atoms increase in size they become less stable and more reactive (the further an electron is from a positive nucleus, the more reactive it is)
• Group 2 - Alkaline earth metals


  • Slightly less reactive than group 1
  • Form ions with 2+ charge (lose 2 electrons)
  • As you move down the group the atoms get larger
  • As the atoms increase in size they become less stable and more reactive.
• Transition metals - Groups 3-12:

  •All metals including our most useful and valuable metals such as gold, silver, copper and iron.

  
  

  They all have very similar properties:

  -Relatively hard

  -High melting points

  -Some are magnetic
• Electron shells

  • Electrons spin around the nucleus in a region known as electron shells or energy levels
  • First electron shell is closest to the nucleus and so attraction between the nucleus and the electrons is strongest
  • Shells can only contain a certain number of electrons
• Electron shells (𝑛 squared) x 2

  • 1 (2 electrons)
  • 2 (8 electrons)
  • 3 (18 electrons)
  • 4 (32 electrons)
• Electron configuration

  • Ground state= lowest energy state
  • Electronic configuration shows how the electrons are arranged in the shells of an atom when in ground state
• Excited electrons

  • Outer shell electrons can jump from one energy shell to another. This jump requires energy.
  • This energy comes from heat and we refer to electrons which have jumped to a higher energy level as excited.
  • When the electrons return to their ground state they release the energy previously absorbed as light.
• Electromagnetic spectrum

  • Shows the light released by the electrons
  • Unique for each element
• In periods:

  • The number of complete electron shells stays the same
  • The number of outer shell electrons increases by one
• In groups: 

  • The number of complete electron shells increases by one
  • The number of outer shell electrons stays the same 
• Alkali Metals: As the atom increases in size, the force of attraction weakens, making it easier to lose an electron, thus increasing reactivity down the group.

  Halogens: As the atom increases in size, the force of attraction weakens, making it harder to attract and gain an electron, thus decreasing reactivity down the group.