History of the periodic table

• Early Periodic Tables


  In the early 1800s, elements were primarily organised by ATOMIC WEIGHT and their PHYSICAL AND CHEMICAL PROPERTIES.

  • It was INCOMPLETE and some elements were placed in INAPPROPRIATE GROUPS if the strict order of ATOMIC WEIGHTS was followed.
• Early periodic table


  ATOMIC WEIGHT was the only measurable data early scientists could use, as the sub-atomic particles was not yet discovered, which led to where PROPERTIES kept repeating PERIODICALLY, (hence the term periodic table).

  • The modern name for atomic weight is Relative Atomic Mass.
• Mendeleev's Periodic Table

  In 1969, DMITRI MENDELEEV, improved earlier tables by arranging 50 known elements by ATOMIC WEIGHT but also their PROPERTIES to maintain consistency within groups.

  • He didn't include the noble gases as they hadn't been discovered yet and they are unreactive.
• Mendeleev's Periodic Table

  Mendeleev left GAPS for elements that he thought would fit the groups but had NOT been DISCOVERED yet.

  E.g. Mendeleev PREDICTED the existence and PROPERTIES of EKA-ALUMINIUM (now known as GALLIUM)

  
• Mendeleev's Periodic Table

  Mendeleev CHANGED the ORDERS of some elements, even though their atomic weights did not fit this order.

  
  

  E.g. Usually, elements are lined up by their weight, and Iodine (I) should be before Tellurium (Te) because it's lighter. But Mendeleev switched them, because its properties were more similar to the other elements in its column.
• Mendeleev's changes were right because:

  The elements he left gaps for were discovered later and Filled the gaps.

  Mendeleev had predicted properties of missing elements.

  properties matched Mendeleev's predictions.
• The Modern Periodic Table

  The elements are arranged in order of increasing atomic number.

  METALS are located to the left and NON-METALS to the right, with a staircase line usually marking the division between them.

  
• • GROUPS are the vertical columns.
  • GROUP NUMBER equals to the number of ELECTRONS in the outer shell.

  
• The Modern Periodic Table

  Elements in the same group have SIMILAR CHEMICAL PROPERTIES because they have the SAME number of ELECTRONS in their OUTER SHELL.
• • PERIODS are the horizontal rows.
  • The PERIOD NUMBER equals to the NUMBER OF SHELLS an atom has.

  
• Group and Period Trends

  Elements in the same group have similar properties and react in SIMILAR ways due to having the same number of electrons in their outer shell.

  • GROUP 1 elements (alkali metals) have one electron in their outer shell, making them highly reactive and they become MORE REACTIVE as you move down the group.
• Predicting Element Properties

  If you know one element's properties, you can make predictions about others in the same group.

  • For example, knowing the properties of Lithium (Li) helps predict the behaviour of Sodium (Na) and Potassium (K).
• explain why the early periodic tables placed iodine (I) before tellurium (Te), but then Mendeleev placed tellurium before iodine.

  early periodic tables were arranged with elements in order of their atomic weights and iodine has a lower atomic weight than tellurium, so Mendeleev placed iodine with elements with similar properties.

  
• Differences between group 1 of Newlands' table and the periodic table:

  hydrogen, fluorine and chlorine are not in the same group of the periodic table.

  Lithium, sodium, potassium are in group 1 of the periodic table.

  
• atomic number is the number of protons and protons were not discovered until later.

  Correct reason was so their properties matched the rest of the group.

  
• Mendeleev knew that there must be undiscovered elements, as some elements did not fit the pattern, or the elements would be in the wrong group, as it had the wrong properties.

  So he left gaps when he designed his periodic table.
• Mendeleev's improvements on Newlands' table:

  • separates elements with dissimilar properties and grouped them with similar properties
  • The elements he left gaps for were discovered later and Filled them.
  • He didn't follow the order of RAM, so elements could go in a group with other similar elements, and it's in order of the number of protons.
  • He placed elements based on their electronic structure, so the group number was equal to the number of outer electrons.
  • It has 8 groups, instead of 7.
  • It separates metals and non-metals.
  • It establishes trends in groups / across periods.
• Mendeleev had predicted properties of missing elements like:

  • That they were metals
  • Their atomic mass
  • Their reactivity
  • Their chemical properties
  • Their physical properties
• The modern periodic table does not show hydrogen in group 1

  Although hydrogen also has one electron in outer shell:

  Hydrogen:

  • A non-metal
  • A gas at room temp
  • Does not react with water
  • Has one electron shell
  • Gain an electron and form a negative ion
  • Forms covalent bonds / chares electrons

  While Group 1 elements:

  • Are metals
  • Are solid at room temp
  • Can react with water
  • Has at least 2 electron shells
  • Form positive ions
  • Form ionic bonds / lose electrons
• The periodic table is now based on atomic structure as:

  • number of shells increases down the group
  • number of protons / electrons increases across each period
  • atomic number = number of protons
  • number of protons is the same as the number of electrons in atoms

  Elements:

  • are arranged in proton / atomic number order.
  • in the same group have the same number of outer electrons.
  • in the same period have the same number if shells / energy levels.
• Hydrogen is an element that's difficult to fit into a suitable position in the periodic table, it could be placed either group 1 or group 7.


  Group 1:

  • Hydrogen has one electron in its outer shell and it can lose the one electron to form a positive ion of H+.
  • H+ ions are discharged at a negative electrode.

  Group 7:

  • Hydrogen is a non metal and a non-conductor
  • It can form:
  • covalent bonds / compounds
  • diatomic molecules
  • H- ions
  • It has a low melting and boiling point.