periodic table chem topic 6

• Dmitri Mendeleev arranged elements in order of relative atomic mass
• Mendeleev left gaps in his periodic table, because he assumed that these elements hadn't been discovered yet
• Mendeleev predicted the properties of some undiscovered elements before they were discovered from the properties of elements in the same column
• Within fifteen years of Mendeleev's predictions, the new elements discovered fitted in with the existing groups
• Mendeleev continued to work on his table and settled on a table in which elements with similar chemical properties were arranged in the same vertical column, just as today
• To make his ideas work, Mendeleev reversed the order for some pairs of elements, so that the elements with similar chemical properties were in the same group rather than in order of atomic mass
• Mendeleev put tellurium (atomic mass 128) before iodine (atomic mass 127) in his table because iodine had similar chemical properties to the Group 7 elements
• Mendeleev started to arrange the elements in order of increasing atomic mass
• When protons, electrons and neutrons were discovered in the 20th century, the elements in the modern periodic table were arranged in order of increasing atomic number
• The Periodic Table is called a Periodic Table because a property/trend is repeated across each period
• In the Periodic Table elements are arranged in order of increasing atomic number, in rows called periods
• Elements with similar chemical properties are placed in the same vertical columns called groups
• The atomic number is the number of protons in the nucleus of an atom
• Each element has a unique atomic number and it gives the position of an element in the periodic table
• All the elements in a group have similar chemical properties because they have the same number of electrons in the outer shell they have the same number of valency electrons
• Oxygen and sulfur have similar chemical properties because they have six electrons in the outer shell
• Elements are identified as metals and non-metals, and as you go across a period, the elements change from metallic to non-metallic
• The compounds formed by the elements change from ionic compounds on the left of the period to covalent compounds on the right
• Metals are clearly separated from non-metals. The non-metals are packed into the top right corner above the stepped line. Some elements close to the stepped line are metalloids (semi-metals) eg silicon
• The group number is the same as the number of electrons in the outer shell
• The period number is the same as the number of occupied shells
• Group 1 metals are called Alkali metals because they react with water to produce strongly alkaline solutions (pH 14)
• Group 1 metals are stored under oil to prevent them from reacting with oxygen in the air
• Group 1 metals react with water to produce hydrogen and a metal hydroxide solution
• Li, Na and K are less dense than water and float over the surface of water
• Group 1 metals react with non-metals to form ionic compounds in which the metal ion carries a +1 charge
• Melting points decrease as you go down group 1 because the metallic bonding becomes weaker
• Group 1 compounds are white ionic solids. They dissolve in water to form colourless solutions
• Reactivity increases as you go down group 1 (Li, Na, K, Rb, Cs)
• As you go down the group the atom gets bigger, because there is one more shell of electrons
• The outer electron in caesium is less strongly attracted to the nucleus, so is lost more easily
• Ionic solids


  They dissolve in water to form colourless solutions
• As you go down group 1 (Li, Na, K, Rb, Cs)

  Reactivity increases
• Potassium is bigger than sodium


  Because it has one more shell of electrons than sodium
• What happens when a group I metal is added to water

  1. Fizzing is observed (because hydrogen gas is formed in the reaction)

  2. The metal melts and moves over the surface of water

  3. In the case of K, Rb and Cs a flame is observed
• Products


  Metal hydroxide and hydrogen
• The more reactive the metal, the more vigorous is its reaction with water
• Test for hydrogen


  1. Test the gas with a lighted splint

  2. The gas extinguishes the lighted splint with a pop sound
• Products formed when potassium reacts with water

  Potassium hydroxide and hydrogen
• Chemical properties group 1 elements have in common

  • They react with water to produce an alkaline solution
  • They react with water releasing hydrogen gas
  • They react with acids/They react with oxygen/They react with chlorine