Development of The Periodic Table and Trends

What makes a group of elements? Dobereiner Triads – groups of three elements with similar chemical properties and middle element has atomic mass that is the average of first and third elements. Newlands Law of Octaves - arranged first 16 elements in order of increasing atomic mass (excluding hydrogen and noble gases). He discovered that chemical and physical properties repeated with every eighth element.

What makes a group of elements? Dimitri Mendeleev – Periodic Law First orderly arrangement of the known 63 elements in 1870 Arranged elements by increasing atomic mass Left gaps so elements with similar chemical and physical properties could be in the same column. Predicted properties of these yet undiscovered elements and was found to be correct when elements were finally discovered.

What makes a group of elements? There were exceptions in Mendeleev’s table of elements. He felt it was more important to have elements group by properties than by increasing atomic mass. Henry Mosley discovered atomic number (number of protons in nucleus) using x-ray diffraction and now PTE is arranged by increasing atomic number (Modern Periodic Law)

What makes a group of elements? Overview of the modern periodic table Period - a series of elements that form a horizontal row Group – a series of element that form a vertical column (also referred to as a family) All elements can be classified as a metal, non-metal or metalloid Most of the elements on the PTE are metals Mendeleev saw patterns in the properties of the elements–did you??

Trends on the

Important Trends Atomic radius – distance from the nucleus to the edge of the electron cloud. How do you think it changes as you go across a period? Decreases due to increase in size of nucleus. As the nucleus gets bigger, it is able to pull electrons closer as they are same distance (energy level) How about down a family (group)? Increase due to additional energy levels. Adding layers of electrons makes radius increase.

Important Trends Electron Affinity – ability of an atom to attract additional electrons while bonded to another atom. How do you think it changes as you go across a period? Increases due to increase in size of nucleus. As the nucleus gets bigger, it is able to pull electrons closer. Makes them harder to take away and easier to attract additional electrons. How about down a family (group)? Decrease due to further distance of the electrons from the nucleus. Adding layers of electrons makes radius increase which decreases the pull of the nucleus on the valence electrons.

Important Trends 3. Ionization Energy – amount of energy needed to remove the most loosely held electron in an atom How do you think it changes as you go across a period? Increases due to increase in size of nucleus. As the nucleus gets bigger, it is able to pull electrons closer. Makes them harder to take away. How about down a family (group)? Decrease due to further distance of the electrons from the nucleus. Adding layers of electrons makes radius increase which decreases the pull of the nucleus on the valence electrons. The bigger the atom the easier to take away electrons.

Important Trends 4. Electronegitivity – Energy needed to attract a pair of electrons while bonding. How do you think it changes as you go across a period? Increases due to increase in size of nucleus. As the nucleus gets bigger, it is able to pull electrons closer. Makes them harder to take away. How about down a family (group)? Decrease due to further distance of the electrons from the nucleus. Adding layers of electrons makes radius increase which decreases the pull of the nucleus on the valence electrons. The bigger the atom the easier to take away electrons.

What is the most important factor in determining these trends? Size of the atom. The larger the atom the harder to hold electrons and attract additional electrons. This is called the shielding effect.

Problems Which has a larger atomic radius Ca or Cl? Which has a greater ionization energy Mn or As? Which has a larger electronegitivity I or Li? Which element is the most reactive metal? Why? Which element is the most reactive nonmetal? Why?