C4 - Summary Notes

History of the Periodic Table

Modern Periodic table - Dmitri Mendeleev

• arranged elements into groups (vertical columns = groups , horizontal rows = periods)
• based on their atomic mass and patterns in their properties
• left groups where undiscovered elements and predicted their properties
• physical and chemical properties of elements were related to their atomic mass in a 'periodic way'

Dobereiner noticed triads - linked patterns of relative atomic masses for 3 elements

• when elements were arranged in order of their increasing atomic mass it was approximately the mean of the other 2 elements in the traid.

Newlands noticed octaves - every eighth element had similar properties

Scientists rejected triads and octaves - most elements didn't fit the patterns

New elements discovered fitted Mendeleev's predictions

Lines of Discovery

Some elements emit distinctive flame colours

• coloured lines can be split into a line specturm (unique to each element)
• discovery of He happened because of the development of spectroscopy
• (when chemists looked at the line spectrum for the Sun)

Inside the atom

Protons + Neutrons (nucleus)

Electrons (Shells)

• all atoms of an element have the same number of protons and electrons
• modern periodic table - atoms are in order of their proton number
• number of protons + number of neutrons = relative atomic mass

Sorting Electrons

1st shell = 2 electrons

2ns and 3rd shell = 8 electrons

Same number of protons and electrons (number of electrons is the same as the proton number)

for the first 20 elements - the 3rd shell holds 8 electrons

• electrons in different shells have different energy levels (closer the electron shell is to the nucleus, the lower the energy level is)

Finding Elements

• Elements arranged in order of proton number
• number of electrons in the outer shell increases across a period
• left - right along a period - elements change from metals to non-metals
• left - right along a period - each element has one more proton and one more electron

Properties change across a period (trends)

• Group 0 elements have full electron shells - they are very unreactive **
• Number of electrons in the outer shell of an atom is the same as its group number
• Atoms…