Periodic Table Trends
From MyMCAT
In awe of that ansewr! Really cool!
Valence Electrons Give Elements Their Properties
Electrons fill atoms in shells of increasing energy. As one goes left to right across a period another electron is added to the atom to compensate for the additional proton (increasing atomic number). It is the state of this outermost shell of electrons which will determine how the atom reacts. Once the end of the period is reached, the valence shell is filled with the maximum number of electrons it is capable of holding and so another period begins repeating the filling order and reactivity of the previous.
The strange shape of the periodic table (only 3 elements on the first row and two extra rows squeezed in at the bottom) is a result of how many electrons can fit into each shell. The further down one goes, the larger the shells and the more electrons it can hold.
See Electron Configuration for details on how electrons fill atoms and how this relates to any element on the periodic table.
