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GCSE Chemistry coursework: Research Study on ‘Francium and the alkali metals’
Text 3
The Bohr model of the atom is named after the twentieth-century Danish scientist, Niels Bohr. The
model says that an atom has a positively charged nucleus surrounded by electrons that have circular
orbits around the nucleus. This model was inspired by the workings of the solar system and says that
electrons are kept attracted to the nucleus by electrostatic forces rather than gravity. [9]
This is an example of the Bohr atomic structure model of a lithium atom. [10]
The Bohr model can explain the reactivity of the alkali metals because it says that atoms are at
their most stable when they have a stable octet and seeing as the alkali metals only need to lose one
electron to gain a stable octet they are very reactive.
The reactivity increases further down the group as the electron that needs to be lost is further
away from the positively charged nucleus and so the electrostatic attraction between the electron
and the nucleus is much weaker. The attraction is weakened as the elements further down the
group have an increased atomic radius and so the shielding effect decreases the attraction between
the outer electron and the nucleus. This means that the ionisation energy (the energy needed to
make the element an ion by losing it's outer electron) decreases descending down the group.
Therefore, the Bohr model of atomic structure can explain that the reactivity of the alkali metals
increases descending down the group as it shows that as less energy is needed for a reaction to
happen, the chances for example of caesium reacting is much more likely than lithium reacting.[11]