Group 2

3.1.2 Group 2 Melting points Down the group the melting points decrease. The metallic bonding weakens as the atomic size increases. The distance between the positive ions and delocalized electrons increases. Therefore the attractive forces between the positive ions and the delocalized electrons weaken. Atomic radius Atomic radius increases down the Group. As one goes down the group the atoms have more shells of electrons making the atom bigger Electronic Structure Group 2 metals all have the outer shell s2 electron configuration. The first ionisation energy is Energy needed to remove an electron from each atom in one mole of gaseous atoms This is represented by the equation: H(g)
H+ (g) + eAlways gaseous Ionisation energy The first and second ionisation energies decrease down the group. The outermost electrons are held more weakly because they are successively further from the nucleus in additional shells In addition, the outer shell electrons become more shielded from the attraction of the nucleus by the repulsive force of inner shell electrons Group 2 reactions Reactivity of group 2 metals increases down the group The reactivity increases down the group as the atomic radii increase there is more shielding. The nuclear attraction decreases and it is easier to remove (outer) electrons and so cations form more easily The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge Second ionisation energy This is represented by the equation: Ti+ (g)
Ti2+ (g) + eWhen the group 2 metals react they lose the outer shell s2 electrons in redox reactions to form 2+ ions. The energy to remove these electrons are the first and second ionisation energy Mg will also react slowly with oxygen without a flame. Mg ribbon will often have a thin layer of magnesium oxide on it formed by reaction with oxygen 2Mg + O2
2MgO This needs to be cleaned off by emery paper before doing reactions with Mg ribbon If testing for reaction rates with Mg and acid, an un-cleaned Mg ribbon would give a false result because both the Mg and MgO would react but at different rates. Mg + 2HCl
MgCl2 + H2 MgO + 2HCl
MgCl2 + H2O Reactions with oxygen. The group 2 metals will burn in oxygen. Mg burns with a bright white flame 2Mg + O2
2MgO MgO is a white solid with a high melting point due to its ionic bonding