THERMO - EXAM MEMORISATION

Ions (atomic radii)

• same number of protons

• valence electrons in the same shell

• ions - electrons added into the valence shell increase the electron-electron repulsion and the electrons move further apart

• increases the size of the valence shell and the distance of the electrons from the nucleus

EN statement

EN depends on the strength of the attractive force between the nucleus of an atom and the bonding electrons, which are in the valence electron shell

EN - Same row

• same period, bonding electrons are in the same energy level, same inner shell shielding from inner electrons

• number of protons increase across a period. greater electrostatic attraction between the nucleus and the bonding electrons

• hence EN of X is larger than EN of Y

EN - Same Group

• same group on periodic table, but bonding electrons in X are in an energy level further from the nucleus than for Y

• There is more shielding from inner electrons

• Although number of protons increase, the effect of the greater nuclear charge is outweighed by the increasing distance and shielding between the nucleus and the bonding electrons

Polar Explanation

X is more electronegative than Y, therefore the X-Y bonds are polar covalent bond. Due to the K shape, the dipoles are not symmetrically arranged around S atom. As a result, the dipoles do not cancel out, and therefore XSY is a polar molecule

HB Bond - Explanation

HF and HBr have permanent dipole attractions between their molecules, but HF also has strong hydrogen bonding due to the large electronegativity difference between H and F, so it has a higher melting point

Ionisation Energy - Same period

• same period

• although the valence electron to be removed is in the same energy level, with the same repulsion/shielding from inner energy levels,

• number of protons increase across a period.

• Electrostatic attraction between the positive nucleus and the valence electrons increases across a period

• therefore more energy is required to remove the outermost electron

• Therefore…

Ionisation energy - Same group

• same group

• Ionisation energy decreases down the group because

• the valence electron to be removed is in an energy level further from the nucleus with greater repulsion/shielding from inner energy levels.

• Although the number of protons increases down a group, this is offset

• by the increasing distance between the nucleus and the valence electron to be removed

• electrostatic attraction between the positive nucleus and the valence electrons decreases down the group

• so less energy is required to remove the outermost electron

• therefore…

Larger electron cloud - Explanation

• SO2 has a larger electron cloud.

• This means that SO2 has stronger/more temporary dipole attractions between the SO2 moelcules and therefore a higher boiling point

SA - Explanation

• a longer molecule

• it has a greater surface area for interaction, meaning stronger temporary dipole attractions.

• Even though there are strong permanent dipole attractions between the polar SO2 molecules

• , more energy is needed to break the attractions between the pentane molecules,

• so pentane has a higher boiling point.

Atomic Radii - Explanation

• Going down a group,

• valence electrons are added to an energy level further from the nucleus with increased repulsion / shielding from inner shells.

• Although the number of protons increases down a group,

• this attraction is offset by the increasing distance between the nucleus and the valence electrons

• So, the electrostatic attraction between the positive nucleus and its valence electrons decreases,

• therefore the atomic radius of Ga is bigger than the atomic radius of B.

Cations

• Ca atom loses its two valence/outer shell electrons when it forms the Ca2+ ion.

• Therefore the Ca atom has four occupied energy levels, whereas the Ca2+ ion only has three occupied energy levels.

• As a result, the Ca2+ ion has a smaller radius