3.2 Periodic trends
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What is a covalent radius?
Half the distance between two neighboring nuclei
Does atomic radius decrease or increase across a period and why?
It decreases as: • number of protons increases=higher nuclear charge • number of electrons increase but electron shielding effect remains the same • Results in stronger attraction between nucleus and valence electrons so radius decreases
Does atomic radius increase or decrease down a group?
It increases as valence electrons occupy a main energy level further from nucleus and so less attraction
Do cations and anions lose or gain electrons?
Cations lose electrons while anions gain
Isoelectronic
Elements with same electronic configuration eg Na+ and Mg 2+
Does ionic radius increase or decrease across a period? Are there any exceptions?
It generally decreases because: • Proton number increases while electron number remains the same • There is still the same shielding effect so higher effective nuclear charge • Stronger attraction to nucleus • Exception: When it shifts from cation to anion as electron no.>proton no. so effective nuclear charge decreases and larger shielding effect. Starts decreasing after increase however.
Does ionic radius increase or decrease down a group?
Increases as shielding effect increases so there is less attraction.
Are cations bigger or smaller than parent atom?
Smaller as they occupy one less main energy level
Are anions bigger or smaller than parent atom?
Bigger as electrons are more than protons so more repulsion and hence bigger radii
Effective nuclear charge
Strength of attraction between positive nucleus and negative electrons
What does effective nuclear charge depend on?
• Size of atomic radius and number of shielding electrons
What is effective nuclear charge of Sodium and Magnesium?
Sodium has ENC of +1 as it has 10 shielding electrons and Magnesium has ENC of +2
Trend of effective nuclear charge down a group
Remains constant as increase in protons and hence attraction of nucleus is offset by increase in occupied energy levels
Trend of effective nuclear charge across a period
Increases since shielding effect remains the same as it’s the same energy level
What is first ionization energy?
Energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Equation for first ionization energy
X (g)–> X+ (g) + e-
Trend of first ionization energy across a period
Increases as: • effective nuclear charge increases so electrons are more difficult to remove • atomic radius decreases and same shielding effect means more attraction
Trend of first ionization energy down a group
Decreases as: • atomic radii decreases • shielding effect increases reducing effective nuclear charge so less attraction makes it easier to remove
Electron affinity
Energy released when one mole of electrons are added to one mole of atoms to form one mole of gaseous 1- ions
Is first electron affinity positive or negative?
Negative as energy is released so it’s exothermic
Trend of electron affinity across a period
Increases because: - Shielding effect is same - Proton number increases - Effective nuclear charge increases so attraction is stronger and more energy is released Moving right means likelihood of pairing electrons increases as they try to get a more stable configuration. Metals often have more tendency to lose rather than gain
Trend of electron affinity down a group
Decreases as: • Electron gained is entering an energy level further (increase in atomic radii) • Increase in shielding effect + decrease in effective nuclear charge = weaker attraction so less release of energy
Electronegativity
Attraction of an atom for shared pair of electrons in a covalent bond
Which elements have the highest and lowest electronegativity?
Fluorine is highest at 4.0 while Francium is lowest at 0.7