1.7 TRENDS IN THE PERIODIC TABLE

Important Definitions Trend: predictable change in a

particular direction Electron Shielding : inner

electrons shield outer electrons from the full attractive force of the nucleus

Effective Nuclear Charge (ENC): charge felt by the valence electrons after you have taken into account the# of shielding electrons

Atomic RadiusAtomic radius: half of the distance between the nuclei of two atoms of the same element bonded together.

Atomic Radius increases as you move down a groupelectrons occupy consecutively higher energy levels, farther from the nucleus

ENC decreases due to increased shielding

H

Li

Na

K

Rb

Atomic Radius decreases as you move across

a period Electrons are in the same energy

level there is more nuclear charge.

(protons are added as you move left to right)

Outermost electrons are pulled closer. Na Mg Al Si P S Cl Ar

Atomic Radius Trends

Radius of Cations Metals form cations. Cations form by losing

electrons. Cations are smaller than

the atom they came from – not only do they lose electrons, they lose an entire energy level. (ENC increases, shielding decreases)

Radius of Anions Nonmetals form anions. Anions form by gaining

electrons. Anions are bigger than the

atom they came from – have the same energy level, and nuclear charge (more repulsion between electrons)

Ionic Radius Trends increases as you move down a groupdue to decreased ENC (increased shielding)

decreases as you move across a perioddue to increasing nuclear charge, whether the ion is positive or negative

Ionic Radii Trends

Ionization Energy Ionization energy: energy required to remove an electron from an atom or ionMust overcome attraction between electron and nucleus

2nd Ionization Energy: The energy required to remove a second electron from a cation

Ionization Energy Group 1: easily loses its 1

valence electron Low first ionization energy Second ionization energy will be

very high since it is “happy” with losing 1 electron.

Group 2: easily lose 2 valence electrons Low first and second ionization

energies High third ionization energy

I.E. for the 1st 20 elements

Symbol First Second ThirdHHeLiBeBCNO F Ne

1312 2731 520 900 800 1086 1402 1314 1681 2080

5247 7297 1757 2430 2352 2857 3391 3375 3963

11810 14840 3569 4619 4577 5301 6045 6276

Why did these values increase so much?

Ionization

Ionization Energy Trends

decreases as you move down a groupdue to electron shielding (decreasing ENC)

increases as you move across a perioddue to the increase in nuclear charge (less shielding)

Ionization Energy Trends

Electron Affinity Electron Affinity: the

energy given off when an electron is added to an atom to make an anion.elements with high electron affinities form negative ions in ionic compounds.

elements with low electron affinities form positive ions in ionic compounds.

Electron Affinity Trends

decreases as you move down a group due to increased electron shielding (decrease in ENC)

increases as you move across a period due to increasing nuclear charge, less sheiding (easy to gain e-)

Electron Affinity Trends

Electronegativity Electronegativity: measure

of the ability of an atom in a compound to attract electrons

higher electronegativity means a stronger pull on electrons

Ranges from 0 to 4

Electronegativity Trendsdecreases as you move down

a groupdue to decrease in ENC, lots of shielding

increases as you move across a period increasing atomic number, so increase in ENC, less shielding

Metals have a low electronegativity; non-metals have a high electronegativity

Electronegativity Trends