Periodic Trends There are several important atomic characteristics that show predictable trends. Atomic radius Ionization energy Electron affinity Electronegativity Adapted from: staffweb.psdschools.org/rjensen/powerpoints/periodic_trends.ppt

Why Do These Trends Exist? The arrangement of elements in the periodic table results in characteristic changes in the Effective nuclear charge and Shielding effect going across a period or down a group.

Effective Nuclear Charge Effective nuclear charge is the pull that an electron “feels” from the nucleus The closer an electron is to the nucleus, the more pull it feels As effective nuclear charge increases, the electron cloud is pulled in tighter

Shielding Effect As more energy levels are added to an atom, the inner layers of electrons shield the outer electrons from the nucleus The effective nuclear charge on those outer electrons is less, and so the outer electrons are less tightly held

Atomic Radius Radius is the distance from the centre of the nucleus to the edge of the electron cloud This is measured as half the distance between the nuclei of 2 bonded atoms

Atomic Radius

Atomic Radius Atomic radius increases going down a vertical column or group This is because each period has an additional electron shell or energy level The shielding effect increases while effective nuclear charge decreases

Atomic Radius Atomic radius tends to decrease going from left to right within a period Each step adds a proton and an electron, but the energy level does not change Effective nuclear charge increases The increased attraction pulls the electron cloud in, making the atom smaller

Atomic Radius **Heavy arrows point in the direction of an increase **

Ionization Energy The energy required to remove a electron from an atom in the gas phase The 1st electron (1st Ion. E.) is the easiest to remove, the 2nd (2nd Ion. E.) is harder, etc. Ionization energy and atomic radius are inversely proportional Ionization energy is always endothermic; energy is added to the atom to remove the electron

Copyright Pearson Prentice Hall

Ionization Energy

Ionization Energy Ionization energy decreases going down a group because the valence electrons are further from the nucleus, therefore less energy is required to remove one The shielding effect also increases as more energy levels are added

Ionization Energy Ionization energy increases moving from left to right across a period As the number of protons in the nucleus increases, so does the effective nuclear charge; and the energy level does not change

Ionization Energy

Electron Affinity Electron affinity is the energy change that occurs when an atom in the gas phase gains an electron Electron affinity is directly proportional to the effective nuclear charge Where ionization energy is always endothermic, electron affinity is usually exothermic (but not always)

Electron Affinity Electron affinity is exothermic if there is an empty or partially empty sub-orbital for an electron to occupy If there are no empty spaces, a new sub-orbital must be created, making the process endothermic (e.g. alkaline earth metals and noble gases)

Electron Affinity Electron affinity decreases going down a group Each time a new energy shell is added, the electrons are further from the nucleus and the effective nuclear charge decreases, while the shielding effect increases

Electron Affinity Electron affinity increases moving left to right across a period Electron affinity increases as the effective nuclear charge increases

Electron Affinity + +

Electronegativity Electronegativity is a measure of an atom’s attraction for another atom’s electrons while in a chemical bond It uses an arbitrary scale ranging from 0 – 4 Generally, metals are “electron givers” and have low electronegativities Nonmetals are are “electron takers” and have high electronegativities The noble gases have 0 electronegativity

Electronegativity Electronegativity decreases going down a group as the effective nuclear charge decreases and the shielding effect increases Electronegativity increases going from left to right across a period because the effective nuclear charge increases

Electronegativity

Overall Reactivity This ties all the previous trends together However, we must treat metals and nonmetals separately The most reactive metals are the largest, since they are the best electron givers The most reactive nonmetals are the smallest ones, the best electron takers

Overall Reactivity

Ionic Radius Positive ions are called cations Negative ions are called anions Cations are always smaller than the original atom, because the entire outer shell is removed during ionization Conversely, anions are always larger than the original atom, because electrons are added to the outer shell

Cation Formation Valence e- lost in ion formation Na atom 1 valence electron Result: a smaller sodium cation, Na+ 11p+ Remaining e- are pulled in closer to the nucleus & ionic size decreases Effective nuclear charge on remaining electrons increases

Anion Formation One e- is added to the outer shell Chlorine atom with 7 valence e- A chloride ion is produced and it is larger than the original atom 17p+ Effective nuclear charge is reduced and the e- cloud expands