Atomic Radius

What it is: Measure of the size of the atom Distance from nucleus to the boundary of the surrounding cloud of electrons Difficult to define because the electrons do not have defined orbits so their positions are estimated.

Periodic Trends (Going Down a Column) Principle quantum number increases Atomic radius increases examples: F = .71? Cl = .99? Br = 1.14? I = 1.33?

Periodic Trends (Left to Right in a Period) Effective nuclear charge increases Atomic radius decreases examples: Na = 1.86? Al = 1.43? S = 1.04? Ar = .97?

How to measure: No fixed radius Measure the distance between the nuclei of two bonded atoms The radii is determined by the bonds that they form

Example Problem: The atomic radius of F=64, Br=114 and I=138 pm respectively. Based on this information estimate a reasonable atomic radius of Cl. 53 pm 89 pm 126 pm 162 pm 196 pm

Example Problem: Using the periodic table and the known periodic trends, which element has the largest atomic radius? H He Fr Rn Rh

Ions and Ionic Radii: cation anion positive charge negative charge smaller atomic radius than the uncharged atom example: Na = 191 Na + = 95 Li = 156 Li + = 60 anion negative charge larger atomic radius than the uncharged atom example: F = 62 F- = 133 Cl = 102 Cl- = 181

Periodic Trends of Ionic Radii: Decreases across the period (from left to right) Increases within groups (from top to bottom)

Metals & NonMetals Metals: NonMetals: decreases after it ionizes metals lose electrons NonMetals: increase after it ionizes nonmetals gain electrons

Ionization Energy

Ion Atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge Can be created by both chemical and physical means All ions are charged: attracted to opposite electric charges, repelled by like charges, when moving, travel in trajectories that are deflected by a magnetic field

Trends of Ionization Energy The energy that is needed to remove an electron from an atom is a process called ionization energy. Ionization energy generally decreases from top to bottom within a group, while it increases from left to right across a period. Ionization energy can be helpful when predicting what ions may be formed. It is much easier to remove one electron, but when you go to remove a second electron the difficulty involved increases

First Ionization Energy The amount of energy required to remove one electron. For two electrons it is called the second ionization energy. For the “S” block these two ionizations are useful.

Group Trends in Ionization Energy Within each group (column): First ionization energy decreases from top to bottom. For example, see the noble gases listed at each peak; they are in the same group. Within each period (row): FIE increases when moving towards the right-hand side. There are some exceptions, evident if we look at the elements from K-Kr. This can be explained as the nuclear charge increases across the period. At the same time, the shielding effect does not change. Shielding effect: average amount of electron density between one e- and the nucleus This inhibits the e-’s attraction to the nucleus. Nuclear charge: Relative charge minus shielding effect constant The higher the nuclear charge is the less shielding there is.

Visual Chart http://www.angelo.edu/faculty/kboudrea/periodic/trends_ionization_energy.htm Previous Page: http://www.vias.org/genchem/atomstruct_12433_05.html

Shielding In a multi-electron system, electrons are simultaneously attracted to the nucleus and repelled by each other Outer electrons are shielded from nucleus by the core electrons screening or shielding effect outer electrons do not effectively screen for each other The shielding causes the outer electrons to not experience the full strength of the nuclear charge

Effective Nuclear Charge The effective nuclear charge is net positive charge that is attracting a particular electron Z is the nuclear charge, S is the number of electrons in lower energy levels electrons in same energy level contribute to screening, but very little so are not part of the calculation trend is s > p > d > f Zeffective = Z − S

Electronegativity The ability to gain electrons. Generally, decreases from top to bottom Generally, increases from left to right. *Noble Gases