Periodic Trends & Bonding Atomic Radii, Ionization Energy, Oxidation Numbers

Atomic size

Patterns in Atomic Size 23/09/99 Atomic size refers to the distance that the furthest (valence) electrons are from nucleus It can affect the properties of atoms & elements Which Is Bigger? a) Ne or Ar b) B or C c) P or Ge Larger Larger We often only focus on valence electrons: they are involved in reactions and determine size Two factors affect size: n and the balance between attractions & repulsions in the atom.

Decreasing Atomic Size Across a Period 23/09/99 As the attraction between the +ve nucleus and the -ve valence electrons , the atomic size . From left to right, size decreases because there is an increase in nuclear charge and Effective Nuclear Charge (# Prot. - # non-Valence Electrons) Li (ENC = 1) Be (ENC = 2) B (ENC = 3) + + + + + + + + + + + +

Sizes of ions 23/09/99 Ions are atoms that have either gained or lost electrons (so that the # of electrons is not equal to the # of protons) The size of an atom can change dramatically if it becomes an ion Ex. when sodium loses its outer electron to become Na+ it becomes much smaller. Why? Na+ is smaller than Na because it has lost its 3s electron. Its valence shell is now 2s22p6 (it has a smaller value of n) Changing n values is one explanation for the size of ions. The other is …

Sizes of ions: electron repulsion 23/09/99 Valence electrons push each other away When an atom becomes a –ve ion (adds an electron to its valence shell) the repulsion between valence electrons increases without changing ENC Thus, F– is larger than F 9 +

Ionization energy Be B + 23/09/99 Ionization energy Ionization energy is the energy required to remove an electron If n is small & ENC is large, electrons will be difficult to remove (i.e. the IE will be high) Be + B

23/09/99 Ionization energy There are as many IEs as there are electrons – you can remove all electrons in an atom Subsequent IEs are higher than the first because you are removing a -ve charge (electron) from an increasingly +ve atom/ion Subsequent IEs make a huge jump after the electrons in the outer shell are lost - it is not difficult for Mg to lose 12th and 11th electron, but very difficult for it to lose it’s 10th electron.

IE is the opposite of atomic radius Trends in Size & IE 23/09/99 IE is the opposite of atomic radius Larger Larger Size Larger Ionization energy Larger

Electrochemistry

Electron Transfer Reactions Electron transfer reactions are oxidation-reduction or redox reactions. Results in the generation of an electric current (electricity) or be caused by imposing an electric current. Therefore, this field of chemistry is often called ELECTROCHEMISTRY.

Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species (either atom or compound); increase in oxidation number; increase in oxygen. REDUCTION—gain of electron(s); decrease in oxidation number; decrease in oxygen; increase in hydrogen.

You can’t have one… without the other Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons. You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation LEO the lion says GER! ose lectrons xidation ain lectrons eduction GER!

Another way to remember OIL RIG s s xidation ose eduction ain

Review of Oxidation numbers The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H2, O2, P4 = 0 In monatomic ions, the oxidation number is equal to the charge on the ion. Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 The oxidation number of oxygen is usually –2. In H2O2 and O22- it is –1. 4.4

HCO3- O = -2 H = +1 3x(-2) + 1 + ? = -1 C = +4 The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. Group IA metals are +1, IIA metals are +2 and fluorine is always –1. 6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. HCO3- Oxidation numbers of all the atoms in HCO3- ? O = -2 H = +1 3x(-2) + 1 + ? = -1 C = +4 4.4