CHEM. 2AP UNIT 5 CH. 7 Quantum Mechanics 28 TRENDS OR PATTERNS IN THE PERIODIC TABLE The electron configurations show a periodic variation with increasing atomic number. Periodic means that there is a trend or pattern that becomes predictable. A periodic trend is: I come to Chemistry class once a day. The atomic size (radius) of an atom is a periodic trend that has a direct bearing on its ability and tendency to form ions. ATOMIC RADIUS AS YOU GO LEFT TO RIGHT ACROSS A PERIOD, THE RADIUS OF THE ATOM DECREASES. The reason for this is that electrons are being added to the same principal energy level. The nuclear charge increases and pulls the outer electrons closer. AS YOU GO FROM TOP TO BOTTOM IN A GROUP, THE RADIUS OF THE ATOM INCREASES. Electrons are being added to the next principal energy level. The nuclear charge increases but the outermost electrons are farther away. Nuclear attraction for farther electrons is shielded from the outer electrons by occupied levels between the nucleus and the outermost electrons. The inner level electrons shield the outer level electrons from the nuclear attraction (a book between a magnet and a nail). The plot of atomic radii versus atomic number shows a periodic variation EXERCISES: 1. Arrange the following atoms in order of increasing radius: P, Si, N. 2. Arrange the following atoms in order of decreasing radius: C, Li, Be. 3. Arrange the following atoms in order of increasing radius: N, O, P, S. 11/01/2002 CHEM. 2AP UNIT 5 29 CH. 7 Quantum Mechanics ION SIZE VERSUS ATOM SIZE. There is no general trend with respect to ion size as you go across and down the periodic table. There is a trend in cation size when compared to the size of the metal atom, and anion size when compared to the size of the non-metal atom. AN INCREASED NUMBER OF ELECTRONS ARE HELD MORE LOOSELY FORMING A LARGER ION. A DECREASED NUMBER OF ELECTRONS ARE HELD MORE TIGHTLY FORMING A SMALLER ION. POSITIVE IONS ARE SMALLER THAN THE NEUTRAL ATOMS FROM WHICH THEY ARE FORMED. (THINK RADIUS!!) By removing one or more electrons from an atom reduces electron-electron repulsion but the nuclear charge remains the same so the electron cloud shrinks. The electron cloud shrinks because, by losing valence electrons, you are emptying subshells. Ex. Al (#13) 1s2 2s2 2p6 3s2 3p1 Al3+ 1s2 2s2 2p6 The 3s and 3p sublevels are removed thereby reducing the size of the electron cloud. The cation is also smaller than the atom because there is the same number of protons pulling on fewer electrons. A Li+ ion is smaller than the Li atom. The most metallic elements are the ones that lose electrons most easily. They are said to have the highest metallic character. Where are the most metallic elements located? NEGATIVE IONS ARE LARGER THAN THEIR NEUTRAL ATOMS. (THINK RADIUS!!) When anions are formed, electrons are being added. The nuclear charge remains the same but there are more electrons to pull on and the attraction is being dispersed. There is additional repulsion of electrons resulting from the additional electrons and this enlarges the size of the electron cloud. As a result, the anion is larger than its atom. O2- is larger than the O atom. The most non-metallic elements are the ones that gain electrons most easily and have the least metallic character. Where are these elements located? CATION LARGER ANION LARGER 11/01/2002 CHEM. 2AP UNIT 5 Questions: 1. Which has the larger radius: a. Sulfur atom or sulfur ion? b. Rb+ ion or Sr2+ ion? 2. For each of the following pairs, indicate which one of the two species is larger: a. N3- or Fb. Mg2+ or Ca2+ c. Fe2+ or Fe3+ 3. Select the smaller ion in each of the following pairs: a. K+, Li+ b. Au+, Au3+ c. P3-, N3- 30 CH. 7 Quantum Mechanics Comparison of atomic radii with ionic radii. (a) Alkali metals and alkali metal cations. (b) Halogens and halide ions. IONIZATION ENERGY Ionization energy is the minimum energy (in kJ/mole) required to remove an electron from a gaseous atom in its ground state, i.e. it is the amount of energy in kilojoules needed to strip one mole of electrons from one mole of gaseous atoms (gaseous atoms because they are not influenced by other atoms around it). The amount of ionization energy used is a measure of how tightly the electron is held in the atom. The higher the ionization energy, the more difficult it is to remove the electron. TRENDS IN IONIZATION ENERGY IN THE PERIODIC TABLE • • • • • First ionization energy is the energy required to remove the outermost electron from an atom. Think radius!! An atom with a smaller radius holds the electrons more tightly because they are closer to the nucleus. THE IONIZATION ENERGY INCREASES AS YOU GO FROM LEFT TO RIGHT ACROSS THE PERIODIC TABLE (RADIUS GETS SMALLER – MORE TIGHTLY HELD – HARDER TO PULL AWAY). THE IONIZATION ENERGY DECREASES AS YOU GO DOWN THE PERIODIC TABLE (LARGER RADIUS – ELECTRONS FARTHER FROM THE NUCLEUS – LESS TIGHTLY HELD). Metals are characterized by low ionization energy (very little energy needed to remove the electron). Metals want to lose electrons easily so they must have very low ionization energy. It is easy to pull an electron away from a metal. Nonmetals have high ionization energies (smaller radius – electrons are tightly held). Non-metals want to gain electrons, not lose so it is very hard to pull an electron from an atom that wants to gain instead of lose. Increased distance of the outer electrons from the nucleus and the shielding effect of the inner electrons tend to lower the ionization energy. Though it appears that the increased nuclear charge of an element with greater atomic number tends to increase ionization energy, the lowering tendency is greater. The number of electrons in the outermost sublevel is the same for all elements in a column or group. FACTORS AFFECTING IONIZATION ENERGY. 1. Nuclear charge – The larger the nuclear charge, the greater the ionization energy. 2. Shielding effect – The greater the shielding effect the less the ionization energy. 3. Radius – The greater the distance between the nucleus and the outer electrons of an atom, the less the ionization energy. 4. Sublevel – An electron from a full or half-full sublevel requires additional energy to be removed. 11/01/2002