CHM 101OL - Lecture
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This document discusses periodic trends, including how atomic and ionic size, ionization energy, and electron affinity vary across the periodic table. It explains that atomic size increases down a group as more electron shells are added, but increases across a period from left to right as the nuclear charge felt by valence electrons increases. Ionization energy and electron affinity also increase as atomic size decreases, as it is harder to remove or add electrons to smaller atoms. The octet rule is introduced, where main group elements seek to obtain full s and p subshells of eight electrons to achieve stable electronic configurations.
CHM 101OL - Lecture
- 1. Metallic Character Atomic Size Ionization Energy Electron Affinity Ionic Size Patterns in Electron Configuration Periodic Trends
- 2. previously learned characteristics: luster, conductivity, malleability, etc. new characteristic to know: tendency to get oxidized characteristics increase in direction of arrows Oxidized = lose e-’s Are you thinking “I thought oxidized meant to react with oxygen?” Well, that is the historical roots to the term, but now the term is applied to something more fundamental – namely, what the electrons are doing. Thus, a substance can undergo “oxidation” without oxygen being involved at all! Recall that K reacted more vigorously with water than Na – i.e. K has more metallic character, it is more readily oxidized than Na. Let’s look at some other trends and then we’ll come back to explain this one…
- 3. size increases in direction of arrows As move down a group: => more e- shells are present in the atom => e-’s in these additional shells spend more time further from the nucleus, => thus increasing the size of the atom. Size is given as the diameter or radius of the atom. Even though an atom doesn’t have “distinct borders” like a ball, we have several techniques that allow us to measure the approximate size of the atom. Na Rb Did you guess Rb? Good job! In a given group, do you think atoms higher ( 11 Na) or lower ( 37 Rb) in the PT are larger?
- 4. Out instincts tell us to predict that since Ar has more e-’s, it is bigger than Na. Unfortunately in this case, we are wrong… size increases in direction of arrows In a given period, do you think atoms on the left side ( 11 Na) or right side ( 18 Ar) of the PT are larger? Na Ar In a period, the “added” e-’s are located in the same shell, not added to a larger one, so another factor (the approximate nuclear charge in chart above) dictates the size. P number of core e- = 10- 10- 10- approx. number of protons actually felt by the valence e- = 1+ 5+ 8+ Think of it this way: the inner e-’s (core) somewhat “shield” the outer e-’s (valence) from the pull of the p+’s in the nucleus nucleus. As the chart shows, atoms in the same period have the same number of shielding core e-’s but a different number of p+’s, thus resulting in a situation where the valence e’s in Ar feel a larger pull toward the nucleus than those in Na. That stronger pull on the e-’s draws them in closer, thus making the atom smaller overall. explanation for the horizontal trend Na P Ar number of protons = 15+ 18+ 11+
- 5. T or F Atoms with more electrons are larger than atoms with fewer electrons. Sometimes yes (ex. comparing atoms in same group), sometimes no (ex. comparing atoms in same period), thus the statement has to be labeled false. T or F A substance can be oxidized without reacting with oxygen. Oxidized means to lose e-’s, oxygen may or may not be involved in the process. T or F Based on the periodic table, it would be predicted that rubidium is a better conductor of heat, more readily oxidized, and a larger atom than sodium. Conductivity and ease of oxidation are two metallic characters and metallic character increases as move down a group. Size also increases as move down a group.
- 6. ↳ the energy change that occurs when an electron is removed from a gaseous atom ↳ higher IE means “takes more energy” which means it is harder to remove an e- IE increases in direction of arrows Both trends relate back to size: => an e- in a smaller atom is more tightly held => it takes more energy to remove it
- 7. increases in direction of arrows the energy change that occurs when an electron is gained by a gaseous atom ↳ higher EA means “more affinity” which means it is easier to gain an e- EA increases in direction of arrow It relates back to size: => a smaller atom has more attraction for an e- and can hold onto it better => it is easier for the smaller atom to gain an e-
- 8. the energy change that occurs when an electron is gained by a gaseous atom ↳ higher EA means “more affinity” which means it is easier to gain an e- IE increases in direction of arrow but trend does not carry through to noble gases (they have very little e- affinity) Trend relates to the number of valence e’s: => as move to the right in PT, the atoms are getting closer to having a full s & p valence subshell => the closer they get, the easier it becomes to add an e- (It may relate somewhat to size also, but fact that EA trend doesn’t fully follow the size trend indicates that something else must be at play.)
- 9. This leads us to the infamous “OCTET RULE” Statement of the octet rule: Main Group elements tend to react in such a way to obtain full s and p valence orbitals *H and He are exceptions, their “octet” is only a “doublet” since they both only have an s orbital. full valence s and p orbitals are associated with some “special stability” 2 e-’s in a full s orbital + 6 e-’s in a full p orbital = 8 e-’s total (8 = octet)
- 10. largest atoms + atoms closest to octet by losing e- ↳ most “loosely” held electrons ↳ lowest IE and EA ↳ readily lose e-’s to form cations ↳ pronounced metallic properties smallest atoms + atoms closest to octet by gaining e- most tightly held electrons ↲ highest IE and EA ↲ readily gain e-’s to form anions ↲ pronounced non-metallic properties ↲ high IE low EA S T A B L E as is Group 3A & 4A metalloids and nonmetals tend not to form ions
- 11. T or F The higher an element’s electron affinity, the harder it is for an atom of the element to gain an electron, so F gains an e- more readily than Cl or Li do. T or F The higher an element’s ionization energy, the harder it is for an atom of the element to lose an electron, so it is harder to remove an e- from F than from Li for Cl. T or F The octet rule states that all elements tend to react in order to obtain a full valence shell of eight valence electrons. main group s & p valence subshells (octet met even if the d orbitals of the valence shell are not filled, that’s why I specify subshells only) easier
- 12. Which is bigger, a cation or an atom of the same element? Which is bigger, an anion or an atom of the same element? 0.10 0.18 0.18 0.097 An atom that loses e-s while #p+ is the same experiences less e- repulsions and thus shrinks in size. An atom that gains e-s while #p+ is the same experiences more e- repulsions and thus expands in size. atom size: Cl ______nm Na ______nm ion size: Cl - ______ nm Na + ______nm example data