Periodic Trends

1. Periodic Trends • There are many ways to use the periodic table • Trends are based on electronic structure • The trends we will discuss are: • shielding • atomic mass • atomic radius • ionization energy • metallic character • ionic radius • properties of the metals and non-metals • melting point

2. Valence Shell • Def: the orbitals in the highest energy level • on any one row on the periodic table – s & p • valence electrons live in these orbitals • core electrons are ALL the electrons between the valence electrons and the nucleus

3. Valence Shell (Cont.) • for each row on the periodic table, the outermost energy levels are always the s and p orbitals (check it out by comparing/contrasting the electron configurations for S, Se, Te, and Po)‏ • for each period (row), the highest energy levels only have s or p as their orbital paths! • exs: S: [Ne] 3s2 3p4 ← Highest energy level: 3rd only orbitals in level: s & p Se: [Ar] 4s2 3d10 4p4← Highest energy level: 4th only orbitals in level: s & p Te: [Kr] 5s2 4d10 5p4 ← Highest energy level: 5th only orbitals in level: s & p Po: [Xe] 6s2 5d10 4f14 6p4←Highest energy level: 6th only orbitals in level: s & p

4. Shielding • There is a rough cancellation of core electrons and proton charge • Effective nuclear charge is the positive charge felt after cancellation of charge • For example, Na has an effective nuclear charge of +1 • the electron configuration for Na: [Ne] 3s1 • it has 1 valence electron and 10 core electrons • 11 protons – 10 core (shielding) electrons • +1 effective nuclear charge

5. Periodic Trends • Atomic Mass: increases down a group (column)‏ • each time you go down one box in a group, the number of protons increases • as you add protons to an atom, the mass goes up • you also need to add neutrons to keep the nucleus stable, increasing the mass even more • Atomic Mass: increases as you go across a row • each time you go across one box in a row, the number of protons increases • as you add protons to an atom, the mass goes up • you also need to add neutrons to keep the nucleus stable, increasing the mass even more

6. Periodic Trends • Atomic Radius: increases as you go down a group • as you go down through each box in a group, you add a new energy level and more shielding • each new energy level wraps around all the previous energy levels • each new energy level increase the size of the atom • due to the shielding, the protons do not have an increased pull on the outer electrons • as the atom grows, so does the radius of the atom

7. Periodic Trends • Atomic Radius: decreases across each period • as you travel across a row, the maximum number of energy levels does NOT change, nor does the shielding • as you travel across a row, the number of protons DOES increase • as the positive charge increases with the same shielding, it attracts the e–s more strongly • with increased pull from the nucleus, all energy levels are pulled in closer to the nucleus • with the energy levels closer to the nucleus, the atomic radius shrinks

8. Periodic Trends • Atomic Radius comments: • even though the number of protons increases as you go down a group the atomic radius also increases • the atomic radius increases in this case because the e−s in the new energy level have to be significantly away from the electrons in the last energy level • across a row, the shrinking caused by the increase in protons is substantially smaller then adding a whole new energy level

9. Periodic Trends • Ionization Energy: the amount of energy needed to remove an e– from an atom • even though ions can be made by adding or subtracting e–s to(from) an atom, we are only considering removal of e–s here • as you travel across any period, the maximum energy level does not change (see last slide) • the attraction between the opposite charges in the atom changes based on the distance between the nucleus and the electrons • Big distance changes result in even larger energy changes: going out one energy level results in a big drop in energy holding electrons in place.

10. Periodic Trends • Ionization Energy: decreases as you go down a group • as you add energy levels, the distance between the nucleus and the outermost electrons increases • this increase in distance decreases the ability of the nucleus to hold the electrons in the atom • that makes it easier to remove the electrons, decreasing the amount of energy needed to remove an electron from the valence shell • that means the ionization energy drops

11. Periodic Trends • Ionization Energy: increases as you go across a period • remember, as you go across a row, the atomic radius shrinks AND the number of protons increases • as the electrons get closer to the nucleus with an increased positive charge, the attraction between the electrons and the nucleus increases • this requires more energy to remove any electron • that means the ionization energy increases, as a general rule…

12. Periodic Trends • Going from the s orbital to the p orbital, the ionization energy drops, slightly, due to the slightly higher energy of the p orbitals (they are a little further from the nucleus) • The ionization energy drops very slightly after the fourth electron is added to the p orbital: the first pair of electrons in one orbital path • The e−- e− repulsion increases b/c there are now 2 e−s in the orbital. The extra pushing means the electrons are trying to get away from each other and less energy is required to remove the electron from the atom

13. Periodic Trends • Reactivity – the ability of an element to form ionic compounds • For bonding purposes, metals lose electrons when bonding to form compounds • The more easily a metal will lose an electron, the more metallic it is (the more reactive it is) • Reactivity is a measure of how easily an element will form a compound • from all the other trends it should be clear that Fr is the most reactive of all the metals: • it is the largest element; it has the lowest ionization energy • so, it will lose a valence electron most easily • meaning it will form ionic bonds easily • this makes it the most reactive metal • Next slide…

14. Periodic Trends • Reactivity (Continued) • As a trend, the further you get from the steps on the periodic table on the metals’ side, the more reactive the element is • This should make sense, b/c the steps are the metal/non-metal divider • Non-metals next…

15. Periodic Trends • Reactivity (Continued) • For NONMETALS, there is also a definition for reactivity • The further you get from the step, the reactivity also increases • This is because the atoms there can more easily add an electron because of the small radii and high ENC • making them more reactive…

16. Periodic Trends • Ionic radius – the radius of an ion as compared to its neutral atomic radius • As you make positive ions, the size tends to decrease • The number of protons has not changed • With fewer electrons, there is less repulsion pushing the electrons apart • With the same positive charge but lower repulsive forces, the protons are able to draw the electrons closer • This shrinks the atomic radius • This is all based on the balance between repulsion between the e−s and the attraction between the e−s and the nucleus

17. Periodic Trends • Ionic radius (continued) • As you make negative ions, the size tends to increase • The number of protons has not changed • With more electrons, there is greater repulsion pushing the electrons apart • With the same positive charge but higher repulsive forces, the electrons push away from each other more • This increases the atomic radius • This is again based on the balance between repulsion between the e−s and the attraction between the e−s and the nucleus

18. Properties of the Metals • Metals tend to be more malleable (able to be pounded or rolled out in to thin sheets) • Metals tend to be shiny • Metals tend to be more lustrous (have a shimmer) • Metals tend to have a heavier feel (have higher density) for the same volume of material • There are always exceptions to trends: • some metals are dull, feel light, and/or crumble when you try to make them into thin sheets, but this is not the majority of metals • Remember the majority of the metals behave as stated above

19. Properties of Non-Metals • Non-metals tend to have a duller appearance when solid • Non-metals tend to crumble when pounded or rolled out • Non-metals tend to have a lighter feel (lower density) when compared to the same volume of a metal • They are better electrical and thermal insulators • They have more variety in their state, as a group