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Periodic Trends

Periodic Trends. The periodic law states that when the elements are arranged by atomic number, their physical and chemical properties vary periodically. We will look at three periodic properties: Atomic radius Ionization energy Electron affinity. 200 pm. C l. C l. 100 pm.

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Periodic Trends

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  1. Periodic Trends The periodic law states that when the elements are arranged by atomic number, their physical and chemical properties vary periodically. • We will look at three periodic properties: • Atomic radius • Ionization energy • Electron affinity

  2. 200 pm Cl Cl 100 pm Atomic Radius • Estimate of atomic size • ½(homonuclear bond length) • Cl = 100 pm (Cl2 bond = 200 pm)

  3. Atomic radius • Two factors determine the size of an atom: • n - the principal quantum number. • The larger “n”, the larger the size of the orbital. • Zeff - the effective nuclear charge. • The larger Zeff, the smaller the size of the orbital. Zeff = Z – σ • σ = screening (~core electrons)

  4. Effective Nuclear Charge screening ACROSS A PERIOD: Add an electron….in the same shell (inc.Z, but not σ). The e- cloud contracts from increased (effective) nuclear charge DOWN A GROUP: An electron adds….to a new outer shell (increase n). Shielding increases with Z, so the Zeff is constant

  5. Atomic Radius decrease Zeff increase (Z inc,  constant) increase Zeff constant Z and  increase

  6. Atomic Radius Periodicity Trend in Atomic Radius group period BIG increase from noble gas to next row element. Transition metals – little change

  7. Periodic Trends: Atomic Radii Atoms grow down a group. • Larger shell (larger n) added in each new row. Atoms shrink across a period • e- add to the same shell and p+ add to the nucleus. Big Jump from noble gas to alkali metal • A new shell (with larger n) is added.

  8. Ionic radii – Main Group Elements The periodic group trend in ionic radii parallel the trends in atomic radii. Within a period, we need to distinguish between cations and anions.

  9. Ionic radii – Main Group Elements Anions are always larger than atoms from which they are derived. More e-/e- repulsion (more e-) so the shell swells. • In general, the size of anions • decrease going across a period.

  10. Ionic radii – Main Group Elements Cations are always smaller than the atoms from which they are derived. The outer shell (main block) is completely removed and e-/e- repulsions reduced (fewer e- ). • In general, the size of the cation • decreases going across a period.

  11. Increasing nuclear charge decreasing size Periodic Trends: Ionic Radii The following anions and cations are isoelectronic with Ne. (The electron configuration is the same.) Isoelectronic Ions O2-F-Na+Mg2+ Ionic radius (pm) 140 133 102 66 Number of protons 8 9 11 12 Number of electrons 10 10 10 10 Practice

  12. Ionization Energy • The first ionization energy is the energy required to remove an electron from the gaseous neutral atom. IE1 = 738 kJ/mol

  13. Ionization Energy – Periodic Trends Down a group: IE ↓. Larger atom = less tightly held e-. Across a period: IE ↑. Smaller atom = more tightly held e-.

  14. Ionization Energy – Periodic Trends

  15. Ionization Energy Why is the ionization energy of beryllium higher than the ionization energy of boron? Be 1s22s2 1s 2s B 1s22s22p1 1s 2s 2px 2py 2pz Be 2s subshell is more stable than the partially filled 2p shell of B, therefore the outer electron is lower energy or…it takes more energy to remove the outer electron.

  16. Ionization Energy Why is the ionization energy of nitrogen higher than the ionization energy of oxygen? Practice N 1s22s22p3 1s 2s 2px 2py 2pz O 1s22s22p4 1s 2s 2px 2py 2pz N is more stable (half-filled p-shell), therefore the outer electron is lower energy or…it takes more energy to remove (n=∞) the outer electron.

  17. Ionization Energy • The first ionization energy is the energy required to remove an electron from the gaseous neutral atom. IE1 = 738 kJ/mol

  18. Ionization Energy Second Ionization energy (IE2) E required to remove a 2nd e- (from A+(g)). Mg+(g) Mg2+(g) + e- Third Ionization energy (IE3) E required to remove a 3rd e- (from A+2(g)). Mg+2(g) Mg3+(g) + e- IE2 = 1450 kJ/mol IE3 = 7734kJ/mol Mg+2 is a noble gas configuration.

  19. Periodic Trends: Ionization Energies • IE3>IE2 > IE1 • Mg+ holds the 2nd e- more tightly. • Mg+2 holds the 3rd e- even more tightly. • Huge increase if e- removal breaks a complete shell (the core). [Ne]

  20. Ionization Energy Core Electrons A big jump in IE occurs when you try to remove more than the valence electrons. (i.e. from a core or noble gas configuration)

  21. Electron Affinity • The electron affinity is the energy change for the process of adding an electron to a neutral atom in the gaseous state to form a negative ion. It is the measure of attraction an atom has for additional electron. Electron Affinity = -349 kJ/mol

  22. Periodic Trends: Electron Affinities F(g) + e- F-(g)ΔE = -328 kJ/mol • Usually exothermic (EA is negative) • Metals have low EA; nonmetals have high EA. • Some tables list positive numbers. (a sign-convention choice)

  23. Electron Affinity increase decrease

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