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Periodic Law

Periodic Law. States: when elements are arranged by increasing atomic number, there is a periodic repetition of their physical and chemical properties. Chemical Bond! Why it is formed. Forces that hold the atoms together in a compound or molecule are called chemical bonds.

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Periodic Law

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  1. Periodic Law • States: when elements are arranged by increasing atomic number, there is a periodic repetition of their physical and chemical properties

  2. Chemical Bond! Why it is formed • Forces that hold the atoms together in a compound or molecule are called chemical bonds. • Atoms form bond whenever there is a net decrease in their energy and an increased attraction.

  3. Electronic Configuration (Arrangement of electrons) Alkali Metals

  4. Alkaline Earth Metals

  5. Halogens

  6. Noble Gases

  7. 6.3 Trends in Atomic Size • The atomic radius is one half of the distance between the nuclei of two atoms of the same element when the atoms are joined.

  8. Atomic radius is ½ the distance between the nuclei of 2 like atoms As you move DOWN a group, atomic size increases As you move LEFT to RIGHT across a period size decreases Trends in Atomic Size

  9. WHY? As you move across the period effective nuclear charge (Zeff) increases. Atom gain electrons and protons. More protons hold electrons tighter. Size Decreases As you move down the group successive shells/orbitals are occupied. atom gain more electrons which are in far orbitals away from the nucleus. The nucleus has less influence and the inner shell electrons shield outer electrons from the attraction of nucleus and the net Zeff decreases. Size Increases

  10. 6.3 Ions & Ionic Radii (IR) • Metals elements lose valence electrons to form cations. Cation radii (IR) are always smaller than atomic radii (AR) [i.e. IR < AR] • The electron/proton attraction has gone UP and so size DECREASES

  11. 6.3 Ions & Ionic Radii (IR) • Non-metal elements gain valence electrons to form anion ions. Anion radii (IR) are always larger than atomic radii (AR) [i.e. IR > AR] • The electron/proton attraction has gone DOWN and so size INCREASES.

  12. 6.3 Trends in Ionic Size • Relative Sizes of Some Atoms and Ions

  13. 6.3 Trends in Ionic Size • Trends in Ionic Size Size generally increases

  14. Ionization Energy is the energy required to remove outermost electron from an isolated gaseous atom. Energy is usually absorbeded (+ve sign) As you move DOWN a group ionization energy DECREASES As you move LEFT to RIGHT across a period ionization energy INCREASES Ionization Energy (Ei) & its Trends

  15. 6.3 Trends in Ionization Energy

  16. Ionization energy vs. atomic number

  17. 11p+ 12n° 11p+ 12n° 10p+ 10n° Why? Na has 11 electrons Na+ has 10 electrons Ne has 10 electrons electron configuration of Na+ resembles Ne Alkali metals become like noble gases. OCTET is completed.

  18. Higher Ionization Energies • Ionization is not limited to the removal of a single electron from an atom. Two, three, or even more electrons can be removed sequentially from an atom. Mg (g) + 735 kJ ---> Mg+ (g) + e- Mg+ (g) + 1451 kJ ---> Mg2+ (g) + e- • successively larger amounts of energy are required for each successive ionization step because it is much harder to remove a egatively charged electron from a positively charged ion than from a neutral atom. Mg2+ (g) + 7733 kJ ---> Mg3+ (g) + e- Energy cost is very high to dip into a lower shell

  19. Electron Affinity (Eea) & its Trends An element’s electron affinity is the energy change that occurs when an electron is added to an isolated atom in the gaseous state. Energy is usually evolved (-ve sign) The more negative the Eea, the greater the tendency of the atom to accept an electron and the more stable the anion that results. Cl(g) + e- ---> Cl-(g) ∆E= -384.6 kJ/mol

  20. As you move DOWN a group electron affinity DECREASES As you move LEFT TO RIGHT across a period electron affinity INCREASES An atom that forms an unstable anion by addition of an electron has, in principle, a positive value of but no experimental measurement can be made in such circumstances. All we can say is that the for such an atom is greater than zero. Trends in Electron Affinity (Eea)

  21. Summary of Periodic Trends

  22. - - - - - - - - + + + + + + + Lattice Energy For an ionic compound the lattice enthalpy is the enthalpy change when one mole of solid in its standard state is formed from its ions in the gaseous state. For an ionic compound the lattice enthalpy is the enthalpy change when one mole of solid in its standard state is formed from its ions in the gaseous state. The lattice enthalpy cannot be measured directly and so we make use of other known enthalpies and link them together with an enthalpy cycle. This enthalpy cycle is the Born-Haber cycle.

  23. - - - - - + + + + L H = -786kJ/mol kJmol-1 Born-Haber Cycle for Sodium Chloride Na+(g) + Cl(g) -348.6 kJ/mol 495.8 kJ/mol Na+(g) + Cl-(g) Na(g) + Cl(g) Na(g) + Cl(g) Na(g) + Cl(g) Lattice Energy for Sodium Chloride 107.3 kJ/mol 122 kJ/mol Na(g) + 1/2 Cl2(g) Na(g) + 1/2 Cl2(g) Na(g) + 1/2 Cl2(g) 107.3 kJ/mol Na(s) + 1/2 Cl2(g) Na(s) + 1/2 Cl2(g) Na(s) + 1/2 Cl2(g) 0 0 0 NaCl(s)

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