1.7 TRENDS IN THE PERIODIC TABLE

1. 1.7 TRENDS IN THE PERIODIC TABLE

2. Important Definitions • Trend: predictable change in a particular direction • Electron Shielding : inner electrons shield outer electrons from the full attractive force of the nucleus • Effective Nuclear Charge (ENC): charge felt by the valence electrons after you have taken into account the # of shielding electrons

3. Atomic Radius • Atomic radius: half of the distance between the nuclei of two atoms of the same element bonded together.

4. Atomic Radius • increases as you move down a group • electrons occupy consecutively higher energy levels, farther from the nucleus • ENC decreases due to increased shielding H Li Na K Rb

5. Atomic Radius • decreases as you move across a period • Electrons are in the same energy level • there is more nuclear charge. (protons are added as you move left to right) • Outermost electrons are pulled closer. Na Mg Al Si P S Cl Ar

6. Atomic Radius Trends

7. Radius of Cations • Metals form cations. • Cations form by losing electrons. • Cations are smaller than the atom they came from – not only do they lose electrons, they lose an entire energy level. (ENC increases, shielding decreases)

8. Radius of Anions • Nonmetals form anions. • Anions form by gaining electrons. • Anions are bigger than the atom they came from – have the same energy level, and nuclear charge (more repulsion between electrons)

9. Ionic Radius Trends • increases as you move down a group • due to decreased ENC (increased shielding) • decreases as you move across a period • due to increasing nuclear charge, whether the ion is positive or negative

10. Ionic Radii Trends

11. Ionization Energy • Ionization energy: energy required to remove an electron from an atom or ion • Must overcome attraction between electron and nucleus • 2nd Ionization Energy: The energy required to remove a second electron from a cation

12. Ionization Energy • Group 1: easily loses its 1 valence electron • Low first ionization energy • Second ionization energy will be very high since it is “happy” with losing 1 electron. • Group 2: easily lose 2 valence electrons • Low first and second ionization energies • High third ionization energy

13. I.E. for the 1st 20 elements Symbol First Second Third 11810 14840 3569 4619 4577 5301 6045 6276 5247 7297 1757 2430 2352 2857 3391 3375 3963 1312 2731 520 900 800 1086 1402 1314 1681 2080 HHeLiBeBCNO F Ne Why did these values increase so much?

14. Ionization

15. Ionization Energy Trends • decreases as you move down a group • due to electron shielding (decreasing ENC) • increases as you move across a period • due to the increase in nuclear charge (less shielding)

16. Ionization Energy Trends

17. Electron Affinity • Electron Affinity: the energy given off when an electron is added to an atom to make an anion. • elements with high electron affinities form negative ions in ionic compounds. • elements with low electron affinities form positive ions in ionic compounds.

18. Electron Affinity Trends • decreases as you move down a group due to increased electron shielding (decrease in ENC) • increases as you move across a period due to increasing nuclear charge, less sheiding (easy to gain e-)

19. Electron Affinity Trends

20. Electronegativity • Electronegativity: measure of the ability of an atom in a compound to attract electrons • higher electronegativity means a stronger pull on electrons • Ranges from 0 to 4

21. Electronegativity Trends • decreases as you move down a group • due to decrease in ENC, lots of shielding • increases as you move across a period • increasing atomic number, so increase in ENC, less shielding • Metals have a low electronegativity; non-metals have a high electronegativity

22. Electronegativity Trends