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Atomic and Ionic Radii Trends: Understanding Elements' Sizes

Learn about atomic and ionic radii trends on the periodic table and why they change across periods and groups. Explore concepts of ionization energy and electronegativity with examples and explanations.

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Atomic and Ionic Radii Trends: Understanding Elements' Sizes

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  1. Atomic Radius • The radius of an atom. • Periods - decreases as you move left to right across the table • Why? Nucleus becomes more positive, while the electrons remain in the same energy level. This change causes the electrons to be pulled toward the nucleus making the radius smaller. • Groups - increase as you move down a group. • Why? More energylevels are added to the atom making it bigger.

  2. Which of the following has the largest atomic radius? • F or Br • Al or S • Na or P • Li or Rb

  3. Ionic Radius • The radius of an ion. • Ion – an atom with a negative or positive charge (gained or lost an electron) • Important: When an atom gains / loses electrons, it becomes more negative / positive and then becomes bigger / smaller. • Periods – • The left side has positively charged ions. • The right side has negatively charged ions. • Where they meet, there is a huge jump in ionic radius. • Groups - increase as you move down a group. • Why? More energylevels are added to the atom making it bigger.

  4. Another way to think of it: The more negative the bigger; the more positive the smaller

  5. Which of the following has the largest ionic radius? • Sr+2 or Rb+1 • Br-1 or F-1 • Ti+2 or Se-2

  6. For the next two trends, remember the octet rule. (All atoms want 8 valence electrons)

  7. Ionization energy • Energy required to remove an electron or how well an atom’s nucleus holds onto its valence electrons. • Periods - increases as you move left to right across the table • Why? more nuclear charge strongly hold on electrons • Groups - decrease as you move down a group. • Why? more electrons are further from the nucleus resulting in less pull. • This is also caused by electronshielding - the blocking of the positive nucleus by inner electrons.

  8. Which of the following has the highest ionization energy? • N or As • B or O • Li or Rb • Be or Sr

  9. Electronegativity • Relative ability of an atom to attract electrons in a chemical bond. • Periods - increases as you move left to right across the table • Why? more nuclear charge attract electrons • Groups – decrease as you move down a group. • Why? Electrons are further from the nucleus resulting in less attraction. • Not counting the noblegases (Group 8A), fluorine is the most electronegative while francium is the least.

  10. Which of the following has the highest electronegativity? • Na or Rb • B or F • K or Ca • F or I

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