Periodic_Trends_Presentation student notes.notebook 1 October 05, 2012 Periodic Trends: Atomic Radius Ionization Energy Electronegativity Metallic Character Ionic Radius Periodic Trends Five main trends in the periodic table will be discussed: The sizes of atoms Ionization energy Electronegativity Metallic character The sizes of ions These five periodic trends are affected by these main factors: Effective nuclear charge Coulomb's Law Shielding from inner electrons Effective Nuclear Charge • In a manyelectron atom, electrons are both attracted to the nucleus and repelled by other electrons. • The nuclear charge that an electron experiences depends on both factors. For example, here's sodium. The effective nuclear charge, Zeff, is found this way: Zeff = Z − S Where Z is the atomic number and S is a screening constant, usually about equal to the number of inner electrons. In this example, the outer electron of sodium is attracted towards the nucleus by an effective charge (Zeff) of 1 proton. Effective Nuclear Charge Moving across the periodic table increases Zeff, and therefore increasing the force of attraction between the electrons and the nucleus. Effective Nuclear Charge • For any given atom, the electrons closest to the nucleus experience the greatest effective nuclear charge (they have the least shielding). • The electrons farthest from the nucleus experience the least effective nuclear charge (they are shielded the most). As you move across the periodic table (from left to right), the effective nuclear charge felt by the outermost electron increases. Atomic Radius • In general, as you move from left to right across the periodic table the atoms have a smaller radius • As you move down from row to row, the radius increases Helium has the smallest radius Francium has the largest radius
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The outer most electrons in an atom are called valence electrons.
Valence Electrons
Number of valence electrons in neutral atoms:1 2 3 4 5 6 7 8
1 4
7 Which ion below has a noble gas electron configuration?
A Li 2+
B Be 2+
C B 2+
D C 2+
E N 2
Ionization Energy
• The ionization energy is the amount of energy required to remove an electron from the ground state of a gaseous atom or ion.
• The first ionization energy is that energy required to remove first electron.
Ca Ca+ + e
• The second ionization energy is that energy required to remove second electron, etc.
Ca+ Ca2+ + e
Trends in First Ionization Energies
• As one goes down a column, less energy is required to remove the first electron.
• For atoms in the same group, Zeff is essentially the same, but the valence electrons are farther from the nucleus, "r" increases, so it is easier to remove the outermost electron
Trends in First Ionization Energies
• Generally, as one goes across a row, it gets harder to remove an electron.
• As you go from left to right, Zeff increases, making it harder to remove an electron
Trends in First Ionization Energies
However, there are two apparent discontinuities in this trend.
• In this case the electron is removed from a p1 orbital rather than an s orbital.
• The electron removed is farther from nucleus, there is a small amount of repulsion by the s electrons, and the atom gains stability by having a full s subshell.
Trends in First Ionization Energies
• The second is between Groups 15 and 16.
• The electron removed comes from doubly occupied p orbital.
• Repulsion from the other electron in the orbital aids in its removal.
• The atom gains stability by having a half full p orbital
Summary of Trends in Ionization Energy
• By adding one proton and one electron you are increasing q1 and q2 in Coulomb's Law
• Therefore, the force of attraction between the nucleus and outermost electrons is strengthened
• Thus, it takes more energy to remove the outermost electron
Ionization Energy increases left to right across a period • The size of orbitals increases significantly
• The distance between the nucleus and outer electrons increases
• So the force of attraction between the nucleus and outer electron is less
• Also, there is more shielding by inner electrons as you go down a group
• And more electronelectron repulsion with increasing number of electrons
Summary of Trends in Ionization Energy
Ionization Energy decreases going down a group
Ionization Energy
• It requires more energy to remove each successive electron.
ie: second ionization energy is greater than first, third ionization energy is greater than second, etc.
• When all valence electrons have been removed, the ionization energy takes a huge jump.
8 Which noble gas has the lowest first ionization energy? Give the atomic number.