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Explore the predictable changes in properties such as atomic radius and ionization energy across the periodic table. Understand how the size of atoms varies and how the charge affects these properties. Learn about the concept of effective nuclear charge and its impact on atomic size. Additionally, discover the formation of ions and their relative size to the parent atom, as well as the concept of oxidation number in redox reactions.
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Trends • More than 20 properties change in a predictable way based on location of element in P.T. • Include: density, melting point, atomic radius, ionization energy, electronegativity
Atomic Radius • Atomic radius: defined as half the distance between neighboring nuclei in a molecule or crystal. • “size” varies a bit from substance to substance.
Use X-ray diffraction to pinpoint the nuclei and measure the distance between them. Cannot measure the electron cloud.
Trends: Atoms get larger as you go down a column – more principal energy levels Atoms get smaller as you move across a series – more “proton pulling power.”
Going down column 1: More principal energy levels as you go down, so it makes sense that the atoms get larger.
previous | index | next Source: Conceptual Chemistry by John Suchocki Li: Group 1 Period 2. Cs: Group 1 Period 6.
But why do the atoms get smaller as you go across the periodic table? • You are still adding electrons!
Going across row 2: Atoms actually get a bit smaller as you go across a row. What’s going on?
What do you remember about charge? • Opposites attract and like charges repel. • The valence electrons are pulled into the atom by the positive charge on the nucleus. • The greater the positive charge, the more pulling power, right?
Well, almost . . . • Except for H and He, the valence electrons don’t feel the full effect of all the protons in the nucleus. • For most atoms, the inner shell electrons “shield” the valence electrons from the nucleus.
Effective nuclear charge • = Atomic Number - # of inner shell electrons.
Effective nuclear charge • Charge actually felt by valence electrons. • = Atomic Number - # of inner shell electrons. • Not usually the same as the nuclear charge or number of protons in the nucleus. • Charge felt by valence electrons is attenuated or shielded by inner shell electrons.
previous | index | next H and He are the only elements whose valence electrons feel the full nuclear charge. Source: Conceptual Chemistry by John Suchocki
previous | index | next Effective nuclear charge = +1. Source: Conceptual Chemistry by John Suchocki The valence electron in Li feels an effective nuclear charge of 3 – 2 = +1.
previous | index | next Source: Conceptual Chemistry by John Suchocki Calculating “effective nuclear charge.”
previous | index | next So as you go across a row the size tends to decrease a bit because of greater “proton pulling power.” Source: Conceptual Chemistry by John Suchocki
previous | index | next Size as you go and size as you go Source: Conceptual Chemistry by John Suchocki
IONS • An ion is a charged particle formed by the gain or loss of electrons. • CATION has a positive charge • ANION has a negative charge
Ionic Size Relative to Parent Atom • Depends on if it’s a positive ion or a negative ion • How do you make a positive ion? • How do you make a negative ion? Remove electrons Add electrons
Oxidation Number • Oxidation Number (State) – number assigned to keep track of electron gain or loss in redox reactions. • Positive number means electrons LOST • Negative number means electrons GAINED
