Trends & the Periodic Table Atomic size & Effective Nuclear Charge

What the periodic table really looks like! previous | index | next                                                                                                                                                                                                                    Source: Conceptual Chemistry by John Suchocki What the periodic table really looks like!

Space-saver. previous | index | next                                                                                                                                                                                                             Source: Conceptual Chemistry by John Suchocki Space-saver.

How is the periodic table arranged? By Atomic Number!

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: Period Element Configuration 1 H 2 Li 2-1 3 Na 2-8-1 4 K 2-8-8-1 5 Rb 2-8-18-8-1 6 Cs 2-8-18-18-8-1 7 Fr 2-8-18-32-18-8-1 More principal energy levels as you go down, so it makes sense that the atoms get larger.

Li: Group 1 Period 2. Cs: Group 1 Period 6. 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!

Biggest Going across row 2: Family Element Configuration IA or 1 Li 2-1 IIA or 2 Be 2-2 IIIA or 13 B 2-3 IVA or 14 C 2-4 VA or 15 N 2-5 VIA or 16 O 2-6 VIIA or 17 F 2-7 VIIIA or 18 Ne 2-8 Biggest 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 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.

Effective nuclear charge = Atomic Number - # of 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

Effective nuclear charge = +1. 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.

Calculating “effective nuclear charge.” previous | index | next                                                                                                                                                                              Source: Conceptual Chemistry by John Suchocki Calculating “effective nuclear charge.”

Row 2 elements: Effective nuclear charge Configuration Nuc. Chg Eff. Nuc. Ch Li 2-1 3 3 – 2 = 1 Be 2-2 4 4 – 2 = 2 B 2-3 5 5 – 2 = 3 C 2-4 6 6 – 2 = 4 N 2-5 7 7 – 2 = 5 O 2-6 8 8 – 2 = 6 F 2-7 9 9 – 2 = 7 Ne 2-8 10 10 – 2 = 8

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

Size  as you go  and size  as you go  previous | index | next                                                                                                                                                                                                                    Size  as you go  and size  as you go  Source: Conceptual Chemistry by John Suchocki