Periodic Properties of Elements

Periodic Trends In this chapter, we will explain observed trends in: Sizes of atoms and ions Ionization energy (losing electrons) Electronegativity (affinity or attraction for electrons)

Atomic Radius or Atomic Size The atomic radius is the distance between the nucleus and the outer edge of the electron cloud. Defined as one-half of the distance between two atoms when they collide.

Trends in Atomic Size In general, atomic size: increases from top to bottom within a group and increases from right to left across a period. This diagram lists the atomic radii of seven nonmetals. An atomic radius is half the distance between the nuclei of two atoms of the same element when the atoms are joined.

Trends in Atomic Size

Trends in Atomic Size This diagram lists the atomic radii of seven nonmetals. An atomic radius is half the distance between the nuclei of two atoms of the same element when the atoms are joined.

Atomic radius vs. atomic number 04/10/99

Ion Size Metals lose electrons: cation (positive charge) Cations are always smaller than the original atoms. When a sodium atom loses an electron, it becomes a positively charged ion. When a chlorine atom gains an electron, it becomes a negatively charged ion. Interpreting Diagrams What happens to the protons and neutrons during these changes? Na+ has valence electrons in energy level 2 more pull on valence electrons – smaller ion.

Ions Non-metals gain electrons: anion (negative charge) Anions are always larger than the original atoms. When a sodium atom loses an electron, it becomes a positively charged ion. When a chlorine atom gains an electron, it becomes a negatively charged ion. Interpreting Diagrams What happens to the protons and neutrons during these changes? More electrons, more repulsion, so electrons “spread out”: larger atom.

Trends in Ionic Size The size of ions: Cations (LOSE ELECTRONS) are always smaller than the original atoms. Anions (GAIN ELECTRONS) are always larger than the original atoms.

Trends in Ionic Size The trend generally follows the atomic size trend. Cannot compare a trend across an entire period – must use separate trends for metals and non-metals.

Ionization Energy Amount of energy required to remove an electron from an atom. First ionization energy: energy required to remove first electron. Second ionization energy: energy required to remove second electron, etc.

Trends in Ionization Energy First ionization energy: increases from bottom to top within a group, and increases from left to right across a period.

Trends in First Ionization Energies As one goes down a column, less energy is required to remove the first electron.

Trends in First Ionization Energies

Trends in Electronegativity Electronegativity is the ability of an atom to attract electrons when the atom is in a compound * * Fluorine has the highest electronegativity

Trends in Electronegativity Fluorine has the highest electronegativity. Noble gases have zero electronegativity (they already have a full valence shell)

Effective Nuclear Charge, Zeff the charge felt by the valence electrons after taking into account the affect of shielding electrons between them and the nucleus.

Effective Nuclear Charge, Zeff Zeff = (# of protons) – (# of inner shell electrons) = (atomic #) – (# non-valence electrons) Zeff = Z - S http://www.youtube.com/watch?v=hwM4mCj8Oh8

Coulomb’s Law Relates energy of attraction of the nucleus to a valence electron with the charge and distance.  

Coulomb’s Law Relates energy of attraction of the nucleus to a valence electron with the charge and distance.     Li has larger relative E, so STRONGER pull on the electrons.

Periodic Trends COMPARE TRENDS IN PERIODS –Al & Na Atomic Radius:       Atomic Radius: smaller larger Ionization Energy: higher lower Electronegativity:

Periodic Trends      

ISOELECTRIC SERIES What is the trend of atomic size for these isoelectric atoms/ions? N3- O2- F- Na+ Mg2+ Al3+ “Isoelectronic” means ions/atoms with the same number of electrons. Each has 10 electrons. Decreasing radius with increasing # of protons. (Due to stronger pull from MORE protons.)

ISOELECTRIC SERIES

Which is larger: Na+ or Na?

Correct Answer: Na+ Na Both have the same number of protons, but Na with one more electron will be larger.

Which is larger: Cl or Cl?

Correct Answer: Cl Cl Both have the same number of protons, but Cl− with one more electron will be larger.

Trends in First Ionization Energies Discontinuities: The first occurs between Groups 2 and 3. Electron removed from p-orbital rather than s-orbital e- is farther from nucleus repulsion by s electrons.

Trends in First Ionization Energies Discontinuities: The second occurs between Groups 5 and 6. e- removed from doubly occupied orbital. Repulsion from other electron in orbital helps its removal.

Ionization Energy It requires more energy to remove each successive electron. When all valence electrons have been removed, the ionization energy shows a HUGE increase.

Ionization Energy Using the data below, determine the number of valence electrons for each atom. # Valence electrons: I1 740 790 580 I2 1,200 1,500 1,800 I3 8,000 3,200 2,700 I4 10,000 4,400 11,600 I5 14,000 16,090 16,000

Atom e- Photoelectron Spectroscopy Ephoton = hv X-rays bombard the atom and electrons are released. The energy required to remove each electron is measured and reported on the Spectrum graph. Photoelectron Spectroscopy Atom Ephoton = hv Monochromatic Beam of X-Rays e-

Photoelectron Spectrum (PES) The x-axis of the PES graph shows the relative “binding energy” (very similar to ionization energy) of electrons in an atom. (Note: many (but not all) PES graphs DECREASE in value from left to right.) The y-axis shows the relative number of electrons for each sub-level.

Photoelectron Spectrum A method to identify the placement of electrons for a SINGLE atom. 3p 2p 3s 1s 2s 4s

Photoelectron Spectrum Identify the sublevel and number of electrons for each peak 1s2 2s2 Boron 2p1

Photoelectron Spectrum Identify the sublevel and number of electrons for each peak. 2p6 3p6 Calcium 1s2 3s2 2s2 4s2

Photoelectron Spectrum Why are Ca peaks higher in energy than K? Ca has more protons and higher Zeff, so stronger pull on e-

Photoelectron Spectrum TRUE OR FALSE: The PES for Ca2+ and Ar are the same. Hint: They are Isoelectric (both have 18 electrons) NO! Ca2+ has more protons, so will have a stronger pull on the electrons. So Ca2+ peaks should have HIGHER binding energy.

Photoelectron Spectrum TRUE OR FALSE: The PES for 16O and 18O are the same. Hint: How many protons and electrons do each have? YES! The only difference between the two isotopes is the number of neutrons, which have neutral charges. It won’t affect the energy of attraction, so no difference.

Photoelectron Spectrum Why is the last peak (on the right) for Scandium assigned to 3d instead of 4p?