Electrons Main Concept: The atom is composed of negatively charged electrons, which can leave the atom, and a positively charged nucleus that is made of protons and neutrons. The attraction of the electrons to the nucleus is the basis of the structure of the atom. Coulomb’s law is qualitatively useful for understanding the structure of the atom.  

How Particles Interact Factors affecting Coulomb’s Law Electrons How Particles Interact – – + + + – Factors affecting Coulomb’s Law 1. 2. Ionization Energy Photoelectron Spectroscopy (PES) What it shows and how it shows it

Coulomb’s law: - force between two charged particles is proportional to the magnitude of each of the two charges (q1 and q2) - inversely proportional to the square of the distance, r, between them - Potential energy is proportional to q1q2/r

Charges of opposite signs = the force is attractive Charges of opposite signs = the force is repulsive

- First ionization energy: minimum energy needed to remove the least tightly held electron from an atom or ion - Generally, ionization energy of any electron is the minimum energy needed to remove that electron from the atom or ion

- relative magnitude of ionization energy can be qualitatively estimated through Coulomb’s law - farther electron is from nucleus = lower ionization energy - comparing two species with the same arrangement of electrons  higher nuclear charge = higher ionization energy of an electron in a given subshell

Photoelectron Spectroscopy

Photoelectron Spectroscopy (PES) provides direct evidence for the shell model Light -made of photons -energy of photon E=hv -h is Planck’s constant -v is the frequency of the light Photoelectric Effect: - light ejects electrons from a material - requires the photon to have sufficient energy to eject the electrons

- Measurement of energies provides a way to deduce the shell structure of an atom - Intensity of the photoelectron signal for given energy is the relative number of electrons in that energy level

The electronic structure of atoms can be inferred from evidence provided by PES ex: both electrons in He are identical, about the same distance from the nucleus as in H vs two shells of electrons in Li, outermost electron is further from the nucleus than in H or He H Li He