Atomic Structure and the Periodic Table Notes
Atomic Structure and the Periodic Table
Introduction
In the 17th through 19th centuries, scientists realized Earth is composed of many elements with different properties.
The structure of matter is more complex than the ancient model of earth, air, fire, and water.
Chemists identified more elements and learned about their properties, noticing patterns.
The Periodic Table
Elements are arranged according to how electrons arrange themselves around the nuclei of atoms.
Electron arrangement determines an element's chemical behavior.
The quantum mechanical model explains the periodicity of chemical properties.
Understanding electron arrangement helps engineers develop new technologies.
Example: Lithium is a reactive element in Group 1.
Its reactivity is due to electron arrangement and location relative to the nucleus.
Lithium reacts with water.
Lithium is used in batteries for various devices.
Multi-electronic Atoms
The Bohr-Rutherford model has limited explanation of electron configuration.
Useful for the first 20 elements (up to calcium).
The quantum mechanical atomic model describes all atoms and allows theoretical predictions.
Consider helium, with 2 protons and 2 electrons in the 1s orbital, to see how the quantum mechanical model applies to multi-electronic atoms.
Three energy contributions for helium:
Kinetic energy of electrons.
Potential energy of attraction between the nucleus and electrons.
Potential energy of repulsion between the 2 electrons.
The Schrödinger wave equation cannot be solved exactly for helium due to electron repulsions.
This is called the electron correlation problem which occurs with all multi-electronic atoms.
Approximation: treat each electron as moving in a field of charge that is the net nuclear attraction and average repulsions of other electrons.
Consider sodium with 11 electrons: 10 in the first and second shells, 1 in the third shell.
The outermost electron is attracted to the nucleus but repelled by the 10 inner electrons.
The outer electron is shielded from the nuclear charge by the repulsions of other electrons.
Orbitals of multi-electronic atoms have the same general shapes as hydrogen orbitals, but different sizes and energy values due to the interplay between nuclear attraction and electron repulsions.
For hydrogen, all orbitals of a given principal quantum level (shell) have the same energy and in multi-electronic atoms for a given principal quantum level, n, the energies of electrons in the different orbitals vary as follows:
E{ns} < E{np} < E{nd} < E{nf}Electrons fill orbitals in order of increasing energy: s, p, d, f.
In multi-electronic atoms, for a given principal quantum level, n, the energies of electrons in the different orbitals vary as follows:
E{ns} < E{np} < E{nd} < E{nf}
Penetration
An electron in a 2p orbital has its maximum probability closer to the nucleus than a 2s orbital electron.
There is a small increase in electron density in the 2s orbital very near the nucleus.
The 2s electron is closer to the nucleus for a very short time than the 2p electron.
This effect, called "penetration," causes a 2s electron to be attracted to the nucleus more strongly than a 2p electron.
Electrons in the 2s orbital have less energy than electrons in the 2p orbital of a multi-electronic atom.
The relative energies of electrons in n = 3 orbitals are: E{3s} < E{3p} < E_{3d}.
The more effectively electrons of an orbital penetrate the shielding electrons, the lower the energy of the electrons in that orbital.
The Aufbau Principle and Writing Electron Configurations
Apply the quantum mechanical model to show how electron arrangements in the ground state account for the organization of the periodic table.
Electron configuration: the location and number of electrons in the electron energy levels of an atom or ion.
Atoms are arranged in order of increasing atomic (proton) number from left to right across the periodic table
In a neutral atom, the atomic number equals the number of electrons so, as one moves across the periodic table from left to right, the electron number of the elements also increases.
The electron configuration of an atom can be determined using the aufbau principle.
Aufbau principle: an atom is "built up" by progressively adding electrons.
Electrons assume their most stable condition (lowest energy orbital) by filling the lowest available energy orbitals before filling higher energy orbitals.
Aufbau is German for "building up".
Examples of Electron Configurations
Hydrogen (1 electron): 1s¹
Helium (2 electrons): 1s²
Lithium (3 electrons): 1s²2s¹
Beryllium (4 electrons): 1s²2s²
Boron (5 electrons): 1s²2s²2p¹
All the electrons in the 2p orbitals have the same energy, so it does not matter which 2p orbital the fifth electron occupies. By convention, we write it in the left-hand orbital.
Hund's Rule
Hund's rule: the lowest energy configuration for an atom is achieved when electrons occupy separate orbitals with parallel spins, within a particular set of orbitals of the same energy.
Before any 2 electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain 1 electron.
Electrons filling a subshell will have parallel spin before the shell starts filling up with electrons having the after the first orbital gains a second electron
For carbon (6 electrons): 1s²2s²2p² (electrons occupy separate 2p orbitals).
Energy of the 4s orbital is lower than the energy of the 3d orbitals, so the 4s orbital must be filled before the 3d orbitals.
Number of orbitals in each subshell: s = 1, p = 3, d = 5, f = 7.
Procedure for Writing Electron Configurations
Determine the number of electrons in the atom or ion.
Assign electrons by main energy level and then by sublevel, using an energy-level diagram or an aufbau diagram.
Distribute electrons into orbitals that have the same energy according to Hund's rule.
Fill each sublevel before starting with the next sublevel. Continue until all electrons are assigned.
For anions (negatively charged ions), add an appropriate number of additional electrons.
For cations (positively charged ions), remove an appropriate number of electrons.
Nitrogen (7 electrons): 1s²2s²2p³ (3 electrons in the 2p orbitals occupy separate orbitals with parallel spins).