Electron Configuration Aufbau Hund and Pauli Exclusion Rules Khan Academy

Atomic Structure & Electron Configuration

In the first unit of AP Chemistry, we often learn about the periodic table, the atom, and the patterns in the periodic table. In this unit, we learn about how the number of protons and electrons in an atom will affect its mass, its charge, its size, and its interactions with other atoms.

Note: All information in this unit can be found on Khan Academy's AP Chemistry unit. This is simply a compilation of the unit into written summaries which can be used as an additional guidance for notes. For more information, please refer to their website .

Periodic Table & Isotopes

The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from the innermost shell outwards.

A given element's reactivity is highly dependent upon its electronic configuration

Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells. This makes them more important in determining how the atom reacts chemically and behaves as a conductor, because the pull of the atom's nucleus upon them is weaker and more easily broken.

In the periodic table, we learn about alkali metals, alkaline earth metals, transition metals, halogens, and noble gases. Each column is a group, and each row is a period. Each group tells you something about an element's chemical characteristics while each period lets you know which valence shell you are looking at, whether that be 1n, 2n, 3n etc. Each valence shell actually cannot harbor all 2 or 8 electrons at a time, so they are broken down into their individual subshells, where up to two electrons can harbor.

Coulomb's Law

E = Energy

Q_1 and Q_2 are the magnitudes of positive and negative charges

R is the distance between electrons

Usually, we can use the fact that the Coulombic attraction can be estimated by the effective nuclear charge (number of protons minus the number of core electrons) over the square of the radius.

The number of core electrons provide the shielding effect that repulses the valence electrons, causing a larger radius.

Each period defines common elements that have the same electron shells.

Each electron shell, however, consists of different subshells, from s, p, d, to f subshells, each containing 2 electrons each.

Each subshell, in order to contain the total number of electrons in the electron shell, contains their own orbitals, from 1, 3, 5, 7 respectively.

Example:

The ground-state electron configuration for an element contains three unpaired 4p electrons.

There are 4 Quantum Numbers that describe an atom:

1. Principal quantum number

   1. The principal quantum number (n) tells us what period the atom is in, how many electron shells it has

2. Orbital Angular Momentum Quantum Number

   1. The orbital angular momentum (l) is a number from 0 to n - 1. It tells us about the shape of the electron's orbit.

3. Magnetic Quantum Number

   1. The magnetic quantum number (m_1) tells us about the magnetic field, a number from -l to l.

4. Electron Spin Quantum Number (ms)

   1. The electron spin quantum number (m_2) is either –½ or +½

Moles and Solution Composition

Avogadro's number (6.02 x 10^22) describes the number of atoms that are in a single mole of any given element, whose mass is the element's molar mass in grams.

The average atomic mass (molar mass) of an element is the average mass of an atom of that element. For example, carbon's average atomic mass is 12.01 amus (atomic mass units).

In order to get exactly 12.01 grams of carbon, we need exactly 6.02214076 x 10^22 atoms of carbon.

An isotope is an atom that has a different number of neutrons than other atoms of the same element.

The identity of an atom is determined by the number of protons contained in its nucleus.

The molar mass of an atom is determined by the number of protons and neutrons in the nucleus.

The charge of an atom is determined by the number of protons and electrons in the atom.

Know the difference between the "molar" formula versus the "empirical" formula. The molar formula is the exact numbers of atoms in a molecule while the empirical formula tells you the simplified version.

Molarity is the concentration of a solution in terms of the total volume. It tells you how many moles of the solute are in a liter of the solution, the percentage of the element in the mixture.

When we see a chemical symbol in brackets, this means we are talking about molarity. For example, when we see "[C]" we are talking about the molar concentration of sodium ions.

In the periodic table, the radius of an atom decreases as we move along the period, since there are more protons inside the nucleus with the same shell; meaning, there is a greater pull toward the nucleus and smaller distance between the valence electrons and the nucleus. This means it is harder to pull an electron away from the atom.

Radius of Atom and Periodic Trends

The radius of an atom increases as we move down the groups because this means we are increasing n or the number of shells, which increases the distance from the valence electrons to the nucleus.

Thus, the ionization energy depends on the effective charge and radius. The effective charge is usually the same among elements of the same group, so they cancel out. Ionization energy's deciding factor is usually the radius, and the maximum radius is on the bottom left (francium) while minimum is top right (helium).

Atomic radii and ionic radii measure the distance from the valence electrons to the nucleus. In a cation, where an atom has lost an electron, the ionic radius decreases because there no longer is an electron in the same shell that repels the others remaining, so the electrons are pulled closer to the nucleus. In an anion, where an atom has gained an electron, the ionic radius increases because there is an extra electron that pulls the current electrons away from the nucleus.

Quantum Forces in Electron Shells

Based on Aufbau's principle, we write electron configurations based off the ground-state configuration, which states that we must fill the lowest energy level possible.Hund's rule states that when an electron is added to a subshell, it will always occupy an empty orbital if one is available; only when there are no empty ones available will the electron begin to pair up. The Pauli Exclusion Principle states that the two electrons which share an orbital cannot have the same spin; one must spin counterclockwise and the other spins clockwise.

Summary

IMPORTANT!

The first ionization energy increases as we go to the right because there are more electrons we need to pluck off to make it want to become an ion.

A larger first ionization energy means that it is HARDER to pluck electrons and make it a cation.

The higher the frequency, the more the energy. The longer the wavelength, the less energy.

Nonmetals are to the right of the periodic table. They have higher ionization energy because they are closer to filling a full octet.

Transition metals always lose their electrons in the s subshell first.

The first ionization energy decreases as we go down because there is a greater distance from the nucleus to the valence shell, so it requires less energy to pluck off an electron.

The atomic radius decreases and electronegativity increases as we go to the right because we have more protons and the same number of core electrons.

The atomic radius increases and electronegativity decreases as we go down because we have more core electrons providing our shielding/screening effect and greater radius (meaning electronegativity lessens).

The electronegativity of an element generally increases as we go right, and decreases as we go down the periodic table.

Electron Configuration Aufbau Hund and Pauli Exclusion Rules Khan Academy

Source: https://teensinhealth.org/science-now/2021/7/4/khan-academy-ap-chemistry-atomic-structure-amp-electron-configuration-unit-1

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