Atomic Structure, Periodic Trends, and Nuclear Chemistry: Study Notes

Atomic Structure and Models

Bohr Model and Electron Configuration

The Bohr model is a historical model of the atom that describes electrons orbiting the nucleus in fixed energy levels. The electron configuration of an atom shows the arrangement of electrons in these energy levels and sublevels.

• Bohr Model: Electrons move in circular orbits around the nucleus; each orbit corresponds to a specific energy level.

• Electron Configuration: The distribution of electrons among the atom's orbitals, following the Aufbau principle, Pauli exclusion principle, and Hund's rule.

• Example: The electron configuration of carbon (atomic number 6) is 1s2 2s2 2p2.

Equation:

$E_n = -\frac{R_H}{n^2}$ Where $E_n$ is the energy of the nth level, $R_H$ is the Rydberg constant, and $n$ is the principal quantum number.

Periodic Trends

Ionization Energy and Atomic Size

Ionization energy is the energy required to remove an electron from a gaseous atom or ion. Atomic size (atomic radius) refers to the distance from the nucleus to the outermost electron shell.

• Periodic Trends in Ionization Energy: Ionization energy generally increases across a period (left to right) and decreases down a group (top to bottom).

• Periodic Trends in Atomic Size: Atomic radius decreases across a period and increases down a group.

• Example: Sodium (Na) has a lower ionization energy than chlorine (Cl).

Equation:

$\text{Ionization Energy} \propto \frac{Z_{\text{eff}}}{r^2}$ Where $Z_{\text{eff}}$ is the effective nuclear charge and $r$ is the atomic radius.

Valence Electrons and Charge

Valence Electrons and Ionic Charge

Valence electrons are the electrons in the outermost shell of an atom and determine chemical reactivity. The charge of an ion is based on the loss or gain of electrons relative to the neutral atom.

• Identifying Valence Electrons: For main group elements, the group number often equals the number of valence electrons.

• Determining Ionic Charge: Atoms lose or gain electrons to achieve a stable electron configuration, often resembling the nearest noble gas.

• Example: Oxygen (O) has 6 valence electrons and typically forms a 2- ion.

Electron Configuration and Subatomic Particles

Electron Configuration and Counting Subatomic Particles

The electron configuration provides information about the arrangement of electrons. The number of protons, neutrons, and electrons can be determined from atomic number and mass number.

• Protons: Equal to the atomic number (Z).

• Neutrons: Equal to mass number (A) minus atomic number (Z).

• Electrons: Equal to protons in a neutral atom; adjust for charge in ions.

• Example: For $^{23}_{11}$Na, protons = 11, neutrons = 12, electrons = 11 (neutral atom).

Nuclear Chemistry

Fission, Fusion, and Radioactive Decay

Nuclear chemistry involves changes in the nucleus, including fission, fusion, and various types of radioactive decay.

• Fission: Splitting of a heavy nucleus into smaller nuclei, releasing energy.

• Fusion: Combining of light nuclei to form a heavier nucleus, releasing energy.

• Radioactive Decay: Includes alpha, beta, gamma, positron emission, and electron capture.

• Example: Uranium-235 undergoes fission when bombarded with a neutron.

Equations:

$\text{Fission:}\quad ^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3^{1}_{0}\text{n}$ $\text{Fusion:}\quad ^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow ^{4}_{2}\text{He} + ^{1}_{0}\text{n}$

Vocabulary

• Ionization Energy: Energy required to remove an electron from an atom.

• Atomic Radius: Size of an atom, typically measured from the nucleus to the outermost electron.

• Valence Electrons: Electrons in the outermost shell.

• Bohr Model: Model of the atom with electrons in fixed orbits.

• Electron Configuration: Arrangement of electrons in an atom.

• Orbital: Region of space where an electron is likely to be found.

• Subshells: s, p, d, f – types of orbitals within energy levels.

• Alpha, Beta, Gamma Decay: Types of radioactive decay.

• Positron Emission: Type of beta decay where a positron is emitted.

• Electron Capture: Process where an inner electron is captured by the nucleus.

Table: Types of Radioactive Decay

Decay Type	Symbol	Change to Nucleus	Example
Alpha Decay	$^{4}_{2}\text{He}$	Loss of 2 protons and 2 neutrons	$^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$
Beta Decay	$\beta^{-}$	Neutron converts to proton	$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + \beta^{-}$
Gamma Decay	$\gamma$	No change in nucleon number	$^{60}_{27}\text{Co}^* \rightarrow ^{60}_{27}\text{Co} + \gamma$
Positron Emission	$\beta^{+}$	Proton converts to neutron	$^{22}_{11}\text{Na} \rightarrow ^{22}_{10}\text{Ne} + \beta^{+}$
Electron Capture	EC	Proton converts to neutron	$^{40}_{19}\text{K} + e^{-} \rightarrow ^{40}_{18}\text{Ar}$

Additional info: Academic context and examples have been expanded for clarity and completeness.