Chapter 1 Atomic Structure and Periodic Trends

Chemistry Foundation: The study of matter is fundamental in chemistry.
Definition of Matter: Matter is any substance that occupies space and has mass. For instance, atoms have matter and interact with one another in predictable ways.
Content Overview: This chapter discusses the characterization of atoms and trends that can help predict their behavior, emphasizing the importance of a solid understanding of these concepts for future topics in chemistry.

Molecules: Atoms are the basic building blocks of molecules.
Elements: A pure substance composed of like atoms (e.g., gold, helium).
Compounds: Molecules composed of different elements (e.g., ammonia (NH3), carbon dioxide (CO2)).

Electron Shell Model: Outdated model where electrons are depicted in spherical orbits around the nucleus.
Quantum Mechanical Model: Proposed by Erwin Schrödinger, focuses on the electron cloud concept where electrons are more likely to be found in certain regions of space rather than fixed orbits.
Valence Electrons: Electrons furthest from the nucleus vital for chemical bonding.

Atomic Number (Z): Number of protons in an atom, defining its unique identity (e.g., nitrogen has Z = 7).
Mass Number (A): Total number of protons and neutrons (A = Z + N).
Isotopes: Same element with different neutron numbers; e.g., carbon-12 and carbon-13 (6 protons with 6 and 7 neutrons respectively).
Atomic Mass Units: 1 amu = 1/12 the mass of a carbon-12 atom.

Mole Definition: One mole of atoms is defined as 6.022 × 10²³ particles (Avogadro's number).
Molar Mass: The mass in grams of one mole of an atom corresponds to its mass in amu.

Neutral Atoms: Equal number of protons and electrons.
Cations and Anions:
- Cations: Positively charged ions formed by loss of electrons (e.g., Ca²⁺).
- Anions: Negatively charged ions formed by gain of electrons (e.g., F⁻).

Monoatomic Cations: Named after the original atom (e.g., Na⁺ = sodium ion).
Cations with Multiple Charges: Indicated using Roman numerals (e.g., Fe²⁺ = ferrous; Fe³⁺ = ferric).
Simple Anions: Root name + suffix -ide (e.g., Cl⁻ = chloride).
Polyatomic Ions: Composed of multiple atoms (e.g., ClO₃⁻ = chlorate).

Overview: Describes electrons orbiting the nucleus in spherical shells.
Energy Levels: Electrons closer to the nucleus are more stable and have lower energy. Excited state occurs when energy is absorbed, moving electrons to higher energy levels, while energy emission returns them to lower levels.
Photon: Energy emitted during electron transitions corresponds to electromagnetic radiation.

Equations:
- Energy of a photon: $E = hf$ where
  - $h$ is Planck's constant (approximately 6.63 × 10⁻³⁴ J.s)
  - $f$ is frequency.
- Using wavelength ($\lambda$): $E = \frac{hc}{\lambda}$
- Energy change during electron transitions is expressed via the Rydberg equation: $\frac{1}{\lambda} = R<em>H \left( \frac{1}{n^2</em>1} - \frac{1}{n^2_2} \right)$
- Where $R_H$ is the Rydberg constant.

Calculate energy of a photon for an electron transition from energy level n = 2 to n = 4.
Result: $\Delta E = 4.09 × 10^{-19} J$ .

Four quantum numbers define electron position:
- Principal Quantum Number (n): Indicates energy level.
- Azimuthal Quantum Number (l): Describes shape of orbital.
- Magnetic Quantum Number (m_l): Orientation of orbital in space.
- Spin Quantum Number (m_s): Spin state of the electron; can be +1/2 or -1/2.

Structure: Combination of principal number, subshell letters, and superscript (indicating the number of electrons).
Example for chlorine (Cl): Electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^5$ .
Aufbau Principle: Electrons fill lower energy orbitals first.
Hund's Rule: Electrons fill orbitals to maximize spin before pairing.

Adjust configurations for cations by removing electrons from the highest energy subshell first. For Ca²⁺, $1s^2 2s^2 2p^6 3s^2 3p^6$ becomes $[Ar]$ .

Stability is increased for half-filled and fully filled subshells, influencing electron arrangement.

Mendeleev's Contribution: Proposed the first periodic table sorted by atomic weight, predicting future element properties.
Modern Periodic Table: Arranged by atomic number, with rows as periods and columns as groups.
Groups: Elements in the same group share chemical properties, especially in valence electron counts.

Atomic Radius: Increases down groups and decreases across periods.
Ionization Energy: Increases across periods and decreases down groups.
Electron Affinity and Electronegativity: Both increase across periods and decrease down groups.

Defined as the net positive charge experienced by valence electrons after accounting for shielding. Increased Z_eff leads to smaller atomic radii and higher ionization energy.

Matter is comprised of atoms, each with subatomic particles.
Atomic Number (Z) determines the chemical identity; Mass Number (A) is the sum of protons and neutrons.
Ions: Positively charged (cations) and negatively charged (anions).
Bohr Model: Electrons occupy quantized energy levels around the nucleus.
Quantum Numbers, Electron Configurations, and PAuli Exclusion Principle must be embraced for MCAT preparation.
Periodic Table Trends: Understand trends of atomic radius, ionization energy, electron affinity, and electronegativity increases.

Engage in realistic MCAT practice; focus on understanding fundamental chemistry concepts and relationships.
Use Must-Knows to create study sheets and test your knowledge on each topic.