Structure of Atoms: Subatomic Particles, Atomic Theory, and Periodic Properties

Section 3: Structure of Atoms

Learning Outcomes and Topics Covered

This section introduces the fundamental structure of atoms, including subatomic particles, atomic models, electron configurations, quantum numbers, and periodic properties such as atomic size and ionization energy.

• Subatomic Particles: Protons, Neutrons, and Electrons

• Atomic Number and Mass Number

• Isotopes and Atomic Mass

• Atomic Models and Orbitals

• Electron Configurations and Orbital Diagrams

• Periodic Properties: Atomic Size and Ionization Energy

Subatomic Particles

Protons, Neutrons, and Electrons

Atoms are composed of three main subatomic particles:

• Proton (p+): Positively charged particle found in the nucleus. Mass ≈ 1 atomic mass unit (u).

• Neutron (n0): Neutral particle found in the nucleus. Mass ≈ 1 atomic mass unit (u).

• Electron (e-): Negatively charged particle found outside the nucleus in energy levels. Mass ≈ 0.000545 u.

The nucleus contains protons and neutrons, while electrons occupy regions called orbitals around the nucleus.

Atomic Number, Mass Number, and Isotopes

Definitions and Calculations

• Atomic Number (Z): Number of protons in the nucleus; defines the element.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons (thus different mass numbers).

Example: Chlorine has two common isotopes: chlorine-35 and chlorine-37.

Formula:

• $A = Z + N$ (where N = number of neutrons)

Atomic Mass and Density Calculations

Average Atomic Mass

The atomic mass listed on the periodic table is a weighted average of all naturally occurring isotopes of an element.

• Example: Lithium's atomic mass reflects the relative abundance of lithium-6 and lithium-7 isotopes.

Density Calculation Example

To determine if a liquid will float or sink in water, compare its density to that of water (1.00 g/mL).

• Convert mass and volume to standard units:

• $0.85 ext{ g} / 1.7 ext{ mL} = 0.5 ext{ g/mL}$

• Since 0.5 g/mL < 1.0 g/mL, the liquid will float.

Atomic Theory Timeline

Development of Atomic Models

The understanding of atomic structure has evolved over time:

Year	Model	Scientist	Description
1803	Solid Sphere	John Dalton	Atom as a solid, indivisible sphere.
1897	Plum Pudding	J.J. Thomson	Negative electrons embedded in a sea of positive charge.
1911	Nuclear	Ernest Rutherford	Positive charges concentrated in a central nucleus.
1913	Planetary	Niels Bohr	Electrons orbit the nucleus in fixed energy levels.
1926	Quantum	Erwin Schrödinger	Electrons described by probability distributions (orbitals).

Additional info: The quantum model incorporates wave-particle duality and probability, replacing fixed orbits with orbitals.

Early Atomic Theory

Greek Philosophers and Elements

Leucippus and Democritus proposed that matter is composed of small, indivisible particles called atomos. Aristotle suggested that matter consists of combinations of four elements: fire, air, earth, and water.

Quantum Numbers and Atomic Orbitals

Describing Electron Location

Quantum numbers specify the properties of atomic orbitals and the electrons in them:

Name	Symbol	Allowed Values	Physical Meaning
Principal quantum number	n	1, 2, 3, ...	Energy level (shell)
Angular momentum quantum number	l	0 to n-1	Orbital shape (subshell)
Magnetic quantum number	ml	-l to +l	Orbital orientation
Spin quantum number	ms	+1/2 or -1/2	Electron spin direction

• Orbitals are regions of space where electrons are likely to be found.

• Subshells (s, p, d, f) have different shapes and capacities for electrons.

Example: For n = 2, possible l values are 0 (s) and 1 (p).

Electron Configurations and Orbital Diagrams

Filling Order and Rules

• Electrons fill orbitals in order of increasing energy (Aufbau principle).

• Each orbital can hold a maximum of two electrons with opposite spins (Pauli exclusion principle).

• Hund's rule: Orbitals of equal energy are filled singly before pairing occurs.

Example: The electron configuration for calcium (Ca, atomic number 20): $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2$

Abbreviated configuration: [Ar] 4s2

Periodic Table and Classification of Elements

Organization and Groups

• Elements are arranged by increasing atomic number.

• Rows are called periods; columns are groups or families.

• Main group elements: Groups 1, 2, and 13–18

• Transition metals: Groups 3–12

• Inner transition metals: Lanthanides and actinides

Types of elements:

• Metals: Shiny, malleable, good conductors

• Nonmetals: Poor conductors, brittle

• Metalloids: Properties intermediate between metals and nonmetals

Periodic Properties: Atomic Size and Ionization Energy

Trends Across the Periodic Table

• Atomic Radius: Decreases across a period (left to right), increases down a group.

• Ionization Energy: Increases across a period, decreases down a group.

Ionization energy is the minimum energy required to remove an electron from a gaseous atom.

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

Ions and Ionic Radius

Cations and Anions

• Cation: Positively charged ion (loss of electrons)

• Anion: Negatively charged ion (gain of electrons)

• Cations are smaller than their parent atoms; anions are larger due to electron repulsion.

Example: Na → Na+ (loses one electron); Cl → Cl- (gains one electron)

Summary Table: Electron Capacity of Subshells

Subshell	Number of Orbitals	Maximum Electrons
s	1	2
p	3	6
d	5	10
f	7	14

Key Equations and Concepts

• Mass Number: $A = Z + N$

• Density: $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$

• Electron Configuration Notation: $1s^2 2s^2 2p^6 ...$

Review Questions (for Practice)

• How many orbitals are in the 2p sublevel?

• What is the maximum number of electrons in the 3d subshell?

• Which element has the electron configuration [Ne]3s23p1?

• Arrange Na+, O2-, and F- in order of increasing atomic radius.

Additional info: These review questions help reinforce understanding of atomic structure, electron configurations, and periodic trends.