Ch. 2 Lecture Notes: Atoms and the Periodic Table: Structure, Properties, and Classification

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Ch. 2 Lecture Notes: Atoms and the Periodic Table

Chapter Overview

This chapter introduces the foundational concepts of atomic structure, the properties of subatomic particles, isotopes, atomic mass, and the organization of elements in the periodic table. It also explores the relationship between electron arrangement and chemical properties.

Atomic Theory and the Structure of Atoms

Modern Atomic Theory

Modern atomic theory is based on four key assumptions:

All matter is composed of atoms.
Atoms of a given element differ from those of other elements. Each element has unique atoms.
Chemical compounds consist of atoms combined in specific ratios. Only whole atoms combine; for example, CO and CO2 are possible, but not C1/2O1/2.
Chemical reactions change only the way atoms are combined in compounds; the atoms themselves remain unchanged.

Definition of an Atom

Atom: The smallest particle of an element that retains its identity in a chemical reaction. The term comes from the Greek atomos, meaning "indivisible."

Subatomic Particles

Protons (p): Positively charged particles found in the nucleus.
Neutrons (n): Electrically neutral particles with a mass similar to protons, also in the nucleus.
Electrons (e-): Negatively charged particles with a mass about 1/1836 that of a proton, found outside the nucleus.

Table: Comparison of Subatomic Particles

Name	Symbol	Mass (amu)	Charge
Proton	p	1.007276	+1
Neutron	n	1.008665	0
Electron	e-	0.0005486	-1

Atomic Mass Units (amu)

Atomic mass units are used to express the mass of subatomic particles and atoms.
1 amu is defined as 1/12 the mass of a carbon-12 atom:

Structure of an Atom

Nucleus: Dense core containing protons and neutrons.
Electron cloud: Electrons move rapidly in a large volume of space around the nucleus.
The nucleus is extremely small compared to the overall size of the atom (like a pea in a stadium).

Elements, Atomic Number, and Mass Number

Atomic Number (Z)

Atomic number (Z): The number of protons in the nucleus of an atom, which determines the element.
For neutral atoms, the number of electrons equals the number of protons.
Examples:
- Hydrogen (H): Z = 1, 1 proton, 1 electron
- Nitrogen (N): Z = 7, 7 protons, 7 electrons

Mass Number (A)

Mass number (A): The total number of protons and neutrons in an atom.
Relationship:

Example Calculation

Phosphorus: Z = 15, A = 31 Number of neutrons =

Isotopes and Atomic Weight

Isotopes

Isotopes: Atoms of the same element with identical atomic numbers but different mass numbers (different numbers of neutrons).
Examples: Hydrogen has three isotopes:
- Protium: 1 proton, 0 neutrons (A = 1)
- Deuterium: 1 proton, 1 neutron (A = 2)
- Tritium: 1 proton, 2 neutrons (A = 3)

Nuclear Symbol Notation

Isotopes are represented as: where X is the element symbol, A is the mass number, and Z is the atomic number.

Atomic Mass (Atomic Weight)

Atomic mass: The weighted average mass of an element's atoms, based on the relative abundance and mass of each isotope.
Formula:
Example: Copper has two main isotopes:
- 69.09% Cu (mass = 62.9298 amu)
- 30.91% Cu (mass = 64.9278 amu)

The Periodic Table

Organization of the Periodic Table

Elements are arranged by increasing atomic number.
Periods: Horizontal rows (7 total).
Groups: Vertical columns (18 total), elements in the same group have similar chemical properties.

Table: Periods and Number of Elements

Period	# of Elements
1	2
2	8
3	8
4	18
5	18
6	32
7	32

Categories of Elements

Main Groups: Groups 1A, 2A, 3A–8A
Transition Metals: Groups 1B–8B
Inner Transition Metals: Lanthanides and Actinides (bottom rows)

Periodic Trends and Group Characteristics

Periodicity: Properties of elements show repeating patterns across periods.
Group 1A (Alkali Metals): Shiny, soft, highly reactive metals, low melting points, react with water.
Group 2A (Alkaline Earth Metals): Lustrous, silvery, higher melting points than alkali metals, less reactive with water.
Group 7A (Halogens): Colorful, corrosive nonmetals, found in compounds, not free in nature.
Group 8A (Noble Gases): Colorless, inert gases, very low chemical reactivity.

Electronic Structure of Atoms

Quantum Mechanical Model

Electron arrangement determines element properties.
Electrons are organized into shells, subshells, and orbitals.
Shell: Main energy level (n = 1, 2, 3, ...)
Subshell: s, p, d, f (energy and shape)
Orbital: Region of space where electrons are likely found; s = 1 orbital, p = 3, d = 5, f = 7
Each orbital holds up to 2 electrons with opposite spins.

Electron Configuration Rules

Electrons fill lowest energy orbitals first (Aufbau principle).
Each orbital holds two electrons with opposite spins (Pauli exclusion principle).
Orbitals of equal energy are half-filled before pairing (Hund's rule).

Electron Configuration Notation

Write the subshell symbol and superscript for number of electrons: Example:
Condensed notation uses noble gas core: Example:

Valence Electrons and Reactivity

Valence electrons: Electrons in the outermost shell, determine chemical properties and reactivity.
Elements in the same group have the same number of valence electrons.

Electron-Dot Symbols

Dots around the element symbol represent valence electrons.
Each side of the symbol can hold up to two dots (maximum of 8 for main group elements).

Practice and Application

Classify elements as metals, nonmetals, or metalloids.
Provide group names and characteristics for periodic table groups.
Calculate atomic structure using atomic number and mass number.
Write electron configurations and draw electron-dot symbols.

Additional info: These notes are based on textbook slides and cover the essential topics for understanding atomic structure and the periodic table in a General Chemistry course.