Atoms and the Periodic Table: Structure, Properties, and Classification

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Atoms and the Periodic Table

Introduction

This chapter introduces the fundamental concepts of atomic structure and the organization of elements in the periodic table. Understanding these basics is essential for studying chemical properties, reactions, and the behavior of matter in GOB Chemistry.

The Atom

Dalton’s Atomic Theory

Atoms are the tiny, indivisible particles that make up all matter.
Atoms of a given element are identical in mass and properties, but differ from those of other elements.
Compounds are formed when atoms of different elements combine in fixed, simple ratios.
Chemical reactions involve the rearrangement of atoms; atoms themselves are not created or destroyed in these processes.

Subatomic Particles

Atoms are composed of three main subatomic particles:
- Protons (p): Positively charged particles found in the nucleus.
- Neutrons (n): Neutral particles also located in the nucleus.
- Electrons (e-): Negatively charged particles that move around the nucleus.
Protons and electrons have equal but opposite charges, while neutrons have no charge.
Protons repel each other, electrons repel each other, but protons and electrons attract each other.

Comparison of Subatomic Particles

The following table summarizes the properties of the three main subatomic particles:

Particle	Symbol	Mass (grams)	Mass (amu)	Charge
Proton	p	1.672622 × 10-24	1.007276	+1
Neutron	n	1.674927 × 10-24	1.008665	0
Electron	e-	9.109328 × 10-28	5.485799 × 10-4	-1

Atomic Number and Mass Number

Atomic Number (Z)

The atomic number (Z) is unique for each element and defines the element.
It equals the number of protons in the nucleus of an atom.
For a neutral atom, the atomic number also equals the number of electrons.
Example: Lithium (Li) has Z = 3, so it has 3 protons and, if neutral, 3 electrons.

Mass Number (A)

The mass number (A) is the total number of protons and neutrons in the nucleus.
Formula:

Example: If an atom has 27 protons and 33 neutrons, its mass number is 60.

Isotopes and Atomic Mass

Isotopes

Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A).
Isotopes have nearly identical chemical properties but different physical properties (such as mass and stability).
Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

Expressing Isotopes

Isotopes are represented by their atomic symbol, mass number, and atomic number:

Example: represents carbon-14.

Atomic Mass

The atomic mass of an element is the weighted average of the masses of all its naturally occurring isotopes.
It is calculated using the relative abundance and mass of each isotope:

Example: If copper has two isotopes, (69.17%, 62.93 amu) and (30.83%, 64.93 amu):

The Periodic Table

Organization of the Periodic Table

The periodic table arranges elements by increasing atomic number.
Periods are horizontal rows; groups (or families) are vertical columns.
Elements in the same group have similar chemical properties.

Classification of Elements

Elements are classified as metals, nonmetals, or metalloids based on their properties and position in the periodic table.

Type	Properties	Examples
Metals	Shiny, malleable, good conductors of heat and electricity, mostly solids at room temperature (except Hg)	Na, Fe, Cu
Nonmetals	Dull, brittle, poor conductors, can be solids, liquids, or gases	O, Cl, S
Metalloids	Properties intermediate between metals and nonmetals	B, Si, As

Main Groups and Special Groups

Main group elements are found in groups 1A–2A and 3A–8A (1, 2, 13–18).
Transition metals are found in groups 3–12.
Special groups include:
- Alkali metals (Group 1A): Highly reactive, soft metals (e.g., Na, K).
- Alkaline earth metals (Group 2A): Less reactive than alkali metals (e.g., Mg, Ca).
- Halogens (Group 7A/17): Very reactive nonmetals (e.g., F, Cl).
- Noble gases (Group 8A/18): Inert, colorless gases (e.g., Ne, Ar).

Electronic Structure of Atoms

Electron Arrangement

The properties of elements are determined by the arrangement of electrons in their atoms.
Electrons are arranged in energy levels (shells) around the nucleus.
Each energy level can hold a specific number of electrons.

Orbitals

An orbital is a three-dimensional region around the nucleus where an electron is most likely to be found.
Each orbital can hold up to 2 electrons.
Types of orbitals include s (spherical) and p (dumbbell-shaped).

Valence Electrons and Electron-Dot Symbols

Valence electrons are the electrons in the outermost shell of an atom.
These electrons determine the chemical properties and reactivity of the element.
Electron-dot symbols (Lewis symbols) represent the valence electrons as dots around the atomic symbol.
For main group elements, the number of valence electrons equals the group number (1A–8A system).
Maximum of 8 dots (octet rule) around a symbol.
Example: Sodium (Na) has 1 valence electron; Chlorine (Cl) has 7; Neon (Ne) has 8.

How to Draw Electron-Dot Symbols

Write the atomic symbol.
Add one dot at a time to each side (top, right, bottom, left) before pairing.
Continue until the number of dots equals the number of valence electrons.

Summary Table: Key Atomic Properties

Property	Definition	Symbol	How to Find
Atomic Number	Number of protons in nucleus	Z	From periodic table
Mass Number	Number of protons + neutrons	A	A = p + n
Isotope	Atoms with same Z, different A	–	Compare neutron numbers
Atomic Mass	Weighted average mass of isotopes	–	Use abundance and mass of each isotope

Additional info: Some images and diagrams referenced in the slides (such as the structure of the atom or periodic table) are described in text for clarity. All key concepts are expanded with definitions, examples, and formulas for self-contained study.