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

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 principles is essential for studying chemical properties, reactions, and the behavior of matter.

The Atom

Dalton’s Atomic Theory

• Atoms are tiny particles of matter that make up all substances.

• Atoms of a given element are different from those of other elements.

• Atoms of two or more elements combine in specific ratios to form compounds.

• In chemical reactions, atoms are rearranged to form new combinations.

Subatomic Particles

• Atoms are composed of protons, neutrons, and electrons.

• Protons and neutrons are located in the nucleus; electrons occupy regions around the nucleus.

• Protons and electrons have opposite charges and attract each other, while like charges repel.

Table: Comparison of Subatomic Particles

Name	Symbol	Mass (amu)	Charge (Units)
Proton	p	1.007276	+1
Neutron	n	1.008665	0
Electron	e-	0.0005485799	-1

Atomic Number and Mass Number

Atomic Number (Z)

• The atomic number is specific for each element and is equal to the number of protons in an atom.

• In a neutral atom, the number of electrons equals the number of protons.

• Atomic number determines the identity of the element.

Mass Number (A)

• The mass number is the sum of protons and neutrons in the nucleus.

• Formula: $A = p + n$

Example

• Cobalt used in cancer treatments: $Z = 27$, $A = 60$

• Protons = 27, Neutrons = $60 - 27 = 33$, Electrons = 27 (if neutral)

Isotopes

Definition and Representation

• Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons.

• Isotopes are represented by their name (e.g., carbon-14) or symbol (e.g., $^{14}_{6}C$).

• General format: $^{A}_{Z}X$ where $A$ is mass number, $Z$ is atomic number, and $X$ is the element symbol.

Example Problem

• Write the atomic symbol for the isotope with 13 electrons and 14 neutrons.

• Since electrons = protons (neutral atom), $Z = 13$ (Aluminum), $A = 13 + 14 = 27$

• Symbol: $^{27}_{13}Al$

Isotope Identification

• Isotopes of the same element have the same atomic number but different mass numbers.

• Example: $^{19}_{9}X$ and $^{21}_{9}X$ are isotopes (same $Z$).

Atomic Mass and Calculations

Atomic Mass (Atomic Weight)

• Atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element.

• Calculated using:

  • % abundance of each isotope

  • Mass of each isotope

• Formula: $\text{Atomic Weight} = \sum [ (\text{isotope abundance}) \times (\text{isotope mass}) ]$

Example Calculation

• Copper: 69.17% $^{63}Cu$ (62.93 amu), 30.83% $^{65}Cu$ (64.93 amu)

• Calculation: $\text{Atomic Weight} = (0.6917 \times 62.93) + (0.3083 \times 64.93)$

• Result: $\approx 63.55$ amu

The Periodic Table

Structure and Organization

• The periodic table arranges elements by increasing atomic number.

• Periods: Horizontal rows (7 total)

• Groups: Vertical columns (18 total)

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

• Elements in the same group have similar chemical properties.

Classification of Elements

• Metals: Left and center of the table; shiny, good conductors, malleable, mostly solids (except Hg).

• Nonmetals: Right side; poor conductors, can be solid, liquid, or gas, brittle, dull.

• Metalloids: Border between metals and nonmetals; properties intermediate between metals and nonmetals. Includes B, Si, Ge, As, Sb, Te.

Table: Classification of Elements

Type	Properties	Examples
Metals	Shiny, malleable, good conductors	Fe, Cu, Au
Nonmetals	Brittle, dull, poor conductors	O, S, Cl
Metalloids	Intermediate properties	Si, B

Main Groups and Special Groups

• Alkali Metals (Group 1): Li, Na, K, Rb, Cs, Fr; shiny, soft, very reactive with water.

• Alkaline Earth Metals (Group 2): Be, Mg, Ca, Sr, Ba, Ra; lustrous, less reactive than Group 1.

• Halogens (Group 17): F, Cl, Br, I, At; colorful, corrosive nonmetals, found in compounds.

• Noble Gases (Group 18): He, Ne, Ar, Kr, Xe, Rn; colorless, unreactive gases.

Electronic Structure of Atoms

Energy Levels and Orbitals

• Properties of elements are determined by the arrangement of electrons.

• Electrons occupy energy levels (shells) and orbitals (regions of space).

• Each orbital can hold up to 2 electrons.

• Types of orbitals: s (spherical), p (dumbbell-shaped).

Valence Electrons

• Valence electrons are the outermost electrons, located in the highest energy shell.

• They determine an element’s chemical properties and reactivity.

Electron-Dot (Lewis) Symbols

• Electron-dot symbols represent valence electrons as dots around the atomic symbol.

• Steps to draw:

  1. Write the atomic symbol.

  2. Add one dot per valence electron, placing one on each side before pairing.

  3. Maximum of 8 dots (octet rule).

• For main group elements, the number of valence electrons equals the group number (1A–8A system).

Table: Electron-Dot Symbols for Main Group Elements

Element	Symbol	Valence Electrons	Electron-Dot Symbol
Hydrogen	H	1	H•
Carbon	C	4	•C•
Oxygen	O	6	:O:
Neon	Ne	8	:Ne:

Example

• Sodium (Na, Group 1A): 1 valence electron → Na•

• Chlorine (Cl, Group 7A): 7 valence electrons → :Cl:

• Neon (Ne, Group 8A): 8 valence electrons → :Ne:

Summary Table: Key Concepts

Concept	Definition/Explanation
Atom	Smallest unit of an element, made of protons, neutrons, electrons
Atomic Number (Z)	Number of protons in the nucleus
Mass Number (A)	Sum of protons and neutrons
Isotope	Atoms of same element with different numbers of neutrons
Atomic Mass	Weighted average mass of all isotopes
Valence Electrons	Electrons in the outermost shell
Electron-Dot Symbol	Symbol showing valence electrons as dots

Additional info: Some slides referenced radioactive isotopes and nuclear equations (Ch. 11), but the main focus here is on atomic structure and the periodic table (Ch. 2).