Atoms, Molecules, and Ions: Structure, Properties, and the Periodic Table

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Atoms, Molecules, and Ions

Introduction

This chapter introduces the fundamental building blocks of matter: atoms, molecules, and ions. It covers the discovery of subatomic particles, atomic structure, isotopes, atomic mass, and the organization of elements in the periodic table, providing a foundation for understanding chemical properties and reactivity.

Radioactivity and Subatomic Particles

Types of Radiation

α particles: Positively charged particles discovered by Ernest Rutherford.
β particles: Negatively charged particles, similar to electrons.
γ rays: Uncharged electromagnetic radiation.

Discovery of Subatomic Particles

Electron: Negatively charged particle.
Proton: Positively charged particle.
Neutron: Neutral particle with no charge.

Table: Subatomic Particles

Particle	Mass (kg)	Mass (amu)	Charge (relative)	Charge (C)
Proton	1.67262 × 10-27	1.00727	+1	+1.60218 × 10-19
Neutron	1.67493 × 10-27	1.00866	0	0
Electron	0.00091 × 10-27	0.00055	-1	-1.60218 × 10-19

Atomic Structure

Historical Models of the Atom

Plum Pudding Model: Proposed by J.J. Thomson, described the atom as a positive sphere with embedded negative electrons.
Nuclear Model: Rutherford's gold foil experiment showed that atoms have a small, dense, positively charged nucleus with electrons around the outside. Most of the atom is empty space.

Atomic Mass Unit (amu)

Atoms have extremely small masses; the atomic mass unit (amu) is used for convenience.
1 amu = 1.66054 × 10-24 g
1 g = 6.02214 × 1023 amu
Mass of a proton ≈ 1.0073 amu; mass of a neutron ≈ 1.0087 amu; mass of an electron ≈ 5.486 × 10-4 amu

Atomic Number and Mass Number

Atomic Number

Atomic Number (Z): Number of protons in the nucleus; determines the element.
In a neutral atom, number of protons = number of electrons.

Mass Number

Mass Number (A): Total number of protons and neutrons in the nucleus.
Written as a superscript before the element symbol.

Isotopes

Definition and Representation

Isotopes: Atoms of the same element with different numbers of neutrons (and thus different masses).
All isotopes of an element have the same number of protons.
Notation: AZX, where X is the chemical symbol, A is the mass number, and Z is the atomic number.

Table: Some Isotopes of Carbon

Symbol	Number of Protons	Number of Electrons	Number of Neutrons
12C	6	6	6
13C	6	6	7
14C	6	6	8

Table: Neon Isotopes

Symbol	Number of Protons	Number of Neutrons	A (Mass Number)	Natural Abundance (%)
Ne-20	10	10	20	90.48
Ne-21	10	11	21	0.27
Ne-22	10	12	22	9.25

Atomic Mass and Measurement

Average Atomic Mass

Atomic mass is the weighted average of all isotopes of an element, based on their relative abundances.
Formula:
Measured using a mass spectrometer, which separates ions based on mass and charge.

Example Calculation

Gallium has two isotopes: amu

Ions: Losing and Gaining Electrons

Formation of Ions

Atoms can lose or gain electrons during chemical changes, forming ions.
Cations: Positively charged ions (e.g., Na+).
Anions: Negatively charged ions (e.g., F-).

Subatomic Particle Counts Example

O2- anion has 10 electrons (8 protons + 2 extra electrons).

The Periodic Table

The Periodic Law

Mendeleev's periodic law: When elements are arranged by increasing mass, sets of properties recur periodically.

Organization of the Periodic Table

Periods: Horizontal rows.
Groups: Vertical columns; elements in the same group have similar chemical properties.

Table: Names of Some Groups in the Periodic Table

Group	Name	Elements
1A	Alkali metals	Li, Na, K, Rb, Cs, Fr
2A	Alkaline earth metals	Be, Mg, Ca, Sr, Ba, Ra
6A	Chalcogens	O, S, Se, Te, Po
7A	Halogens	F, Cl, Br, I, At
8A	Noble gases	He, Ne, Ar, Kr, Xe, Rn

Reading the Periodic Table

Each element's box lists the atomic number above the symbol and atomic weight below.
Example: K Atomic number: 19 Atomic weight: 39.0983

Classification of Elements

Metals: Left side; shiny, conduct heat/electricity, mostly solids.
Nonmetals: Right side; can be solids, liquids, or gases.
Metalloids: Steplike line; properties intermediate between metals and nonmetals.

Periodicity and Predictable Ion Formation

Periodicity

Chemical properties and reactivity repeat in a regular pattern across periods.

Ions and the Periodic Table

Main-group metals lose electrons to form cations with the same electron count as the nearest noble gas.
Main-group nonmetals gain electrons to form anions with the same electron count as the nearest noble gas.
Alkali metals (1A): lose one electron, form 1+ ions.
Alkaline earth metals (2A): lose two electrons, form 2+ ions.
Halogens (7A): gain one electron, form 1- ions.
Oxygen family (6A): gain two electrons, form 2- ions.

Table: Elements That Form Ions with Predictable Charges

Group	Ion Formed
1A	H+, Li+, Na+, K+, Rb+, Cs+
2A	Mg2+, Ca2+, Sr2+, Ba2+
3A	Al3+
5A	N3-, P3-
6A	O2-, S2-
7A	F-, Cl-, Br-, I-

Summary

Atoms are composed of protons, neutrons, and electrons.
Isotopes differ in neutron number but have the same atomic number.
Atomic mass is a weighted average of isotopic masses.
The periodic table organizes elements by atomic number and recurring chemical properties.
Ion formation is predictable based on element group.

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