Atoms, Ions, and Ionic Compounds: Structure and Properties

Atomic Structure

Subatomic Particles

Atoms are the fundamental building blocks of matter, composed of a dense nucleus surrounded by electrons. The nucleus contains protons and neutrons, while electrons move around the nucleus at relatively large distances.

• Protons (p+): Positively charged particles found in the nucleus. The number of protons defines the atomic number and the identity of the element.

• Neutrons (n0): Electrically neutral particles in the nucleus, with a mass nearly equal to that of protons. Neutrons contribute to the mass number but do not affect the element's chemical identity.

• Electrons (e-): Negatively charged particles that move around the nucleus at a distance of approximately $10^{-8}$ cm. Electrons are much less massive than protons or neutrons.

Key Point: The nucleus contains almost all the mass of the atom, while electrons occupy most of the volume.

Table: Properties of Subatomic Particles

Particle	Mass (kg)	Mass (amu)	Charge (relative)	Charge (C)
Proton	1.6726 × 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 Number, Mass Number, and Isotopes

Definitions

• Atomic Number (Z): The number of protons in the nucleus of an atom. It determines the element's identity.

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

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

Example: Calcium (Ca) has atomic number 20. Most calcium atoms have 20 protons and 20 electrons, but the number of neutrons can vary, resulting in different isotopes (e.g., Ca-40 with 20 protons and 20 neutrons).

Notation for Isotopes:

• General form: $^{A}_{Z}\text{X}$, where X is the chemical symbol.

• For calcium-40: $^{40}_{20}\text{Ca}$

Guideline: For lighter elements, the mass number is often about twice the atomic number, but this is only an approximation.

Ions: Cations and Anions

Formation of Ions

Atoms can gain or lose electrons to form ions, which are charged species. The number of protons does not change during this process.

• Cation: A positively charged ion formed when an atom loses one or more electrons.

• Anion: A negatively charged ion formed when an atom gains one or more electrons.

Examples:

• Potassium cation: $\text{K} \rightarrow \text{K}^+ + e^-$ (Potassium atom loses one electron to form K+.)

• Chloride anion: $\text{Cl} + e^- \rightarrow \text{Cl}^-$ (Chlorine atom gains one electron to form Cl−.)

Ionic Compounds and Ionic Solids

Formation and Properties

Ionic compounds are formed by the electrostatic attraction between cations and anions. The resulting solid is called an ionic solid or salt.

• Simple ions: Single-atom ions such as Cl−, Br−, Na+, K+.

• Polyatomic ions: Ions composed of multiple atoms, such as NO3− (nitrate), SO42− (sulfate), NH4+ (ammonium), and PH4+ (phosphonium).

Example Reaction:

• $2\ \text{Na}_{(s)} + \text{Cl}_{2(g)} \rightarrow 2\ \text{NaCl}_{(s)}$

• Sodium (Na) is a soft metallic solid; chlorine (Cl2) is a greenish gas; sodium chloride (NaCl) is a clear crystalline ionic solid.

Table: Examples of Ions

Type	Examples
Simple Cations	Na+, K+
Simple Anions	Cl−, Br−
Polyatomic Ions	NO3−, SO42−, NH4+, PH4+

Summary of Key Concepts

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

• The atomic number (Z) defines the element; the mass number (A) defines the isotope.

• Ions are formed by the gain or loss of electrons; cations are positive, anions are negative.

• Ionic compounds are formed from the combination of cations and anions, resulting in ionic solids.

Additional info: The study notes have expanded on the definitions, provided standard notation for isotopes, and included general guidelines for atomic structure and ion formation for clarity and completeness.