Atomic Structure, Isotopes, Ions, and Atomic Mass: General Chemistry Study Notes

Atomic Structure

The Atom and Its Subatomic Particles

The atom is the smallest unit of an element and the basic functional unit in chemistry. It consists of a central nucleus surrounded by subatomic particles.

• Nucleus: The center of an atom that contains protons and neutrons.

• Neutrons: Subatomic particles with no charge, found within the nucleus.

• Protons: Subatomic particles with a positive charge, found within the nucleus.

• Electrons: Subatomic particles with a negative charge, found in regions around the nucleus.

Example: Protons and electrons have charges of equal magnitude but opposite signs. The number of protons usually equals the number of electrons in a neutral atom.

Forces Within the Nucleus

Two major forces characterize the behavior of protons and neutrons in the nucleus:

• Nuclear Force: The strong force that holds protons and neutrons together within the nucleus.

• Electrostatic Force: The force of repulsion between protons due to their positive charges.

For a stable nucleus, the nuclear force must overcome the electrostatic repulsion.

Subatomic Particles: Masses and Charges

Comparing Subatomic Particles

The three subatomic particles differ in mass and charge. Their properties are summarized below:

Subatomic Particle	Actual Mass (kg)	Relative Mass (amu)	Relative Charge	Charge (Coulombs)
Neutron	1.67493 × 10-27	~1	0	0
Proton	1.67262 × 10-27	~1	+1	1.60218 × 10-19
Electron	9.10939 × 10-31	~0.0005	-1	-1.60218 × 10-19

Additional info: 1 atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

Atomic Number, Mass Number, and Isotopes

Atomic Number and Mass Number

Atoms are identified by their atomic number (Z) and mass number (A):

• Atomic Number (Z): Number of protons in the nucleus; determines the element's identity.

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

To calculate the number of neutrons:

• Number of neutrons = Mass Number (A) – Atomic Number (Z)

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

• Isotopes have identical chemical properties but different physical properties (such as mass).

Isotope Notation: AZX, where X is the element symbol, A is the mass number, and Z is the atomic number.

Ions and Isoelectronic Species

Ion Formation

Ions are formed when atoms gain or lose electrons:

• Cation: Formed when an atom loses electrons; positively charged.

• Anion: Formed when an atom gains electrons; negatively charged.

Isoelectronic species are atoms or ions that have the same number of electrons.

Practice Problems: Subatomic Particles and Isotopes

Sample Table: Subatomic Particles in Isotopes

Isotope	Mass Number	Atomic Number	Neutrons	Protons	Electrons
Zirconium-94	94	40	54	40	40
Phosphorus-33	33	15	18	15	15
Aluminum-27	27	13	14	13	13

Additional info: For a neutral atom, the number of protons equals the number of electrons.

Atomic Mass and the Periodic Table

Atomic Mass

Atomic Mass is the mass of an element that includes the masses of all three subatomic particles. The atomic mass of an element is an average of all its isotopes, weighted by their natural abundances.

• Atomic masses are listed on the Periodic Table in units of amu (atomic mass unit) or Da (Dalton).

• 1 amu = $1.66054 \times 10^{-27}$ kg

Calculating Atomic Mass

To calculate the atomic mass of an element, use the masses and percent abundances of its isotopes:

• Isotopic Mass: Mass of each isotope.

• Percent Abundance: Percentage of each isotope in a natural sample.

• Fractional Abundance: Percent abundance divided by 100.

Atomic Mass Formula:

$\text{Atomic Mass} = (\text{Isotope Mass}_1 \times \text{Fractional Abundance}_1) + (\text{Isotope Mass}_2 \times \text{Fractional Abundance}_2) + \ldots$

Example Calculation

Given two isotopes of lithium:

• Li-6: Mass = 6.015122 amu, Abundance = 7.59%

• Li-7: Mass = 7.016004 amu, Abundance = 92.41%

Calculate the atomic mass:

$\text{Atomic Mass} = (6.015122 \times 0.0759) + (7.016004 \times 0.9241)$

Summary Table: Atomic Mass Calculation

Isotope	Isotopic Mass (amu)	Percent Abundance (%)	Fractional Abundance
Li-6	6.015122	7.59	0.0759
Li-7	7.016004	92.41	0.9241

Key Terms and Definitions

• Atom: The smallest unit of an element, consisting of a nucleus and electrons.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Total number of protons and neutrons.

• Atomic Mass Unit (amu): Standard unit for atomic mass; $1 \text{ amu} = 1.66054 \times 10^{-27}$ kg.

Additional info:

• Practice questions throughout the material reinforce understanding of atomic structure, isotopes, ions, and atomic mass calculations.

• Tables and diagrams are used to compare properties and calculate values for subatomic particles and isotopes.