Atomic Structure and Electron Configuration: Foundations of Chemistry

Atomic Structure

Electricity and the Atom

Electricity plays a crucial role in understanding atomic structure and chemical reactions. Certain compounds and materials are essential for conducting electricity in chemical systems.

• Electrolyte: A compound that conducts electricity when molten or dissolved in water. Electrolytes are vital in processes such as electrolysis and in biological systems.

• Electrodes: Carbon rods or metallic strips that carry electrical current into or out of a solution or molten compound.

Electrolysis

Electrolysis is a process that uses electricity to drive a non-spontaneous chemical reaction. It is fundamental in the extraction and purification of elements.

• Anode: The positive electrode where oxidation occurs.

• Cathode: The negative electrode where reduction occurs.

• Example: Electrolysis of water produces hydrogen and oxygen gases.

Ions

Ions are charged species formed when atoms gain or lose electrons. They are central to chemical bonding and reactions.

• Ion: An atom or group of atoms with a net electric charge.

• Anion: A negatively charged ion (gains electrons).

• Cation: A positively charged ion (loses electrons).

Discovery of Subatomic Particles

Thomson Experiment

In 1897, Joseph John Thomson discovered the electron by determining the mass-to-charge ratio of cathode rays. This experiment revealed that atoms contain smaller negatively charged particles.

• Key Point: Discovery of the electron, a fundamental subatomic particle.

Goldstein's Experiment

In 1886, Goldstein observed positive rays (canal rays) using a perforated cathode, leading to the discovery of protons.

• Key Point: Identification of positively charged particles in the atom.

Millikan Oil-Drop Experiment

In 1909, Robert Millikan measured the charge of the electron using the oil-drop experiment, quantifying the fundamental unit of electric charge.

• Key Point: Determined the charge of a single electron: $-1.602 \times 10^{-19}$ coulombs.

Radioactivity

Discovery and Types

Radioactivity was discovered by Antoine Becquerel in 1895, with further work by Marie and Pierre Curie. Radioactive substances emit radiation spontaneously.

• Alpha (α) particles: Helium nuclei, mass = 4 u, charge = 2+

• Beta (β) particles: Electrons, mass ≈ 1/1837 u, charge = 1−

• Gamma (γ) rays: High-energy electromagnetic radiation, mass = 0, charge = 0

Name	Greek Letter	Mass (u)	Charge
Alpha	α	4	2+
Beta	β	1/1837	1−
Gamma	γ	0	0

Structure of the Atom

Rutherford’s Gold Foil Experiment

In 1911, Ernest Rutherford’s gold foil experiment demonstrated that atoms have a small, dense, positively charged nucleus, with most of the atom being empty space.

• Key Point: Most alpha particles passed through the foil, but some were deflected, indicating a dense nucleus.

Subatomic Particles

Atoms are composed of three main subatomic particles:

Particle	Symbol	Mass (u)	Charge	Location in Atom
Proton	p+	1	1+	Nucleus
Neutron	n	1	0	Nucleus
Electron	e−	1/1837	1−	Outside nucleus

Atomic Number and Mass Number

• Atomic number (Z): Number of protons in the nucleus; defines the element.

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

• Formula: $A = Z + N$ (where N = number of neutrons)

Isotopes

Isotopes are atoms of the same element (same atomic number) with different numbers of neutrons (different mass numbers).

• Example: Hydrogen has three isotopes: protium, deuterium, and tritium.

Nuclear Symbol

The nuclear symbol represents an isotope as:

• $^{A}_{Z}X$

• X = element symbol, A = mass number, Z = atomic number

Electron Arrangement

Energy Levels and Shells

Electrons in atoms occupy energy levels or shells, which are regions at increasing distances from the nucleus.

Element	1st shell	2nd shell	3rd shell
H	1
He	2
Li	2	1
Na	2	8	1

The Quantum Model

The quantum model describes electrons as existing in probability-based regions called orbitals, organized into principal energy levels and sublevels.

• Principal energy levels (n): Indicate the main energy shells (n = 1, 2, 3, ...).

• Sublevels (subshells): Each principal level contains n sublevels (s, p, d, f).

• Orbitals: Regions of high probability for finding an electron; each sublevel contains one or more orbitals.

Electron Configurations

Electron configurations show the arrangement of electrons in an atom’s shells and subshells.

• Notation: $1s^2 2s^2 2p^6$ (example for neon)

• Order of filling: Electrons fill orbitals in order of increasing energy, following the Aufbau principle.

Name	Atomic Number	Electron Configuration
Hydrogen	1	1s1
Helium	2	1s2
Lithium	3	1s22s1
Neon	10	1s22s22p6
Sodium	11	1s22s22p63s1
Calcium	20	1s22s22p63s23p64s2

Electron Configurations and the Periodic Table

Structure of the Periodic Table

The periodic table organizes elements by increasing atomic number and recurring chemical properties.

• Groups (families): Vertical columns; elements in a group have similar properties and the same number of valence electrons.

• Periods: Horizontal rows; properties change progressively across a period.

Valence Electrons

Valence electrons are the outermost electrons involved in chemical bonding and reactions.

• Elements in the same group have the same number of valence electrons.

• Valence electron configuration determines reactivity.

Special Groups

• Alkali Metals (Group 1A): $ns^1$

• Alkaline Earth Metals (Group 2A): $ns^2$

• Halogens (Group 7A): $ns^2np^5$

• Noble Gases (Group 8A): $ns^2np^6$

Metals, Nonmetals, and Metalloids

• Metals: Shiny, conduct heat and electricity, malleable, ductile (e.g., sodium, copper).

• Nonmetals: Dull, nonconductors, brittle in solid state (e.g., sulfur, bromine).

• Metalloids: Properties intermediate between metals and nonmetals (e.g., silicon, arsenic).

Green Chemistry: Solar Fuels

Photosynthesis and Artificial Photosynthesis

Green chemistry seeks sustainable energy solutions. Plants convert solar energy to chemical energy via photosynthesis. Artificial photosynthesis mimics this process to generate hydrogen fuel.

• Photosynthesis: $\text{carbon dioxide} + \text{water} + \text{sunlight} \rightarrow \text{carbohydrate} + \text{oxygen}$

• Artificial Photosynthesis: $\text{water} \rightarrow \text{hydrogen} + \text{oxygen}$

• Hydrogen fuel: Can be reacted with oxygen to produce heat and electricity.