Atomic Structure

John Dalton's Atomic Theory

John Dalton proposed one of the earliest models of the atom, describing it as a tiny, hard sphere that cannot be split up. This model is often referred to as the Solid Sphere Model or Bowling Ball Model. Dalton's theory laid the foundation for modern atomic theory by suggesting that atoms are indivisible and indestructible particles that make up all matter.

• Atoms are the basic units of matter.

• Each element consists of identical atoms unique to that element.

• Atoms combine in simple whole-number ratios to form compounds.

• Chemical reactions involve the rearrangement of atoms, not their creation or destruction.

Electricity and the Atom

Static electricity has been observed since ancient times, but the concept of continuous electric current was developed in the nineteenth century. The flow of electrons is fundamental to understanding atomic structure and chemical reactions.

• Direct current (DC) involves the flow of electrons from the negative terminal (cathode) to the positive terminal (anode).

• Conventional current is considered to flow from positive to negative, opposite to electron flow.

Properties of Electrical Charges

Electrical charges are fundamental to atomic structure and chemical interactions. The behavior of charges is governed by several key properties:

• Opposite charges attract (e.g., positive and negative).

• Like charges repel (e.g., two positives or two negatives).

• Charges are additive: The sum of positive and negative charges of equal magnitude is zero.

Electrolysis

Electrolysis is a chemical change caused by electricity, such as the decomposition of water into hydrogen and oxygen. This process demonstrates the movement and separation of charges within atoms and molecules.

• Electrolysis requires a voltage source to drive the reaction.

• Hydrogen gas is produced at the cathode, and oxygen gas at the anode.

Discovery of the Electron

J.J. Thomson's experiments with cathode rays in 1897 led to the discovery of the electron, a fundamental subatomic particle. Cathode rays were shown to be streams of negatively charged particles (electrons), which travel from the cathode to the anode in a straight line.

• Electron: Subatomic particle with a negative charge.

• Mass is about 2,000 times smaller than a hydrogen atom.

• Electrons are found in all substances and have the same mass-to-charge ratio.

Measuring the Electron

J.J. Thomson measured the charge-to-mass ratio of the electron to be coulombs/gram (C/g). However, the actual mass and charge of a single electron were determined later by Robert Millikan's oil-drop experiment.

• Charge of a single electron: C

• Mass of a single electron: g

• Calculation:

X-Rays and Radioactivity

Wilhelm Roentgen discovered X-rays in 1895 using a cathode ray tube. X-rays are a high-energy form of light, and their discovery led to further exploration of atomic structure. Radioactivity, discovered by Henri Becquerel and further studied by Marie and Pierre Curie, revealed that atoms can spontaneously emit radiation.

• Gamma rays (\gamma): High energy light, no charge.

• Beta particles (\beta): High-speed electrons, negative charge.

• Alpha particles (\alpha): Helium nuclei, positive charge (2+).

Thomson's Plum Pudding Model

Thomson proposed the "plum-pudding" model of the atom, where electrons are embedded in a positively charged sphere. This model attempted to explain the electrical neutrality of atoms.

• Electrons are like "raisins" in a "plum pudding" of positive charge.

• Atoms are electrically neutral overall.

Rutherford's Gold Foil Experiment and Nuclear Model

Ernest Rutherford's gold foil experiment demonstrated that atoms are mostly empty space, with a dense, positively charged nucleus at the center. Most alpha particles passed through the foil, but some were deflected at large angles, disproving the plum-pudding model.

• Atoms have a small, solid nucleus containing most of the mass and positive charge.

• Electrons are dispersed in the empty space around the nucleus.

• Atoms are electrically neutral because the number of protons equals the number of electrons.

Subatomic Particles

Atoms are composed of three main subatomic particles:

• Protons: Positive charge, found in the nucleus.

• Neutrons: No charge, similar mass to protons, found in the nucleus.

• Electrons: Negative charge, very small mass, found outside the nucleus.

Protons and neutrons have nearly the same mass, while the mass of an electron is so small it is often ignored in calculations.

Nuclear Theory of the Atom

The modern nuclear theory states:

• Most of the atom's mass and all of its positive charge are contained in the nucleus.

• Most of the atom's volume is empty space, with electrons dispersed throughout.

• The atom is electrically neutral because the number of protons equals the number of electrons.

Symbols of Elements and Atomic Notation

Elements are represented by one or two-letter symbols. The atomic number (Z) is the number of protons in the nucleus, and the mass number (A) is the sum of protons and neutrons.

• Atomic number (Z): Defines the element.

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

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

Example: Silicon (Si) has Z = 14 and A = 29, so it has 15 neutrons ().

Isotopes

Isotopes are atoms of the same element with different masses due to varying numbers of neutrons. They have the same chemical properties but different physical properties.

• Example: Hydrogen has isotopes with 0, 1, or 2 neutrons (protium, deuterium, tritium).

• U-235: 92 protons, 143 neutrons ().

Bohr Model of the Atom

The Bohr model describes electrons as orbiting the nucleus in discrete energy levels or shells. Electrons can only gain or lose energy by jumping between these levels, absorbing or emitting electromagnetic radiation.

• Energy levels are quantized.

• Electrons in the ground state have the lowest energy.

• Electrons absorb energy to move to higher levels (excited state) and emit energy when returning to lower levels.

Atomic Spectroscopy and Line Spectra

Atoms emit light at specific wavelengths, producing line spectra unique to each element. These spectra can be used to identify elements.

• Line spectra are the "fingerprints" of elements.

• Energy transitions between levels explain the emission and absorption of photons.

Quantum Model and Electron Arrangement

Electrons occupy principal energy levels (shells) and sublevels (s, p, d, f). The maximum number of electrons in a shell is , where n is the shell number.

• s sublevel: 2 electrons

• p sublevel: 6 electrons

• d sublevel: 10 electrons

• f sublevel: 14 electrons

Electron configurations describe the arrangement of electrons in an atom, using notation such as 1s22s22p6.

Periodic Table and Electron Configurations

The periodic table is organized by electron configurations. Main-group elements (A) and transition elements (B) are distinguished by their electron arrangements. Valence electrons are those in the highest energy level and are responsible for chemical bonding.

• Alkali metals: Group 1A, 1 valence electron

• Alkaline earth metals: Group 2A, 2 valence electrons

• Halogens: Group 7A, 7 valence electrons

• Noble gases: Group 8A, 8 valence electrons

Metals, Nonmetals, and Metalloids

Elements are classified based on their physical and chemical properties:

• Metals: Conduct heat and electricity, malleable, ductile, metallic luster.

• Nonmetals: Dull, brittle, nonconductors.

• Metalloids: Properties intermediate between metals and nonmetals.

Green Chemistry: Solar Fuels

Sunlight is an abundant energy source. Plants store solar energy as chemical energy via photosynthesis. Artificial photosynthesis can generate hydrogen fuel, which can be reacted with oxygen to produce heat and electricity.