Atomic Theory: Development and Models

• Matter consists of atoms, the smallest distinguishable part of an element.
• Scientific models, both mental and physical, are used to represent chemical entities for various purposes.
• Scientific theories, like atomic theory, evolve as new evidence becomes available.
• All atoms/molecules are attracted to each other due to electrical forces.
• Attractions decrease a system's potential energy; repulsions increase it.
• Stable systems are achieved when attractive and repulsive forces are balanced.
• The strength of intermolecular forces influences the temperature at which a phase change occurs.

Existence of Atoms

• Claim: Atoms are the fundamental constituents of matter.
• Evidence: Dalton's Atomic Theory (1803) suggested that elements are composed of indivisible atoms and combine in fixed ratios during chemical reactions.
• Reasoning: The predictable behavior of substances in reactions and the conservation of mass support the existence of atoms.

Existence of Electrons

• Claim: Atoms contain negatively charged particles called electrons.
• Evidence: J.J. Thomson's cathode ray experiment (1897) showed that cathode rays are deflected by electric and magnetic fields, indicating negatively charged particles (electrons).
• Reasoning: The behavior of cathode rays and measurements of electron charge confirm the existence of electrons within atoms.

Existence of Nucleus

• Claim: Atoms have a small, dense, positively charged nucleus.
• Evidence: Rutherford's gold foil experiment (1911) demonstrated that most alpha particles passed through gold foil while some were deflected, suggesting a tiny, dense, positively charged region (nucleus).
• Reasoning: The deflection of particles in Rutherford's experiment indicates that the atom is mostly empty space with a dense, positively charged nucleus causing deflections.

Evolution of Atomic Models

• Dalton's Model (1803): Proposed atoms as tiny, indivisible spheres, with different weights for different elements, marking the initial atomic theory based on experimental evidence.
• Thomson's Model (1897): Proposed the "plum pudding" model, where atoms are positively charged "pudding" with negatively charged electrons scattered inside, atoms aren't solids.
• Rutherford's Model (1911): Atoms have a tiny, dense nucleus at the center containing most of the atom's mass, with electrons orbiting around it, mostly empty space.
• Bohr's Model (1913): Electrons travel in fixed orbits or energy levels around the nucleus, explaining why atoms don't collapse and accounting for observed atomic spectra.

Scientific Theory vs. Everyday Theory

• Scientific theory is a comprehensive explanation based on extensive research and data.
• Scientific theory is highly reliable within the scientific community.
• Everyday theory is a personal opinion or untested idea.

Consistent Aspects of Atomic Theories

• Atoms are the fundamental building blocks of matter.
• Atoms of each element are unique from other element in mass and properties.

Changing Aspects of Atomic Theories

• Dalton's Model (1803): Atoms were solid, indivisible spheres.
• Thomson's Model (1897): Atoms were a positively charged "pudding" with electrons scattered like "plums".
• Rutherford's Model (1911): The atom contains a tiny, dense, positively charged nucleus with electrons orbiting it.
• Bohr's Model (1913): Electrons travel in fixed orbits around the nucleus, explaining atomic spectra.
• Initially, Dalton's theory didn't recognize subatomic particles.
• Later, electrons, protons, and neutrons were discovered, adding complexity to our understanding of atomic structure.
• Dalton's theory had no concept of a nucleus; Rutherford proposed the nucleus, later refined to include protons and neutrons.

Electrostatic Force

• Electrostatic force is the attraction or repulsion between two electrically charged objects.
• Gravitational force is the attraction between any two objects with mass.

Similarities between electrostatic and gravitational forces

• Both mediated by fields
• Both proportional to 1/r²
• Both require 2 objects

Differences between electrostatic and gravitational forces

• Gravitational force is proportional to mass; electrostatic force is proportional to charge.
• Gravitational force is only attractive; electrostatic force can be attractive or repulsive.

Interaction of Two Isolated Atoms

• When two isolated atoms approach, they experience attraction due to the electrostatic attraction between the nucleus of one atom and the electron cloud of the other.
• If they get too close, electron clouds repel each other, causing repulsion.

Energy Changes as Helium Atoms Approach

• As helium atoms approach, potential energy decreases until they reach a stable distance.
• Potential energy beings to shoot up as electron clouds overlap.
• Kinetic energy increases.
• Total energy stays the same (conserved).
• Potential energy increases when atoms are separated (less held together).

Transfer of Energy at the Atomic Level

• Collisions can transfer energy to the system (increase energy) or out of the system (decrease energy).
• Adding energy to the system will cause atoms to move faster or vibrate, increasing kinetic energy which can cause heat.
• Removing energt from the system will cause the atoms to move slower of cool down, resulting in a decrease of kinetic energy.

London Dispersion Forces

• Larger atoms or molecules with more electrons exhibit stronger London Dispersion Forces (LDFs).
• Larger molecules (like iodine) have stronger LDFs, while smaller atoms (like helium) have weaker ones.

LDF Strength and Boiling Points

• Stronger LDFs lead to higher melting and boiling points.
• Larger atoms have more significant electron clouds, leading to stronger temporary dipoles and stronger intermolecular attractions.
• Large electron size = Large electron cloud = stronger dispersion forces = higher boiling points.
• Helium has the lowest boiling point due to weak dispersion forces and small electron size.
• The smaller the electron cloud, the weaker the LDF and the lower the boiling/melting point. Lowest to highest melting/boiling pt: H2,F2, N2, Cl2, Br2, I2, 02.

Helium vs. Hydrogen Combination

• Helium is a noble gas, so if electron shells are full, they don't need bonds.
• There is minimal interaction and essentially no energy change upon "combination".
• When two helium atoms combine, there is essentially no energy change as they do not readily form a bond.
• When two hydrogen atoms approach, their electron clouds overlap, allowing them to share electrons and form a covalent bond, resulting in a decrease in potential energy and energy release.

London Dispersion Forces vs. Covalent Bonding

• London Dispersion Forces (LDFs) are weaker than covalent bonds.
• LDFs occur between molecules (intermolecular), while covalent bonds occur within a molecule (intramolecular).
• LDFs arise from temporary fluctuations in electron distribution, while covalent bonds result from electron sharing between atoms.
• LDFs act between molecules, whereas covalent bonds hold atoms together within a molecule.

Explore the evolution of atomic theory through key experiments like Rutherford's gold foil experiment and Thomson's cathode ray experiment. Understand how these findings reshaped our understanding of atomic structure and electrical forces, influencing the development of new atomic models. Learn about the role of intermolecular forces in phase changes.