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Copy of Chemistry: 3.3: Introduction to Atomic Structure
Transcript of Copy of Chemistry: 3.3: Introduction to Atomic Structure
Review: Dalton's atomic theory proposes that:
All of matter is made up of particles called atoms
These particles combine to form elements (same type of atoms) and compounds (different types of atoms)
The atoms that make up compounds are always present in the same ratios. This is known as the "Law of Constant Composition".
The theory and law leads to the formation of chemical formulas that are used to represent compounds:
Atoms are represented by symbols
Compounds are represented by "sentences" that are made-up of the symbols of the atoms present in the molecule
More than one type of the same atom in a compound is represented by a subscript next to the atomic symbol
The Structure of the Atom
Dalton's theory revolutionized our understanding of what matter was, and started a new era in chemical research.
This new era sought to answer a series of questions that Dalton's theory posed:
What causes atoms to come together to form compounds?What is the actual structure of an atom?What causes certain atoms to be different than others?
Over the last 200 years, five major theories have developed that attempt to explain Dalton's theory about atoms, elements and compounds.
These theories arose as new technology from one time period to the next offered new insights into the structure of the atom.
New technologies, such as electricity, were also developed as our understanding of the atom expanded.
Understanding the background and history of the structure of the atom is important for placing chemistry in context with our lives.
Theories of Atomic Structure
J.J. Thomson's plum pudding model
Earnest Rutherford's nuclear model
Niels Bohr's planetary model
John Dalton's solid sphere model
However, in the 1890's J. J. Thomson used a cathode ray tube to show that atoms were themselves made up of even smaller particles.
These smaller particles were soon called electrons!
Thomson knew they were negatively charged because he discovered that they were repelled by a negative charge.
Because electrons were negatively charged, Thomson deduced that there must also be a positively charged part of an atom. Thomson's discovery further revolutionized our understanding of the atom.
Thomson's discovery defined all future theories about atomic structure.
Each theory now had to explain the atom in terms of both a positively charged particle, and a negatively charged particle.
Thomson himself developed a theory, known as the "plum pudding' model.
Each model built on previous models for development.
John Dalton himself proposed answers to these questions
He suggested that the atom was solid sphere.
Earnest Rutherford said, "No way"! He showed that Thomson's "pudding" model was not accurate with his own experiment in the early 1900's.
Rutherford discovered that heavy alpha particles were scattered by a sheet of thin foil.
Rutherford concluded that there had to be a heavier, centralized positive particle in the atom that would cause the positive alpha particles to "bounce" off the foil, instead of pass right through it, as Thomson's theory would predict.
His new model was called the nuclear atom, since it contained a heavy "nucleus" of positive particles called protons.
Neils Bohr wasn't satisfied with this. He proposed his own model that expanded on Rutherford's nuclear model.
Now the atom has a centralized positive particle.
Rather than having electrons suspended in a "soup" around the centralized positive particle, Neils described the atom as electrons that orbit in defined paths around the positive nucleus. This is similar to the way planets orbit around the sun.
Today, we know that even Neils model is close, but not entirely correct. The modern concept of the atom is similar to Neil's model, but some important differences.
The electrons that orbit the nucleus do not follow definite circular orbits, but occupy spherical domains that extend outward from the nucleus. This is like layers of an onion extending outward from the center.
So, where have we gone?
John Dalton's Atomic Theory: Solid Sphere Model
J.J. Thomson's cathode ray experiment: Plum Pudding Model
Rutherford's foil experiment: nuclear model
Neils Bohr: Planetary model
Modern Science (Earnest Schrodinger): Quantum Mechanical Model
1926 and on...