Atom Models (pg 58 - 62)
What is a Model?
Model - a representation or mental picture of something we cannot see directly
Chemists examine new matter to obtain new information and expand their hypotheses or representations of matter.
Development of atomic models (description of the atom) experience change to explain new observations.
The First Idea of Atoms
Greek Philosopher - Democritus
Democritus developed the initial idea of an atom
Believed that matter is composed of small individual particles.
Called the particles atoms
Derived from word ‘atomos’ = invisible
Wasn’t questioned again until the 1800s
Daltons Atomic Theory
British Scientist - John Dalton
Dalton’s invisible atom model.
Dalton formulated his own atomic theory
All matter is made up of hard, invisible particles called atoms
described these particles as being a solid sphere that looks like a billiard ball
The atoms of the same elements are identical.
The atoms of different elements are different.
Therefore - each element is unique
Law of Multiple Relationships
Compounds are formed when atoms are combined in a specific ratio
A chemical reaction involves the rearrangement of atoms.
It changes the way they are joined together.
Atoms cannot be created or destroyed, and during a chemical reaction, cannot change from one element to another.
Model provides a simple explanation for the fixed relationships which connect atoms.
NO explanation of the forces that hold atoms together in compounds.
NO explanation for the electrical nature of matter.
After Thompson, the invisible atom model was discarded.
The Electrical Nature of Matter
English Chemist Davy
Assisant Micheal Faraday
Swedish Chemist Berzelius
Discovered that chemical compounds broke up when an electrical current was sent through them.
This indicated that matter must have an “electrical nature”.
Scientist Stoney (later)
Predicted that electricity consisted of separate units or particles.
Proposed the name “Electron” for the unit of electrical charge.
Thompson Atomic Model
English scientist J.J. Thompson
Discovered that electric charge is made up of small negatively charged particles.
These particles were identified as electrons (predicted by Stoney)
Calculations were done to show: The electrons of substances all have the same mass and charge.
Led to the conclusion that all the electrons of all matter are identical.
Thus, substances don’t differ because their electrons differ, but because the electrons are arranged differently inside the atom.
Still proposed that the atom was a solid sphere.
But one where equal positive and negative charges are evenly distributed throughout the atom.
Believed that an atom looked like a raisin cake
Negatively charged electrons surrounding a positively charged mass.
After this model, Dalton’s invisible atom model was discarded
Thompson’s model was rejected after the discovery of the atomic nucleus (Rutherford)
Discovery of Radio Activity
Radioactivity is the property of some unstable atoms (radionuclides) to spontaneously emit nuclear radiation, usually alpha particles or beta particles often accompanied by gamma-rays.
French Physicist Henri Becqueral
Discovered that uranium salts emitted gamma rays that could penetrate the paper coverings of photographic plates and affected those plates inside.
Phenomenon was called “Becqueral radioactivity”
This discovery caught the interest of Pierre Curie and Marie Curie
They did research on various chemicals that spontaneously decomposed and gave off radiation that was extremely permeable.
Such substances are called radioactive materials
Was discovered that radioactive substances released three types of radiation:
Alpha Particles (α-particles)
Beta Particles (β-particles)
Gamma rays (γ-particles)
Gave way to the theory:
Atoms cannot be created or destroyed a final death blow, because in radioactivity, atoms destroy themselves continuously to create other atoms
Rutherfords Atomic Model
New Zealand Ernest Rutherford
Studied particles shot out by radioactive particles because he wanted to find out how they originated.
Far-reaching consequences on the atomic model.
After this Thompson’s model was rejected.
1. Rutherford’s gold foil experiment
Ernest Rutherford
Geiger
Marsden
Performed a gold foil experiment
Used Alpha Particles (α-particles).
The purpose was to determine where the protons (positive particles) were located in an atom.
The charge of the Alpha Particles (α-particles) were positive and would be repelled by the other positive particles.
A thin piece of golden foil was bombarded with Alpha Particles (α-particles), and a fluorescent zinc sulfire screen was used as a finder for the Alpha Particles (α-particles).
How the Alpha Particles (α-particles) moved through the atom and which direction change they underwent could thus be determined
The Alpha Particles (α-particles) were released by natural radioactivity.
Selected due to high kinetic energy and mass.
It was later found that Alpha Particles (α-particles) were actually helium nuclei.
With a double positive charge (+2)
Mass four times that of a proton.
2. Observations during the experiment
Most Alpha Particles (α-particles) showed no deviation and moved unchanged through the gold foil.
Some Alpha Particles (α-particles) changed direction slightly.
Some Alpha Particles (α-particles) were completely reflected.
3. Rutherford’s conclusions
The first observation [Most Alpha Particles (α-particles) showed no deviation and moved unchanged through the gold foil.] indicates that the largest volume is filled with a very small mass.
Observations two and three [Some Alpha Particles (α-particles) changed direction slightly. Some Alpha Particles (α-particles) were completely reflected.] indicate that the positive particles in the atom are grouped together in the nucleus and thus were strong enough to reflect the heavy positive Alpha Particles (α-particles).
This nucleus is very small compared to the rest of the atom but has a large mass.
4. Rutherford’s atom model is therefore based on the following
The positive charge of an atom is concentrated in a very small volume
The nucleus.
The nucleus is surrounded by empty space consisting of negatively charged electrons of very small mass that occur at relatively large distances around the nucleus.
The volume of an atom is mainly created by the rapid moving of electrons.
The mass of the atom is mainly located in the core.
Protons and neutrons (nucleons) are heavier than electrons.
Later, Rutherford predicted that the nucleus is composed of protons (positive particles).
Believed that there are as many +protons as -electrons in the atom.
According to the Rutherford Model electrons move around the nucleus.
The movement of electrons around the nucleus was unacceptable.
When electrical charges move under the influence of force, they are accelerated and lose energy.
This loss of energy would cause electrons to move slower and as a result of the force of attraction towards the nucleus - would spiral towards it.
This would result in the collapse of the atom and since atoms do not collapse nut remain stable the model was rejected.
Bohr’s Atom Model
Phenomenon of Line Spectra
Lines that present the spectrum of a glowing gas element when its light is passed through a prism/
Each line in the spectrum coincides with the light with definite energy.
Implies that the existence of line spectra and shows that atoms can only radiate lights of certain energies.
Danish Scientist Niels Bohr
Developed a theory that explains thee existsistence of line spectra and an explanation that atoms with moving electrons don’t collapse.
Predicted that the total energy (kinetic and potential) of an electron in an atom is quantized.
Thus, this energy can only have certain values.
Theory laid the foundation for quantum mechanics
The study of the law of motion that controls the motion of atomic particles.
Bohr proposed an atomic model similar to the solar system.
Electrons move around the nucleus like planets orbit the sun.
Aka the planetary atomic model.
His theory goes as follows:
Electrons move around the nucleus of an atom in one of several concentric circles (shells) or energy levels.
Electrons with the same energy move in the same energy level.
The energy levels further from the nucleus have higher energy than the energy levels closer to the nucleus.
Energy levels closer to the nucleus are first filled with electrons.
Each energy level is indicated by the letters K, L, M, N, O, P, Q, or by the numbers 1, 2, 3, 4, 5, 6, 7.
Each energy level can only take a certain amount of electrons.
Level 1 takes 2 electrons
Level 2 takes 8 electrons
Level 3 takes 18 electrons
Etc…
Electrons in orbits close to the nucleus are in lower energy levels than orbits away from the nucleus.
If electrons are in the lowest available energy level, the atom is in ground state.
State of lowest possible energy.
When electrons absorb energy, they move to a higher energy level.
Electron is then in the excited state.
The excited state is unstable, and electrons fall back to lower energy levels when they release energy.
Line Spectra by Bohr
Electrons in the ground state absorb energy and move to a higher energy level (excited state)
Electrons in the excited state are unstable
Electron falls back to its ground state and radiates excess energy as light.
Since energy is seen as separate coloured lines this shows that electrons in an atom can only have a certain given energy. Only energy equal to the difference in energy between the levels is possible and not in between energy.
Energy of electrons is thus quantified.
Energy being quantified
Means that energy only occurs in certain quantities that are equal to the difference in energy between two energy levels in the atom.
Each element has its own unique line spectrum (aka the line emission spectrum) that is a set of coloured lines that the element radiates when it is in the gaseous phase.
The Wave Mechanical Atomic Model
Bohr’s model couldn’t explain the line spectra of atoms with more than one electron. (Only described the structure of the hydrogen atom).
French Physicist Louis de Brogile.
Compiled a theory that each moving particle contains both particle and wave properties.
American CJ Davison and Brit GP Thompson
Demonstrated that moving electrons have a wave nature,
Austrian Erwin Schrodinger
Presented a wave mechanical model where the moving electron forms a three dimensional wave (wave space) that surrounds an atom.
This is called an Orbital
The Discovery of the Neutron
British Scientist James Chadwick
Discovered a small particle with mass practically equal to the proton.
These are called neutrons (and are neutral)
Revealed structure of the nucleus.