Chapter 1 - Atomic Theory Flashcards
Democritus
- Greek philosopher
- 2400 years ago
- ‘Could matter be divided into smaller and smaller pieces forever or was there a limit?
Atomos
- Matter could not be divided into smaller and smaller pieces forever, eventually we would come to the smallest piece at the end.
- The piece would be indivisible
- Atomos means ‘indivisible’ in Greek
Democritus’ Atomic Model
- Atoms: small, hard particles made of the same material but were different shapes and sizes
- Atoms were infinite in number and capable of joining together
This theory was ignored for 2000 years because of two eminent philosophers named Aristotle and Plato.
They believed that the 4 main elements were made of Earth, Water, Fire, Air.
Dalton’s Atomic Theory
1) All element contain indivisible particles named atoms
2) Atoms cannot be created or destroyed
3) Atoms of the same element are exactly alike, they have the same masses
4) Atoms of different elements have different masses
Law of Constant Compositions
- A compound always contains atoms of two or more elements combined in definite proportions by mass.
Law of Multiple Compositions
- Atoms of two or more elements may combine in different ratios to produce more than one compound.
Modern Objections to Dalton’s Theory
1) Atoms are not indivisible, they contain subatomic particles
2) Not all atoms of an element have exactly the same mass
3) Some nuclear transformations alter (destroy) atoms
Crooke’s Experiment
- Put a magnet next to a light beam caused it to be deflected
- Experiment: passing an electrical current through a gas at very low pressure caused the gas to glow.
Lavoisier
- Proposed the law of conservation of matter
- Matter cannot be destroyed or created
Proust
- Law of Definite proportions
- Law of Multiple proportions
- Dalton used these to develop his theory
J.J. Thompson
- Discovered the electron
- Proposed that negative charges came from within the atom
- From this he reasoned that there must be positive charged subatomic particles as well to keep a neutral charge.
- E/m= 1.76 * 10^8
Protons were first discovered by E. Goldstein in 1896
- Mass of Proton = 1.673*10^-24 grams
Then was officially proven by Rutherford.
- This lead to the discovery of neutrons
His experiment required a gold foil and noticed that once atoms were shot directly at the sheet some bounced off.
- Protons bounced off of the gold sheet as the sheet had protons as well (positive charges repel positive charges)
Neutrons were officially discovered by Chadwick
The Bohr Model
- Electrons move in definite orbits around the nucleus like planets moving around the sun
- Each electron moves in a specific energy level
- Electrons that are further away from the nucleus have higher energy levels
- When electrons jump from a higher energy level to a lower energy level, atoms radiate energy
- When electrons jump from a lower energy level to a higher energy level, atoms absorb energy
Planck
- Suggested that energy consists of small paricles known as photons
- photons can only have discreet energies
Einstein
- Matter and Energy are not different things but different expressions of the same thing
- E (energy)= m (mass) c (speed of light)^2
De Broglie
- If energy could be thought of as having particle properties along with wave like properties
- Then matter can be thought of as having wave like properties along with particle properties
- mc^2 = h (planck’s constant) v (frequency)
Heisenberg Uncertainty Principle
- It is impossible to know the location and the momentum of a high speed particle such as an electron
- .by finding out one, the other os changed
Wave Mechanical Model
- Atom is more like a cloud
- Atom orbitals around the nucleus define the places where electrons are most likely to be found
Compound
Elements chemically bonded together to produce different properties that its original elements
- E.g. Hydrogen (flammable)+ Oxygen (flammable) –> Water ( a substance that puts out fire)
Mixture
Elements not chemically bonded together to retain properties of its original elements.
The Functions:
- Protons –> The identity of the element
- Neutrons –> Stability, changing amount of neutrons make elements unstable and radioactive
- Electrons–>Chemical radioactivity, electrons are on the outside of the atom.
Isotopes
- Isotopes of an element have the same chemical properties because they have the same number of electrons.
- Isotopes of an element however have different physical properties
- Isotopes with fewer neutrons will have:
a) lower masses and densities
b) faster rate of diffusion
c) lower melting and boiling points
Used to determine the Average Atomic Mass of an Element
m (mass in a.m.u.) * r (relative abundance in decimals) + m2*r2 = average atomic mass
The Mass Spectrometer
Used to:
- measure the relative masses of isotopes
- find the relative abundance of the isotopes in a sample of an element
Vaporization
the sample is heated to form a gas.
Ionisation
- sample is bombarded by a stream of high energy electrons from an electron gun.
- an electron is knocked from an atom
- a positive ion forms
Acceleration
accelerates postiveions and creates a narrow beam of ions
