8.1 - Probing Matter Flashcards
What/when was the first mention of atoms in history
Around 400 BCE, Democritus was the first to suggest the existence of atoms
What was Daltons model of the atom and when was it made
In 1803, Daltons model of the atom was made. Dalton determined atoms are distinguished by their masses and can be combined to produce compound molecules.
What and when was Thomson’s plum pudding model of the atom made
Thomson’s plum pudding model of the atom was made in 1906. Suggested negative electrons are scattered through a positive ‘dough’
What and when was Rutherford model of the atom made
Rutherford’s nuclear model of the atom was made in 1911.
Suggested a large amount of the charge and most of the mass is concentrated in a tiny central nucleus. Most of the atom is empty space, and electrons orbit at the edge.
What and when was Bohrs model of the atom made
Bohr’s model of the atom was made in 1913, suggested limited electrons to certain fixed energy orbits, which they could jump between.
Tell me all about atomic theories through history
Mankind has long wondered about what materials around us are fundamentally made out of. There have been many scientists, from Democritus in the fifth century to dalton in the early 19th century, who have suggested the idea of a tiny indivisible particle from which everything else is constructed. The basic model for one of these atoms was published In a paper in 1803 and it was simply a hard solid sphere. When Thomson discovered that tiny negatively charged electrons could be removed from an atom, leaving behind a positively charged ion, he produced the plum pudding model. This has the main body of the atom being composed of a nebulous positively charged material (the pudding ‘dough’) with electrons (the ‘plums’) randomly scattered through it. This theory was superseded by Rutherfords model of the atom, which has a tiny, charged nucleus carrying most of the mass of the atom, surrounded at some distance by the electrons, with most of the atom as empty space. Bohrs later refined the nuclear model to have electrons located around the nucleus in fixed orbits. Electrons could move from one orbit to another depending on the energy they gained or lost. This idea is still not an exact perfect model lol.
Tell me about Alpha particle scattering /how the plum pudding model was discarded
Geiger and Marsden, (students of Rutherford) undertook an experiment in which they aimed alpha particles at an extremely thin gold foil. Their expectations was that all the alpha particles would pass through, possibly with little deviation. The results generally followed this pattern - the vast majority passed straight through. However, a few alpha particles had their trajectories deviated by quite large angles. Some were even repelled back the way they had come. It was a shocking result based on the model of the atom at the time. Only conclusion Rutherford came to was that the model of an atom must be a small nucleus which had most of the charge and mass of the atom in it.
How was it determined the nucleus was positively charged
Rutherford could not tell for certain whether the charge on the nucleus was positive or negative. Within two years, Henry Moseley had published a law about x ray spectra that implied the nucleus must be overall positive.
Why were particles deflected in the alpha particle scattering experiment
If the alpha particle hit the nucleus instead of empty space, it would be deflected rather than pass straight through. It would be a little deflected, but still deflected by going through any part of the atom. As the positively charged nucleus would repel alpha particles, it was not deflected when going through just the vacuum.
Tell me about the alpha particle scattering and observations and conclusions
When the angle of deflection was between 0-10 degrees, most alpha particles pass straight through with little deviation. Shows most of he atom is empty space
When angle of deflection is 10-90 degrees, some alpha particles were deflected through a large angle, suggested a large concentration of charge was in one place.
When the angle was between 90-180 degrees, found a few alpha particles are repelled back towards the source side of the foil. Suggested most of the mass of the atom and a large concentration of charge is in a tiny, central nucleus.
Tell me about Chadwick’s discovery of the neutron
Rutherford determined that most of the atoms mass and all the positive charge was held in a very small nucleus in the centre, and that electrons were at the edge of the atom. The difference between the nuclear mass and known number of protons in it caused a problem though. Nuclei were too massive for the number of protons they contained. Rutherford suggest that additional proton-electron pairs, bound together, formed thus extra mass in the nucleus.
In 1930 it was found that alpha particles striking beryllium would cause it to give off an unknown radiation. Difficult to detect, this unknown, uncharged radiation could knock protons out of paraffin and these were detected by a Geiger-Müller tube.
People tried to explain the unknown radiation as gamma rays, but as these rays have no mass, this was a breach of the conservation of momentum. James Chadwick repeated the experiments using other target materials as well as paraffin. By considering momentum transfer and conservation of kinetic energy in the collisions between the particles, Chadwick concluded that the beryllium radiation was a neutral particle which had a mass about 1% more than that of a proton.
In 1932, he published a proposal for the existence of this new particle, which he called a neutron, and in 1935 we was awarded the Nobel prize for this discovery 🏳️🌈🌠🌠🌸🤩
Paraffin is an oily/waxy material, alpha particles were fired at beryllium, and neutrons were shot out of the beryllium and passed though a layer of paraffin and to a detector
What are nucleons
The nucleus contains two types of particles: protons and neutrons. Collectively, these particles, when in a nucleus are known as nucleons.
What does the number of protons in a nucleus determine
Determines which element the atom will be.
What is the periodic table
The periodic table is a list of the elements ordered according to the number of protons in each atoms nucleus. This number is called the proton number.
What is the proton number
Number of protons in an atoms nucleus
What’s the proton number also known as
Atomic number (Z)
What are isotopes
The number of neutrons can vary, and we call atoms of the same element with different numbers of neutrons, isotopes.
For small nuclei, up to about atomic number 20 (which is calcium), the number of neutrons in the nucleus is generally equal to the number of protons.
Why are more nurturing needed in atoms above atomic number 20
Above atomic number 20, to be stable, more neutrons than protons are generally needed in the nucleus. The neutrons help bind the nucleus together as they exert a strong nuclear force on other nucleons, and they act as a space buffer between the mutually repelling positive charges of the protons. The buffering action means that as we progress through the periodic table to larger and larger nuclei, proportionately more and more neutrons are needed. By the time we reach the very biggest nuclei, there can be over 50% more neutrons than protons.
How do we write chemical symbols
Eg radium has symbol Ra
Above ‘Ra’ is the mass number (nucleons + protons) and below is the proton number.
How can we work out neutron number (useful for isotopes)
Mass number - proton number = neutron number
What’s the nucleon number also known as
The mass number (A)
What is the nucleons number
Refers to the total number of nucleons - neutrons and protons - in a nucleus.
So to find the number of neutrons, we must subtract the atomic number from the mass number.- this is the neutron number.
What’s the quantum mechanical model of an atom
In the 1920s, Werner Heisenberg altered the model of the atom, which had electrons in orbits like planets in a solar system. This uncertainty principle holds that we cannot know the exact position and velocity of anything at a given moment. Instead of specific orbits, his new version of the atom has regions around the nucleus in which there is a high probability of finding an electron, and the shapes of these ‘probability clouds’ represent what we currently refer to as the electron orbits.
Define the plum pudding model
It’s a pre 1911 model of the atom, in which the main body of the atom is composed of a nebulous positively charged material (the pudding dough) with electrons (the plums) randomly scattered through it.
Define nucleons
Nucleons are any of the protons and neutrons comprising a nucleus
Define isotopes
Isotopes are atoms of the same element with different numbers of neutrons in the nuclei.
Define the proton number
The proton number is the number of protons in the nucleus of the atom
Define the atomic number
The atomic number is an alternative name for the proton number
Define the strong nuclear force
The strong nuclear force is an extremely short range force between hadrons (such as protons and neutrons)
Define the nucleon number
The nucleon number is the total number of all neutrons and protons in a nucleus
Define the mass number
The mass number is an alternative name for the nucleon number
Define the neutron number
The neutron number is the total number of neutrons within a given nucleus.
What do free conduction electrons in metals need if they are to escape the surface of the metal structure
A certain amount of energy. This energy can be supplied by a beam of photons, as seen in the photoelectric effect.
The electrons can also gain enough energy simply through heating of the metal. The release of electrons from the surface of a metal as it is heated is known as thermionic emission.
What are the two ways free conduction electrons in metals can escape the surface of the metal structure
Photoelectric effect
Thermionic emission
If, when electrons escape the surface of a metal via the photoelectric effect or thermionic emission find themselves in an electric field, what will happen
The electrons will be accelerated in the electric, moving in the positive direction. The kinetic energy they gain will depend on the p.d, V, that they move through according to the equation:
E(subscript k) = eV
Where e is the charge on an electron and V is potential difference
How can you find the velocity of an electron in an electric field after escaping from the surface of a metal
E(subscript k) = eV
Find kinetic energy, as KE = 1/2 mv^2
And solve for v
What is a cathode ray and how can it be produced
Using thermionic emission to produce electrons, and applying an electric field to accelerate them, we can generate a beam of fast moving electrons, traditionally known as a cathode ray.
This beam of electrons will be deflected by the force produced on it if it passes through a further electric field or a magnetic field. If a fast moving electron hits a screen painted with a certain chemical, the screen will fluoresce - it will emit light.
These are the principles by which cathode ray oscilloscopes (CROs) and old style televisions operate. The electron beam in a CRO is moved left and right, up and down, by passing the beam through horizontal and vertical electric fields. These are generated by electric plates so the strength and direction can be altered. Thus the point on the screen which is emitting light can be changed quickly and easily.
What will happen when electron beams are fired at a crystal
They will produce scattering patterns that can tell us about the structure of the crystal.
Unlike the patterns found by Geiger and Marsden in Rutherfords alpha particle scattering experiments, it was shown by Davisson and Germer that an electron beam can produce a diffraction pattern.
It was shown that an electron beam can produce a diffraction pattern, what did this provide evidence for
This provided the experimental evidence to prove a novel theory that had been suggested 3 years earlier by Louis de Broglie. He was bemused that light could be shown to behave as wave in some situations and as a particle in other circumstances.
By taking an analogous mathematical route to that of finding the momentum of a photon, de Broglie had proposed that the wavelength, lamder, of a particle could be calculated from its momentum using the expression:
Lamder = h/p
Where h is the Planck constant and p = mv
When an electron beam was fired to produce a diffraction pattern, when could this only happen
The Diffraction pattern was obtained when a cathode ray hit a crystal could only be produced if the electrons in the beam had a wavelength that coincided with the de Broglie wavelength. As a consequence, Louis de Broglie was awarded the 1929 noble prize for physics.
How can we investigate electron diffraction
You may have the equipment to observe electron diffraction. By measuring the radius of the circular pattern for a certain accelerating voltage, you can perform a calculation to confirm de Broglies hypothesis.
The idea of electrons acting as waves has allowed scientists to study what?
The structure of crystals, along similar lines to X ray crystallography. When waves pass through a gap which is about the same size as their wavelength, they are diffracted - they spread out.
What does the degree of diffraction spreading depend on
The ratio of the size of the gap to the wavelength of the wave. If a beam of electrons is aimed at a crystal, the gaps between atoms in the crystal can act as a diffraction grating and the electron waves produce a diffraction pattern on a screen. measuring the pattern allows the spacings between the atoms to be calculated.
How does electric diffraction and alpha particle scattering both highlight the idea that we can study the structure of matter by probing it with beams of higher energy particles?
The more detail - or smaller scale - the structure to be investigated has, the higher energy the beam of particles need to be. This means that very high energies are needed to investigate the structure of nucleons, as they are so incredibly small. Accelerating larger and larger particles to higher and higher energies has been the aim of particle physicists since Thomson discovered the electron in 1897.
Define the photoelectric effect
It’s the phenomenon in which electrons are released from a metal surface as a result of its exposure to electromagnetic radiation.
Define thermionic emission
It’s the phenomenon in which electrons are released from a metal surface as a result of its exposure to thermal energy.
Define cathode ray
Beam of electrons
