Atomic Model and the Nucleus Flashcards
What is a model?
Scientific models are representations of objects, systems, or objects (can help scientists communicate their ideas, understand processes, and make predictions). They are used as tools for understanding the natural world, and they use familiar objects to represent unfamiliar things.
The atomic model
*See diagram
Dalton (1803 [all matter made of indivisible atoms]), Thomson (1904 [positive and negative charges]), Rutherford (1911 [the nucleus]), Bohr (1913 [energy levels; incorporates the subatomic theory of matter, which was starting to develop in the late 1800s]), Schrödinger (1926 [electron cloud model]). Don’t take Ruth’s beer, sis.
Our knowledge is still developing; we will continue to refine it as we better our technology (e.g. [improvements in instrumentation made it possible to understand the structure of the atom]).
Thomson’s cathode ray experiment
Created “cathode rays”, which were charged particles, and then sent the charged particles through oppositely charged plates and through a magnetic field. Showed that the “cathode ray” was deflected away from the negatively charged plate (so it was composed of negatively charged particles that could separate from the atom) and determined that the mass of the particles was much smaller than any known atom and was constant for different elements (this disproved Dalton’s theory that atoms are indivisible).
Rutherford’s gold foil experiment (*important)
Fired positively charged alpha particles (two protons and two neutrons [identical to a He nucleus]) at gold foil. Most of the alpha particles went through the empty space of the gold atom, but some of them hit the nucleus and were deflected. Results suggested that an atom was made mostly of empty space with the positive charges grouped together in a very small, dense nucleus).
Bohr’s quantized shell model
Theorized that Rutherford’s model had a problem with the placement of the electrons (if the electrons were stationary, then they would be attracted to the nucleus: if the electrons were spinning randomly around the nucleus, they would lose energy and spiral into the nucleus [which they don’t] because a charged particle moving on a curved path emits electromagnetic radiation).
His theory fixes this problem by requiring that the electrons move in “permitted orbits” where they don’t lose energy. The energy of an electron depends on the size of the orbit and it lower for smaller orbits; radiation can occur only when the electron jumps from one orbit to another; and the atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.
Current model (Schrödinger’s “electron cloud” model)
Is based on mathematical wave functions and describes the regions in space—or orbitals—where electrons are most likely to be found. Describes the probability that an electron can be found in a given region of space at a given time (tells us where an electron might be [not where it is], and allows the electron to occupy three-dimensional space). Electrons exist in clouds (in groups)?
Atomic structure
An atom consists of a positively charged dense nucleus composed of protons and neutrons. Negatively charged electrons occupy the pace outside the nucleus in 3D orbitals.
Mass number
The # of protons + # of neutrons
Symbol: A
Atomic number
The # of protons in the nucleus
*An atom is neutral, to the # of protons = # of electrons
*Each element has a fixed # of protons
Symbol: Z
Relative charge, mass
*Actual mass, charge in databook (1.627 x 10^-27 kg for proto and neutron: 9.109 x 10^-31 kg for electron [1.602 x 10^-19 C for proton and electron])
Mass: For proton and neutron, 1. For electron, 1/1836 (negligible).
Charge: For proton, +1. For electron, -1 (0 for neutron).
Isotopes
Atoms of the same element with the same atomic number and different mass number (neutron count differs). Chemical properties are determined by the # of electrons in the highest energy level (outer shell), and isotopes have the same electron config, so they have the same chemical properties.
Physical properties differ, though: rates of diffusion differ b/c they depend on the mass of the particles; nuclear properties such as radioactivity and the ability to absorb neutrons differ; and they have slightly different boiling points.
Radioisotopes (radioactive isotopes) are used in nuclear medicine for diagnosis and treatment; as tracers for biochem research (and with 3D imaging to detect cancer); and for historical dating.
Relative atomic mass
Weighted average mass of an atom, taking into account the masses of all the isotopes of an element relative to 12C (decided standard [gas would be hard to isolate/work with]; figured out mass, divided by 12 to get mass of protons [so if something has six protons, they divided it by six]).
Ar = (mass of isotope 1)(% isotope 1)/100 + (mass of isotope 2)(% isotope 2)/100 + …
- Whichever the # is closer to is the more abundant
- No unit b/c it’s relative to something else (for atomic mass, g/mol)
Symbol: Ar
Mass spectrometer
Can show the presence of isotopes of elements in a sample, abundance of each isotope (allowing calculation of relative mass [splits stuff up based on their mass]). Five processes that take place.
- Vaporization
Sample is injected into a chamber where it it heated and vaporized, producing gaseous atoms/molecules.
- Ionisation
High energy electrons fired from an electron gun knock out the electrons from the atoms in the gaseous sample, ionizing them. Positive ions are formed.
M(g) + e^- to M^+(g) + 2e^-
(or M(g) to M^+(g) + e^-)
Some doubly charged ions can be produce, but in smaller amts b/c more energy is required for the gun to knock out two electrons. Molecules may also be broken into many different fragments by the high-energy electrons from the gun. Mass not really changed.
