Atomic Structure - NMSU Astronomy

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Atomic Spectra – What Do We See From Atoms?

What makes up an atom? An atom is composed of a heavy nucleus of protons (positively charged particles, written as p+) and neutrons (neutral particles, written as n0), around which orbit a cloud of extremely light electrons (negatively charged particles, written as e-) .

What defines an element? The number of protons in the nucleus of each atom.

  • Hydrogen (H) atoms have 1 proton.
  • Hydrogen atoms with 1 proton and 1 electron are neutral hydrogen (1H1).
  • Hydrogen atoms with 1 proton, 1 electron, and 1 neutron are a heavy isotope of hydrogen called deuterium (2H1).
  • If a proton is added to hydrogen, we then have a different element - helium (4He2).

  • Nomenclature: For each element, the superscript denotes the number of protons and neutrons, and the subscript the number of protons.
  • How many neutrons are there in neutral carbon (12C6)?
  • How many neutrons are there in the radioactive isotope called carbon-14 (14C6)?

    Legend: blue dot represents a proton, p, a nuclear particle with a positive charge; yellow dot represents a neutron, n, a nuclear particle with a neutral charge; brown dot represents an electron, e, a tiny particle which orbits the nucleus with a negative charge. Figure 1: hydrogen atom, with a central blue dot (proton) with a brown dot (electron) in orbit. Figure 2: helium atom, with two central blue dots and two central yellow dots (neutrons), with two brown dots in orbit.
    [NMSU, N. Vogt]

How does the make-up of the atom or element tell us what its spectrum will look like?

  • Electrons exist in stationary states within atoms, each defined by a discrete, unique level of energy. Only certain energy levels, like orbits with certain radii, are allowed.
  • Light, or radiation, emitted or absorbed by atoms as electrons move from one energy level to another can be thought of as a stream of quanta called photons. Each photon carries an energy E = h × v. We define these energy levels as follows, saying that the electron is in an excited state when it has extra energy (think of a child bouncing off the walls with excitement).

    A representation of a hydrogen atom, showing the nucleus (a blue dot, for a proton), surrounded by four energy levels shown as circles of increasing radii. The circle closest to the nucleus is labeled ground state, the next one as first excited level the next one as second excited level, and the last, furthest from the nucleus, as third excited level. An electrom (a brown dot) is shown in the ground state orbital circle.
    [NMSU, N. Vogt]
  • The ground state, the lowest energy level possible
  • The first excited state, the next highest allowed energy level
  • The second excited state, the next highest allowed energy level
  • The third excited state, the next highest allowed energy level
  • ...
  • Till the point at which the electron is no longer bound to the atom

  • An atom usually has the same number of protons and electrons. Because protons have a positive charge and electrons have a negative charge, it carries no charge in this state. When the atom loses (or gains) an electron we say that it is ionized, and it then carries an electrical charge.

Entropy tells us that all things are naturally drawn to the lowest possible energy state:

  • Logs and water roll downhill.
  • Bouncing balls slow to a halt.
  • People collapse into bed at night and find it hard to get up in the morning.
  • In the same fashion, hydrogen atoms tend to be in the ground state.
What happens when we add energy to a hydrogen atom, by bombarding it with photons?
  • Most of the photons zip right past without interacting with the atom.
  • But photons with just the right energy get absorbed by the atom.
  • In this case, right means that the energy of the photon corresponds to the energy level difference between allowed orbits in the hydrogen atom, and absorbed means that the energy of the photon will be taken into the atom (leaving the atom in a higher energy state).

    A representation of a helium atom, showing the nucleus (two blue dots for protons and two yellow dots for neutrons, clustered tightly together), surrounded by four energy levels shown as circles of increasing radii. There is an electron (shown as a brown dot) in each of the two lowest energy levels. A stream of photons is shown as horizontal arrows of various colors passsing through the atom. A blue arrow (high energy photon) points at the electron in the lowest energy level, and a black arrow points from the electron to the highest energy level to indicate that the epectron absorbs the photon and uses the energy to jump up three levels. A red arrow (low energy photon) points at the electron in the first excited energy level and a short black arrow points from this electron to the next energy level above it to indicate that the electron absorbs the photon and uses the energy to jump up one level. The figure annotation reads: most wavelengths of light pass through the atom unhindered; an electron absorbs a photon only if it contains exactly the amount of energy needed to jump between levels.
    [NMSU, N. Vogt]
  • A photon with frequency v will be absorbed by an atom if the energy of the photon corresponds to an energy level difference between allowed states in the atom.

What happens next?

  • Remember that entropy seeks the lowest available energy level for all things, so the electron which has been raised to an excited orbit will eventually drop back to the ground state.
  • Conservation of Energy, tells us that the energy difference between the excited state and ground state must appear somewhere when the electron makes the transition. It is emitted by the atom as a photon, with the same energy of the original one which was absorbed.

    The same helium atom shown in the previous figure is shown here. For the electron in the ground state, a second black arrow pointing from the highest energy level back to the ground state indicates that the electron first jumps up and then drops back down, and a second blue arrow pointing away from the electron shows the photon leaving the electron in a random direction. For the electron in the first excited state, a second black arrow pointing from the second excited state back to the first indicates that the electron first jumps up and then drops back down, and a second red arrow pointing away from the electron shows the photon leaving the electron, again in a random direction. The figure annotation reads: Certain photons are absorbed by electrons in the atoms. They are later re-emitted, as the electron drops back down.
    [NMSU, N. Vogt]

Here is a schematic diagram of the allowed orbits in a hydrogen atom. If you can answer the questions listed below, you've got the right idea!

    A hydrogen atom, shown as a proton in the nucleus (blue dot) with four energy levels shown as circles surrounding it. The difference in radius is shown to decrease with each additional energy level shown, indicating that the gap between levels is largest between the ground state and the first excited level and is smaller between the first and second excited levels, and smaller still between the second and third excited levels. An electron (brown dot) is shown in the ground state to emphasize that the energy levels can be populated by electrons. An arrow labeled A points from the ground state up to the first excited level; one labeled B points from third excited level down to the first excited level; one labeled C points from the third excited level down to the ground state; one labeled D points from the ground state up to the third excited level. These transitions (A through D) are those referred to in the text below this figure.
    [NMSU, N. Vogt]
  • Which transition(s) correspond(s) to the absorption of a photon? A & D
  • Which transition corresponds to the highest energy photon emitted ? C
  • Which transition corresponds to the shortest wavelength photon emitted? C
  • Which transition corresponds to the lowest energy photon absorbed? A
  • Which transition corresponds to the highest frequency photon emitted? C
Thanks to Mike Bolte (UC Santa Cruz) for the base contents of this slide.

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