Ions

An ion is a particle that is similar to an atom but has more or less electrons than protons in the nucleus. In most cases, ions are present in compounds that have a metal and a non-metal (like NaCl, common table salt), but can also be present in hot gases or plasmas, like the Sun.


If an ion has more electrons than protons, i.e. it is negatively charged, we call that an anion. In the above example (NaCl), the chlorine ion has one more electron than proton (18 electrons vs. 17 protons) and therefore, has a charge of -1 e (-1.602e-19 coulombs). If the ion has less electrons than protons, it is a cation. The sodium ion in NaCl has one less electron than proton, so has a charge of +1 e.


Why are ions are important? They are found in the spectra of stars, in nebulae, and as stated above many compounds that we use everyday. In the Sun or any star, the gas is so heated, that it can strip electrons from the atoms, ionizing the gas, converting it into a plasma.


There are some elements that do not ionize easily. We call these elements inert and they are found on the far right of a periodic table. These are the noble gases: Helium, Neon, Argon, Krypton, Xenon, and Radon. This does not mean that they cannot be ionized. Under normal temperatures and pressures, the outer electrons are not readily removed from their orbits. However, under extreme temperatures and pressures, electrons can be stripped from the outer shells. In fact, the alpha particle is the bare nucleus of a helium atom with a charge of +2 e. We find helium ions in the core of the Sun, as it is the final product of the proton-proton chain. Electrons do not easily combine with the bare nucleus to form a stable helium atom.


Next time, we will learn about ion engines and how they may be a future propulsion system for solar system travel.

Curvature of the Universe

Last time, we talked about how the universe might end since we really don't know what exactly will happen. There is not enough information for us yet to tell us whether the universe will come collapsing back in on itself or if it will just continue on expanding forever.


What do we know? We do know that the universe is nearly flat. By comparing the overall mass/energy density of the universe to what is called the critical density, we see that the ratio between the two (actual/critical) is very nearly one. As mentioned last time, a ratio less than one will result in an open universe, a ratio of one gives a flat universe, and a ratio greater than one will end up with a closed universe.


Both open and flat universes result in expansion continuing on forever, though a flat universe will asymptotically approach a physical limit for the size of the universe. A close universe will reach a maximum, then as gravity takes over, will begin to pull all matter and energy back towards each other.


Looking at the picture below, we see how the three types of universe relate to each other. The circled area is about where we are now, and you can see how it is difficult to tell if we are in an open universe, a flat universe, or a closed universe.

We can also describe the three different universes in reference to their shapes, which will go into more detail for the three. A flat universe is considered to have zero curvature, hence, it is flat like a piece of paper. Open universes have a negative curvature, like a saddle. Closed universes are said to positive curvature, like the surface of a sphere. Negatively curved space and positively curved space can lead to some strange phenomena which we will learn more about later.