- Atoms consist of a positively-charged nucleus of protons and neutrons surrounded by negative electrons. When an atom's protons and electrons balance out, it is electrically neutral. When atoms bond together into molecules, charges become more complicated. Negative charges in one atom may repel the negative charges of another. If the negative charges stay the same all around the molecule, it stays neutral and non-polar. If charges bunch up on one side, the atom becomes polarized.
- An atom's electrons occupy distinct areas called shells. While these shells have well-defined shapes, they are elastic. In a single atom, the shells assume symmetrical, rounded shapes. In molecules, they press on one another like tied-up balloons, causing bulges and distortions. When this forces negative charges to pile up on one side of the molecule, the other side becomes positively charged. This results in a polar molecule. When electrons shells stay symmetrically arranged, as in carbon dioxide, the molecule is non-polar.
- In water, a polar molecule, the hydrogen atom's electrons are pulled away from the hydrogen atom slightly, exposing its positive proton. The water molecule shows a positive charge on the hydrogen side and, negative charge on the oxygen side. The positive charges on one water molecule attract negative charges on neighboring water molecules, causing them to line up in distinct patterns. As water freezes, these patterns lock together in the crystal shapes of ice. Non-polar molecules do not have these mutual attractions.
- A microwave is an electromagnetic wave, having a wavelength of about 1 cm, and frequencies in the neighborhood of 1 to 100 GHz. The natural vibration frequencies of molecules lie in this range. Inside a microwave oven, microwaves cause polar molecules, such as the water in food, to vibrate and heat up, while leaving non-polar molecules, like a plastic container, alone.
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