Atoms, molecules and light

Atoms:

Like hydrogen, all atoms consist of a nucleus (protons ^? and neutrons) surrounded by a number of electrons. In an atom in the ground state, the number of electrons is equal to the number of protons ^? . For example, the oxygen atom, with symbol O, has 8 protons ^? and 8 electrons. This number can be found in the periodic table below.

There are 118 chemical elements, representing all combinations of protons ^? , electrons, and neutrons. The heaviest are unstable and have very short lifetimes. A chemical element is a class of atoms with the same number of protons ^? . The number of neutrons, which varies, defines the isotopes. Of these 118 elements, 94 exist naturally and 24 are synthetic.

Molecules:

Molecules are assemblies of atoms.

Spectrum: 

Each chemical element, each molecule, can emit or absorb light, but only in certain very specific colors, called spectral lines ^? . Each element or molecule is therefore associated with a set of lines, which constitutes its own signature. Since the position of these spectral lines ^? is universal, analyzing the spectrum of a body, a star for example, by identifying these families of lines, we can determine which chemical elements or molecules it contains and in what quantity.

Depending on the configurations, these can either be emission lines (top of the figure) or absorption lines (bottom of the figure):

Spectrum of a body:

An object, a star for instance, emits radiation which comes from all the physical and chemical phenomena taking place within it (green curve in figure below). To study it, we separate the spectrum into a smooth curve (blue on the figure), which depends on the star’s temperature, and the absorption lines which depend on the composition of its atmosphere. The blue curve is called the black body spectrum.