Why would you expect sodium (Na) to react strongly with chlorine (Cl)?
They both need to lose one electron. They both need to gain one electron. Sodium needs to lose one electron, and chlorine needs to gain one electron. Sodium needs to gain one electron, and chlorine needs to lose one electron
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I haven't taken chemistry in a long time, so the words I'll use may not be what your chemistry class is looking for, but I'll try to explain.
Na wants to lose an electron. It has 1 electron in its valence shell, and could form a stable bond by "giving" this to another element, giving it an overall positive charge, and forming an ionic bond.
Cl wants to gain an electron. It has 7 electrons in its valence shell. If it can, it wants to take an electron from another atom, achieving an overall negative charge, and form an ionic bond.
Thus, these two elements are made for one another! The Na will give its electron to the Cl. The Na will then be positively charged, and the Cl will be negatively charged. They will form an ionic bond, and go about their business as the molecule NaCl, sodium chloride, salt.