Na is the symbol for the element sodium. Sodium has an atomic number of 11 and an atomic mass of 23 atomic mass units.
The number 11 indicates the number of protons in the nucleus, which in turn indicates the number of electrons in orbitals around the nucleus. The first level can hold 2 electrons; the next can hold 8. The valence electrons are in the unfilled orbital, which will be 1 electron.
It is the valence electrons that make an element react with other elements. Atoms react together to create a valence shell that is complete. Since sodium has 1 valence electron, it can lose it to an element that needs one electron in an ionic bond. Chlorine, which has 7 valence electrons, can gain 1 electron from sodium and will form the compound sodium chloride or table salt.
The link I have provided has a diagram showing how sodium and chlorine form salt. It also shows the change in charge on each atom. Since sodium loses one electron, it gets a 1+ charge, and since chlorine gains an electron, it gets a 1- charge.
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