Final answer:
A neutral sodium atom has 11 protons and electrons, and when it forms a cation, it loses one electron. An element with similar reactivity is potassium, which is also an alkali metal with one valence electron. Sodium forms salts such as NaCl by combining with anions in a 1:1 ratio.
Step-by-step explanation:
An element that would most likely have the same chemical reactivity as sodium (Na), which has an atomic number of 11 and an atomic mass of 23.0 atomic mass units (amu), is another element from the same group in the periodic table, specifically the alkali metals group. Elements in the same group have similar chemical properties because they have the same number of valence electrons. For sodium, with 11 protons, 12 neutrons, and 11 electrons, the element with a similar reactivity pattern would be potassium (K) which also has one valence electron just like sodium. When looking at a neutral sodium atom, it contains 11 protons and 11 electrons. If it loses one electron to become a sodium cation (Na+), it has 11 protons and 10 electrons, leading to a +1 overall charge, just as a potassium cation (K+) would also have when it loses its single valence electron. An example of sodium's reactivity is its role in forming table salt, NaCl, where it exists as sodium cations combined with chloride anions in a 1:1 ratio.