1) Chemical properties. The chemical properties of an atom are determined by the number of protons and the number and arrangement of electrons.
Sodium (Na): 11 protons and 11 electrons. 1 out of the 11 electrons is valence electrons.
Oxygen (O): 8 protons and 8 electrons. 6 out of the 8 electrons are valence electrons.
Neon (Ne): 10 protons and 10 electrons. 8 out of the 10 electrons are valence electrons.
Potassium (K): 19 protons and 19 electrons. 1 out of the 19 electrons is a valence electron.
Bromine (Br): 35 protons and 35 electrons. 7 out of 35 electrons are valence electrons.
Sodium and potassium share one characteristic. Both elements have 1 valence electron.