How can you describe the arrangement of sodium ions and chloride ions in a crystal of sodium chloride

Sodium chloride is an ionic substance, it is made up of both cations and anions. These ions combined together to form the crystal lattice structure of sodium chloride. In the crystal lattice, each cation of sodium is surrounded by six chloride anions and each chloride ion is in turn surrounded by six sodium ions. The array of ion merge together to form a face centered cubic structure. These ions are held together by strong electrovalent bond.  

out of these options:

ability to burn paper

pure sodium reacting violently with water

the boiling point of water

helium does not tend to react with anything

i would say this is a physical property:

helium does not tend to react with anything

why? :

i said helium does not tend to react with anything because well, it reacts to ! but ability to burn paper, pure sodium reacting violently with water, & the boiling point of water are chemical or

physical properties: the properties of a substance that can be observed without changing the chemical makeup of the substance

chemical properties: properties of a substance relating to the chemical nature and reactivity of the substance

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