Properties of the Elements Properties of an element are
sometimes classed as either chemical or physical. Chemical
properties are usually observed in the course of a chemical
reaction, while physical properties are observed by examining a
sample of the pure element. The chemical properties of an
element are due to the distribution of electrons around the
atom's nucleus, particularly the outer, or valence, electrons;
it is these electrons that are involved in chemical reactions. A
chemical reaction does not affect the atomic nucleus; the atomic
number therefore remains unchanged in a chemical reaction. Some
properties of an element can be observed only in a collection of
atoms or molecules of the element. These properties include
color, density, melting point, boiling point, and thermal and
electrical conductivity. While some of these properties are due
chiefly to the electronic structure of the element, others are
more closely related to properties of the nucleus, e.g., mass
number. The elements are sometimes