element

Meaning: An element is defined by its atomic number Z, the number of protons in the nucleus. Atoms with different neutron numbers are isotopes of the same element, and atoms that have gained or lost electrons are ions of the same element. Ordinary chemical reactions only rearrange electrons, so they never change Z, which is why an element cannot be broken down chemically. Nuclear reactions can change Z and so can turn one element into another. A compound contains two or more elements in a fixed ratio and can be separated by chemical means.

Equation: A = Z + N, where A is the mass number (unitless count of protons plus neutrons), Z is the atomic number (unitless count of protons), and N is the neutron number (unitless count).

On the MCAT: You may be given a symbol such as ¹⁴₆C and asked for the numbers of protons, neutrons, and electrons. You may be asked to classify a sample as an element, compound, or mixture. Periodic trends (atomic radius, ionization energy, electronegativity) are predicted from an element's position by Z. In nuclear decay questions, you decide whether the product is a different element: alpha and beta decay change Z, while gamma emission does not.

Common confusion: Students call isotopes or ions different elements. Only a change in Z changes the element. Also, O₂ and O₃ are both the pure element oxygen (different molecular forms), while H₂O is a compound.