Chemistry 2026 — Chapter 2

What is an atom?

The smallest unit of an element

What determines an element’s identity?

Number of protons (atomic number)

What is atomic number (Z)?

Number of protons

What is mass number (A)?

Protons + neutrons

Law of conservation of mass

Matter is not created or destroyed

Law of definite proportions

Same compound, same ratio of elements

Law of multiple proportions

Same elements, combine in small whole-number ratios

Proton charge and location

+ charge, in nucleus

Neutron charge and location

0 charge, in nucleus

Electron charge and location

– charge, outside nucleus

What is an isotope?

Same element, different number of neutrons

Why are isotopes still the same element?

Same number of protons

What is a cation?

Positive ion (lost electrons)

What is an anion?

Negative ion (gained electrons)

What is 1 mole?

6.022 × 10²³ particles

Mole relationships

grams to moles (molar mass), moles to atoms (Avogadro’s number)