Chapter 2

Pure substance. Law of constant composition?

Matter that has a fixed composition and distinct properties. Includes elements and compounds.

Law of constant composition: the elemental composition of a pure compound is always the same. Water is always H₂O.

Elements consist of ______ which consist of ______, ______, and ______

Atoms, protons (+), electrons (-), and neutrons.

What holds electrons in the atom?

• Electrostatic charge: opposite charges attract.

• The density (mass) of the nucleus develops a gravity force to pull the electrons in (similar to our universe).

• The electrons are moving around the nucleus always (centripetal force keeps them in orbit so they don’t crash).

Dalton’s Atomic Theory of Matter

1. Each element is composed of extremely small particles called atoms atoms are the building blocks of matter).*

2. All atoms of a given element are identical, the atoms of different elements are different.

3. All atoms of a given element are not changed into different types of atoms by chemical reactions, atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms.

* 1850s → scientists began to discover that the atom is composed of smaller particles called subatomic particles; figured out that the structure of the atom was somehow related to electrical charge.

** Leads to the law of constant composition → in a given compound the relative numbers and kinds of atoms are constant.

Isotopes

Atoms of a given element that differ in the number of neutrons, and consequently mass. The average atomic mass of an element takes into account the mass and relative abundance of each elemental isotope.

E.g. carbon has 6 protons and electrons but can have 5, 6, 7, or 8 neutrons.