atomsAll matter is composed of very tiny particles, which Dalton called atoms. moleculeA molecule is a tightly bound combination of two or more atoms that acts as a single unit.

1 amu = x g

Element: Element: a substance (for example, carbon, hydrogen, and iron) that consists of identical atoms. There are 116 known elements. Of these, 88 occur in nature; the others have been made by chemists and physicists.

Monatomic elements:Monatomic elements: consist of single atoms; for example, helium (He) and neon (Ne). Diatomic elements:Diatomic elements: there are seven elements that occur as diatomic molecules: –H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2 Polyatomic elements:Polyatomic elements: some elements have three or more atoms per molecule: –O 3, P 4, S 8 –diamond has millions of carbon atoms bonded together to form one gigantic cluster.

Compound:Compound: a pure substance made up of two or more elements in a fixed ratio by mass. Formula of a compound:Formula of a compound: tells us the ratios of its constituent elements and identifies each element by its atomic symbol. –NaCl: the ratio of sodium atoms to chlorine atoms in sodium chloride is 1:1 –H 2 O: the ratio of hydrogen atoms to oxygen atoms in water is 2:1

Mixture:Mixture: a combination of two or more pure substances –The substances may be present in any mass ratio. –Each substance has a different set of physical properties. –Mixtures may be homogeneous or heterogeneous. –If we know the physical properties of the individual components of the mixture, we can use appropriate physical means to separate the mixture into its component parts.

Homo- or Heterogeneous mixture? Pure substance or compound?

Law of Conservation of Mass –Matter can be neither created or destroyed. –As Dalton explained, if matter is made up of indestructible atoms, then any chemical reaction just changes the attachments among atoms, but does not destroy the atoms themselves. OWL 2.3

Isotopes:Isotopes: atoms with the same number of protons but a different number of neutrons. –carbon-12 has 6 protons and 6 neutrons; –carbon-13 has 6 protons and 7 neutrons; –carbon-14 has 6 protons and 8 neutrons; Most elements found on Earth are mixtures of isotopes: –chlorine is 75.77% chlorine-35 (18 neutrons) and 24.23% chlorine-37 (20 neutrons).

Mass number:Mass number: the number of protons and neutrons in the nucleus of an atom. –the mass of the electrons in an atom is so small compared to that of its protons and neutrons that electrons are not counted in determining mass number. Atomic number:Atomic number: the number of protons in the nucleus of an atom. a carbon atom of this composition is referred to as carbon-12.

Atomic weight:Atomic weight: the weighted average of the masses (in amu) of the naturally occurring isotopes of an element. –example: chlorine is 75.77% chlorine-35 and 24.23% chlorine-37