Matter

Origins of the Idea

• Democritus (~400 BCE)– Matter is composed of atoms– Atoms are eternal, invisible, indivisible,

and incompressible• Greeks believed in 3 phases or

states of matter– Solid, liquid, gas– We now add plasma as the fourth

phase

Matter in the Universe

• 99% of the known matter in the universe is Hydrogen and Helium

• The majority of the known matter is in the form of a plasma in stellar interiors

• The average density of the known matter in the universe is such that 1 gram would occupy a volume 30 x that of Earth

• The location and characteristics of most of the matter in the universe is unknown – dark matter

Antimatter

• A matter particle and an antimatter particle are the same with the exception of charge and magnetic properties (which are opposite).

• Naturally formed and man-made.• When matter and antimatter particles

collide, they annihilate each other.

Elements

• Fundamental forms of matter

• Can’t be broken apart by normal

means

• 92 occur naturally on Earth

• They are the same everywhere

What Are Atoms?

• Smallest particles that retain the distinguishing

physical and chemical properties of an element

• Made up of subatomic particles:– Protons (+)

– Electrons (-)

– Neutrons (no charge)

Hydrogen Helium

electron

proton

neutron

Examples of Atoms

Chemical Bonds, Molecules, & Compounds

• Bond is union between electron structures of atoms

• Atoms bond to form molecules

• Molecules may contain atoms of only one element - O2

• Molecules of compounds contain more than one element - H2O

Atomic Number

• Number of protons

• All atoms of an element have the same atomic number

• Atomic number of hydrogen = 1

• Atomic number of carbon = 6

6C12 6C13 6C14

Mass Number

Number of protons

+Number of neutrons

Isotopes vary in mass number

6C12 6C13 6C14

Isotopes

• Atoms of an element with different numbers of neutrons (different mass numbers)

• Carbon 12 has 6 protons, 6 neutrons

• Carbon 14 has 6 protons, 8 neutrons

6C12 6C13 6C14

Chemical Origins

• Most chemical elements were created after the Big Bang in subsequent generations of stars.

Nucleosynthesis:

primordial (H, He, Li)

stellar (He -> Fe)

explosive (Co -> …)

Radioisotopes - Radioactivity• Have an unstable nucleus that emits energy and

particles a particles – 2 protons and 2 neutrons b particles – an electron g rays – a high energy photon

• Radioactive decay transforms radioisotope into a different element

• Decay occurs at a fixed rate

• Because decay happens at a fixed rate, you can use it for age dating! (C14 has a half-life of 5,700 years.)

The Age of the Solar System

Ages of rocks can be measured through radioactive dating:

Measure abundance of a radioactively decaying element to find the time since formation of the rock

Dating of rocks on Earth, on the Moon, and meteorites all give ages of ~ 4.6 billion years.