Biology Ch# 2.1

All matter is composed of these

In all atoms

positive charge

One or more in each atom

negatively charged

moving in a cloud around nucleus

one or more found in nucleus

positively charged

one or more are found in an atom's nucleus

no charge

quantity of matter present

> mass = > quantity of matter

unit for measurement of a mass of a proton

(atomic mass unit)

Dalton

1.7 x 10^-24 grams

9 x 10^28 grams

pure substance

contains only one kind of atom

have certain properties and characteristics that distinguish them

Atomic # (on top in periodic table)

On top for ²H

Protons + Neutrons

# at bottom in periodic table

Mass # - Proton #

top number (²H) - bottom # (₁H)

Different version of an element that has the same # of protons but a different # of nuetrons

stable associations that most atoms of living things are organized

The region where the electrons are found

• First, S - 2
• Second, P - 8
• Third and beyond - 8

determines bonds

outermost electron shell

The tendency to have eight electrons in the Valence shell

creates stability

an atom that cannot react with other atoms