# mass

```Atomic Structure
 History: because we cannot see atoms, scientists have developed
theories about their shape and structure; and, over time as we
have learned more, those theories have been improved
 Understanding structure helps us understand behavior
 Atoms are not the smallest particles of matter
 Atoms are made up of protons, neutrons, and electrons
 Basic structure: a nucleus surrounded by one or more electrons
 Nucleus: very small central core of an atom; made of protons and
neutrons; contains most of the mass of an atom
 Protons: positive charge, symbol is p+
 Neutrons: neutral (no charge), symbol is n
 Electrons: negative, symbol is e-, surround nucleus
 Electron cloud model: electrons move rapidly around nucleus in a
sphere-like shape; imagined to be like a cloud b/c e- can be
anywhere; lower E e- are closer to nucleus, higher E e- are
farther from nucleus; contains most of the volume of the atom
 # p+ = # e-  charges equal zero, making atom neutral
Protons
Neutrons
- About 1 amu
- Found in nucleus
- Make up most of
+ charge
no charge
the mass
- Made of quarks
Particles
in an
atom
X
X
- charge
cloud around nucleus
occupy most of the volume
Elements
Electrons
 Each element’s atoms are different from all other elements’
atoms
 An element is identified by the # of p+
 Atomic number = # p+
 Mass number = mass of nucleus = sum of p+ and n (round to
nearest whole number)
 # p+ = atomic number
 # n = mass number – atomic number
 # e- = # p+
Bohr Model
 Shows electrons moving in specific layers, or shells, rather than
randomly
 Each shell/energy level can hold only a certain number of e They fill in order:
o First shell
2
o Second shell
8
o Third shell
18
o Fourth shell
32
 Valence electrons – e- in the atom’s outermost energy level
 Drawing:
o Atomic # - 13
 # p+ = _____
o Mass # - 26.982
 # n = _____
 # e- = _____
```