1. Atomic Theories (Sections 4.1 & 4.3)

2. Ancient Greek Models
Democritus believed that all matter consisted of extremely small particles that could not be divided
Atoms
Means uncut or indivisible
Thought there were different types of atoms with specific properties
Smooth and round for liquids, rough and prickly for solids

3. Ancient Greek Models
Aristotle didn’t think there was a limit to the number of times matter could be divided
Thought that all substances were built up from only four elements
Earth
Air
Fire
Water
His theory was accepted for many centuries

4. Dalton’s Atomic Theory
Gathered evidence by measuring masses when elements combine and form compounds
Developed a theory to explain why the elements in a compound always join in the same way
Elements are like solid spheres
Made wooden spheres to represent the atoms of different elements

5. Dalton’s Theory
Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided
All elements are composed of atoms
All atoms of the same element have the same mass, and atoms of different elements have different masses
Compounds contain atoms of more than one element
In a particular compound, atoms of different elements always combine in the same way

6. Thomson’s Model of the Atom
Used an electric current to learn more about atoms
Electric current is a flow of charged particles
Objects with like charges repel and objects with opposite charges attract
Used a sealed tube of gas with two oppositely charged disks on either side
Cathode Ray Tube
A glowing beam appeared between the two disks
The beam bent toward a positively charged plate outside the tube

7. Thomson’s Model Thomson’s Model “Plum Pudding” model
The particles inside the beam were negatively charged and came from inside atoms
Thomson’s experiments provided the first evidence that atoms are made of even smaller particles
Revised Dalton’s theory to account for these subatomic particles
Thomson’s Model
“Plum Pudding” model
Negative charges are evenly scattered throughout an atom filled with a positively charged mass of matter
Like chocolate chip ice cookie

8. Rutherford’s Atomic Theory
Gold Foil Experiment
Used alpha particles and a screen of gold
Found that many particles were deflected more than 90 degrees, some straight back

9. Rutherford’s Model
Found that the positively charge of an atom is not evenly spread throughout the atom
It is concentrated in a very small central area called the nucleus
Nucleus is a dense, positively charged mass located in the center of the atom
According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus

10. Bohr’s Model of the Atom
Focused on the electrons
In Bohr’s model, the electrons move with constant speed in fixed orbits around the nucleus
Like planets around the sun
Each electron has a specific amount of energy and if it gains or loses energy the energy of the electron can change
Energy levels - the possible energies that electrons in a atom can have

11. Energy Levels Like steps in a staircase
As you move up or down you can measure how many steps you took
You can take one step up or jump up two steps down
Up or down you can only move in whole-step increments, never between steps
An electron in an atom can move from one energy level to another when the atom gains or loses energy
Can move up two energy levels if it gains enough energy
Can move down two energy levels if it loses enough energy
The size of the jump determines the amount of energy gained or lost

12. Evidence for Energy Levels
Scientists can measure the energy gained when electrons absorb energy and move to a higher energy level
They can measure the energy released when the electron returns to a lower energy level
The movement of electrons between energy levels explains the light you see when fireworks explode
Heat from the explosion causes some electrons to move to higher energy levels, when they move back they emit some energy as visible light
Different elements emit different colors of light

13. Electron Cloud Model
Electrons move in a less predictable way than Bohr thought
Electron cloud - a visual model of the most likely locations for electrons in an atom
Denser at locations where the probability of finding an electron is high
Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus

15. Atomic Orbitals
The electron cloud represents all the orbitals in an atom
Orbital - a region of space around the nucleus where an electron is likely to be found
Electron cloud is a good approximation of how electrons behave in their orbitals
Example - your location at school
If you placed a dot on a map every 10 minutes throughout the school day
Your dots would create a model of your orbital at school
The places with more dots are the places you are most likely to be
The places with few dots are the places you are least likely to be

16. Energy Levels, Orbitals, & Electrons
The level in which an electron has the least energy (lowest energy level) has only one orbital
Higher energy levels have more than one orbital
The first four energy levels of an atom
Max # of electrons in an energy level is twice the number of orbitals
Each orbital can contain two electrons at most

17. Electron Configurations
Electron configuration - the arrangement of electrons in the orbitals of an atom
The most stable electron configuration is the one in which the electrons are in the orbitals with the lowest possible energies
Ground state - when all the electrons in an atom have the lowest possible energies
Excited state - when an electron is in an orbital with a higher energy level, less stable than the ground state