1. Atoms

2. Atoms and Elements The basic units of matter are called atoms
Atoms cannot be broken down into smaller pieces by any common methods of separating matter
Atoms are the building blocks of the universe

3. Models of the Atom All atoms share the same basic structure
With new observations or experiments, the model had to be changed over the past 200 years

4. Democritus
In 400 BC, a Greek philosopher named Democritus hypothesized that everything was made up of tiny particles
He called these particles “atomos”
“Atomos” was Greek for cannot be divided
He said that these “atomos” were empty spheres

6. Dalton’s Model
In the early 1808, John Dalton performed experiments with gases and found that matter was made up of tiny, indivisible particles
Dalton observed that the same amounts of hydrogen and oxygen always combined to form a given amount of water
He reasoned that each element must be made of its own unique kind of particle and that these particles combine in simple ways
He called these particles atoms and pictured them as tiny, solid spheres

7. Dalton’s Atomic Theory
All matter is composed of tiny indivisible particles called atoms
Atoms of each element are exactly alike
Atoms of different elements have different masses
Atoms of different elements can join to form compounds

9. Thomson’s Model
In the late 1897, J.J. Thomson discovered that atoms were not just simple, solid spheres
Atoms contained even smaller, subatomic particles that were very small and were negatively charged

10. Thomson’s Experiment
Thomson discovered these particles called electrons while experimenting with a glass vacuum tube containing metal electrodes
When he connected the tube to a source of high-voltage electricity, glowing green rays appeared inside
He discovered that electric charges bent the rays which made him infer that they were made of charged particles
Thomson knew that atoms are electrically neutral so he reasoned that an atom must contain enough positive charge to balance the negative charge of the electrons

12. Rutherford’s Model
By the early 1900s, scientists knew that the positive charge of an atom comes from subatomic particles called protons
A proton is a positive particle with a mass much greater than that of an electron
In 1911, Ernest Rutherford set out to test this theory

13. Rutherford’s Experiment
Rutherford aimed a beam of positively charged particles at a sheet of gold foil only a few atoms thick
Most of the particles went through the sheet but some bounced back
The only explanation for this discovery was that the particles had struck objects larger than single protons
Rutherford reasoned that the protons are concentrated in small area at the center of the atom called the nucleus

15. Bohr’s Model Niels Bohr modified Rutherford’s model in 1913
He proposed that each electron in an atom has a fixed amount of energy and this energy keeps an electron moving around the nucleus within a specific region called an energy level
In Bohr’s model, energy levels surround the nucleus in rings or shells

16. Bohr’s Model

18. Energy Levels
Electrons can move up and down energy levels and occupy space in each level but cannot occupy space in between levels
In order for an electron to move up a level, it must gain energy
In order for an electron to move down a level, it must lose energy

19. Orbitals
Regions in an atom where electrons are found are called orbitals
The s orbital is the simplest kind of orbital. It’s spherical shape enables it to surround the nucleus of an atom.
The p orbital is dumbbell-shaped and can be orientated 3 different ways in space.
There are 5 possible d orbitals and 7 possible f orbitals. They can each hold a maximum of 2 electrons

20. Sublevels 1st energy level: closest to the nucleus
contains only the s sublevel
1 orbital and only 2 electrons
2nd energy level: contains both s and p sublevels
1 s orbital and 3 p orbitals
can hold up to 8 electrons
3rd energy level: contains s, p, and d sublevels
1 s orbital, 3 p orbitals, and 5 d orbitals
can hold up to 18 electrons
4th energy level: contains s, p, d, and f sublevels
1 s orbital, 3 p orbitals, 5 d orbitals, and 7 f
orbitals
can hold up to 32 electrons
An electron in the outermost energy level of an atom is called a valence electron.

21. Electron Cloud Model
The electron cloud model, 1920, electrons dart about within an energy level in an ever-changing path
Most of this path falls into a region called an electron cloud

23. Chadwick’s Model
In 1932, James Chadwick showed that most atomic nuclei contain a third kind of subatomic particle, called a neutron
A neutron has about the same mass as a proton but has no electrical charge

25. Quarks
In 1964, physicists Murray Gell-Mann and George Zweig discovered a new subatomic particle called a quark
Quarks are subatomic particles that are found inside protons and neutrons
There are six types, “flavors”, of quarks:
Up, Down, Charm, Strange, Top, and Bottom