1. What is matter... ? Aristotle (384-322 BCE): I will say
matter is infinitely divisible!
Democritus ( BCE): I maintain that matter is made of tiny indivisible particle, like the grains of sand on a beach! He called them atoms.
Today: we know the the ATOM is the smallest particle of matter. It cannot be devided chemically.

2. How small is an atom? A sheet of paper is about a million atoms thick
A drop of water can contain
as many as atoms.
Atoms are so small that
scientists came up with various
models to represent them... They
have changed over time!
CLIP!

3. John Dalton’s Atomic Theory (1808)
All matter is made of atoms (indivisible)
Atoms of an element are identical
Each element has different atoms
Atoms of different elements combine in constant ratios to form compounds
Atoms are rearranged in reactions

4. Models of the atom
Once it was accepted that an atom must exist, scientists set out to determine what it must look like.
Proposed models:
Dalton’s model (billiard ball)
Thompson’s Model (plum pudding)
Rutherford Model
Bohr-Rutherford Model

5. J.J. Thompson’s model
GAS DISCHARGE TUBE
The electron is one of the particles that make up an atom. In is negatively charged.

6. Red = neon
Green = argon
+ neon

7. J.J. Thompson’s model - 1904 Discovery of ELECTRONS Model
Method of discovery
Discovery of ELECTRONS
When an electric current is applied across the electrodes, a cathode ray is produced. A fluorescent material inside the tube illuminates the cathode ray.
These rays have a small mass and are negative.
Thompson noted that these negative subatomic particles were a fundamental part of all atoms.

9. Then we had x-rays: Röntgen 1895

10. Ernest Rutherford’s Model - 1911
Zinc sulfide screen
Thin gold foil
Lead block
path of invisible -particles (+)
Radioactive substance
Discovery of PROTONS + NUCLEUS
In 1911, Rutherford became interested in the effect of radioactivity on matter.
At that time, scientists had discovered that radioactive substances could emit three types of radiation: alpha (which is positively-charged), beta (which is negatively charged) and gamma (which is electrically neutral)
Rutherford wanted to learn more about the distribution of e- in atoms.
He bombarded a sheet of gold foil with a stream of alpha particles.
He expected that most of the alpha particles would pass easily through the gold foil because it was extremely thin (160 atoms thick – very much thinner than a sheet of paper).
He predicted that only a few of the alpha particles would be deflected slightly through contact with an electron.
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Knowing that like charges repel each other, he concluded that the entire positive charge of an atom must be concentrated in a very small area. Thus, a “nucleus” is positive & holds most of an atom’s mass.

11. WOW! – PROTONS! https://www.youtube.com/watch?v=yQP4UJhNn0I
-An atom is mostly empty space
-An atom contains a very dense and very small nucleus.
-The nucleus of an atom is positively charged.

12. The size of the nucleus!

13. Bohr found that electrons are found in specific areas called orbits!
-An atom is mostly empty space
-An atom contains a very dense and very small nucleus.
-The nucleus of an atom is positively charged.
-The different colour light is know as it’s electromagnetic spectrum

14. Bohr-Rutherford Model - 1922
Electrons orbit the nucleus in orbitals (OR SHELL) which corresponds to a level of energy
Electrons can jump to a higher orbital if receives energy (heated/electrical discharge)
The electron returns rapidly to its original orbit, releasing the absorbed energy in the form of light
Discovery of ORBITALS
Bohr added to Rutherford’s work:
Electrons orbit the nucleus in shells also called orbitals
Electrons can be bumped up to a higher shell if hit by an electron or a photon of light.
Rutherford and Bohr believed that there were neutral particles in the nucleus as well as protons, however the proof of the existence of neutron only came in 1932 by James Chadwick.

15. The Rutherford-Bohr model
-When an atom receives energy,
it becomes excited and can
jump to higher orbit for a short
period of time. When they return to
their original orbit, they release
energy in the form of light!

16. A side note..... Discovery of NEUTRONS
Why does the nucleus not explode, given that it is made up entirely of positively charged particles?
Discovery of
NEUTRONS
James Chadwick

17. The Nucleus Contains proton. Is positively charged.
The nucleus is a small part of the atom and is very dense.
Atoms are (in part) differentiated by their number of protons
The number of protons in an atom is called the atomic number.

18. 2, 8, 8, 2 = RULE!!! Around the Nucleus
Electrons circle the nucleus in orbitals (also called energy levels or shells).
There are up to 7 shells, labeled 1(closest to the nucleus) to 7 (furthest from the nucleus)
Every shell has a specific number of electrons. The order (for the first 20 elements) is 2,8,8,2
Shells may be completely full, partly full or completely empty.
Normally, the formula 2n2 determines how many electrons can fit in each shell (n = shell #)
2, 8, 8, 2 = RULE!!!

19. How atoms are represented in the Periodic Table
The Atomic Number (Z) = the number of protons.
The Symbol of the Element (E)
There are slightly different variations of each atom in nature. They differ in the number of neutrons. Because of this they have slightly different masses. The average of all these masses gives the atomic mass.
Do not talk about isotopes just yet.
The mass number is important for determining the number of protons – on next slide.
The atomic mass = average mass of the atom
Mass Number: Calculated by rounding the atomic mass to the nearest whole number. In this case the mass number would be 23.
Symbol : A