1. Unit 2
Atomic Structure
Read Chapter 4

2. Matter: Something that occupies space and has mass.
Atom: The basic unit of matter.

3. Greek philosopher Democritus believed all matter was made of extremely small particles. (About 400 B.C.)
He used the greek word Atomos, which means “uncut” or “indivisible”, to describe atoms.

4. John Dalton’s Theory – late 1700’s
All elements are made of atoms
Atoms of the same element have the same mass, and different elements have different masses
Compounds have atoms from more than one element
Atoms in particular compounds always combine in the same ratios

5. Joseph John (J.J.) Thomson
Cathode tube experiment

6. Thomson provided the first evidence that atoms were made of smaller particles.
Since an atom was neutral and he discovered negative particles he reasoned in had positive particles as well – Plum Pudding Model

7. J.J. Thomson’s Plum Pudding model
Dalton’s solid sphere model
J.J. Thomson’s Plum Pudding model

8. Ernest Rutherford
Gold Foil Experiment

10. He learned that protons have a positive charge and are located in the nucleus

11. Rutherford Atomic Model

12. The atom is made up of 3 subatomic particles:
Proton
Neutron
Electron

13. The nucleus (Protons & Neutrons) account for 99% of the atom’s mass.
Particle
Charge
Mass
Location
Proton 1
Nucleus
Neutron
Electron -1
Outside
The nucleus (Protons & Neutrons) account for 99% of the atom’s mass.

15. There are 92 naturally occurring elements.
The # of protons = the # of electrons
This makes atoms neutral in charge
Element: A pure substance made up of entirely one type of atom which cannot be broken down by chemical processes into smaller substances.
There are 92 naturally occurring elements.

16. 4 elements which make up 96% of living matter are:
Carbon
Hydrogen
Oxygen
Nitrogen

17. Each element has a one or two letter symbol. (Ex. Na – Sodium)
The Atomic number indicates how many protons are found in that element.
The Atomic mass indicates the average weight of the element.

19. Niels Bohr
Bohr agreed with Rutherford’s model of the nucleus but he focused on the electrons
The electrons moved at a constant speed in fixed orbits (energy levels) around the nucleus

21. Today we use the electron cloud model
Bohr was correct in assigning energy levels but incorrect that they moved like planets around the sun.
Today we use the electron cloud model

22. This model describes the most likely locations for electrons as they travel around the nucleus

23. Energy levels and orbitals are NOT the same thing!
Energy level is the range of distance electrons are from the nucleus
Orbital – a region of space around the nucleus where an electron is likely to be found
Video Clip

24. Each orbital can contain 2 electrons at the most.
Except for the first energy level, an energy level may have more than one orbital
The maximum number of electrons in an energy level is twice that of the number of orbitals

26. Electron Configuration is the arrangement of electrons in an atom
The most stable configuration is the one in which the electrons are in the orbitals with the lowest possible energies

27. An electron can move from one energy level to another as it gains or loses energy

28. When all electrons have the lowest possible energies the atom is said to be in its ground state
When an electron absorbs enough energy to move to an orbital with a higher energy it is referred to as an excited state

29. When excited electrons move to lower energy levels they may emit some energy as light

30. Don’t mess with Protons!
The number of electrons and neutrons within an element may change but not the number of protons.
Don’t mess with Protons!

31. Isotopes: Atoms of the same element that have a different number of neutrons.

32. Isotopes are named according to their weight (Ex. Carbon 14)

33. Uses of Radioactive Isotopes
Determine the ages of rocks and fossils
2. Treat cancer and kill bacteria
which cause food to spoil
3. Used as “tracers” for medical
purposes

34. This image, taken from medical scan of a 47-year old woman, shows leukemia present in the bone marrow before treatment (left) and after.

36. Ions: Positively or negatively charged atoms.
Ions form when electrons are given or taken away from another atom

37. Loses an electron = positive charge
This is called a cation B B
5 (+)
5 (+)
5 (0)
5 (0)

38. B B Gains an electron = negative charge This is called an anion 5 (+)
5 (0)
5 (0)

39. Chlorine Ion with 1 extra electron = Cl¯
Chlorine Ion with 2 extra electrons = Cl2-
Sodium Ion with 1 less electron = Na+
Sodium Ion with 2 less electrons = Na2+