1. The Structure of the Atom
Chapter 4
The Structure of the Atom

2. Atomic Theory of Matter
The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.
Figure 2.1 John Dalton ( )

3. Dalton’s Atomic Theory
Elements are made up of atoms
Atoms of each element are identical. Atoms of different elements are different.
Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom.
Chemical reactions are rearrangement of atoms. Atoms are not created or destroyed.

4. Thomson’s Model Discover the electron. (Cathode Tube)
Said the atom was like plum pudding.
A bunch of positive stuff, with the electrons embedded throughout.

5. Cathode Ray Experiment

6. Rutherford’s Experiment
Used uranium to produce alpha particles.
Aimed alpha particles at gold foil by drilling hole in lead block.
Since the mass is evenly distributed in gold atoms alpha particles should go straight through.
Used gold foil because atoms could be made very thin.

7. Florescent
Screen
Lead block
Uranium
Gold Foil

8. What he expected

9. Because, he thought the mass was evenly distributed in the atom.

10. What he got

11. Gold Foil Experiment

12. The Result
Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.
Most of the volume of the atom is empty space.
Figure 2.12

13. Atomic Structure
Atoms are composed of: 1. 2. 3.

14. Atomic Structure Particle Location Mass Proton Inside nucleus
1.673 x g
Neutron
1.675 x g
Electron
Outside nucleus
9.109 x g

15. Niels Bohr
Discovered that electrons occupy specific rings around the nucleus, while orbiting around it.
These specific rings were moving outwards based on energy.

16. Atomic Number Number of protons in the nucleus
Can be found of top of the symbol
Every element has a different atomic number since every element has a different number of protons in the nucleus (it is the atom’s fingerprint)

17. A NEUTRAL ATOM HAS THE SAME NUMBER OF PROTONS AND ELECTRONS

18. Mass Number Total number of protons and neutrons in the nucleus
Cannot be found on the periodic table
# of neutrons = mass # - atomic #

19. Shorthand Notation X – Symbol A – Mass number (protons + neutrons)
Z – Atomic number (protons)

20. Isotopes
ISOTOPES ARE ATOMS WITH THE SAME NUMBER OF PROTONS (AND ELECTRONS) BUT DIFFERENT NUMBER OF NEUTRONS

21. Isotopes The element hydrogen has 3 isotopes
H ( __ proton, __ electron, __ neutron)

22. Isotopes ISOTOPES OF AN ELEMENT ARE CHEMICALLY THE SAME SINCE
THE ATOMS’ CHEMICAL
PROPERTIES ARE DETERMINED
BY THE NUMBER OF
ELECTRONS.

23. Average Atomic Mass This is an average of the isotopes’ masses
Can be found below the symbol
To calculate the atomic mass of an element:
Atomic mass = (abundance x mass) + (abundance x mass) + …
The units are amu, atomic mass unit

24. Average Atomic Mass Naturally occurring chlorine is 75.78% Cl-35,
which has an atomic mass of amu,
and 24.22% Cl-37, which has an atomic
mass of amu. Calculate the average
atomic mass of chlorine.

25. Ions
Ions are atoms or groups of atoms that have a positive or negative charge
Ions are formed when an atom or groups of atoms gains or loses ELECTRONS

26. Cations Metallic elements tend to form ions by
LOSING ONE OR MORE ELECTRONS
What charge will metallic elements have?
Ions with a _________ charge are known as
CATIONS

27. Anions Nonmetallic elements tend to form ions by
GAINING ONE OR MORE ELECTRONS
What charge will nonmetallic elements
have?
Ions with a _________ charge are known as
ANIONS

28. REMEMBER
THE NUMBER OF PROTONS IN THE NUCLEUS DETERMINES THE IDENTITY OF AN ATOM.

29. Chemical vs. Nuclear
During a CHEMICAL reaction only electrons are exchanged, therefore the atom’s identity remains unchanged.
During a NUCLEAR reaction an atom of an element may change into the atom of another elements.

30. Nuclear Reactions
An atom with an unstable nucleus is a radioactive atom.
A radioactive atom emits radiation because it is unstable.
The unstable nucleus loses energy by emitting radiation in a spontaneous process called radioactive decay.

31. Types of Radiation Property Alpha Beta Gamma Composition Symbol Charge
Helium
Nucleus
Electron
High-energy electromagnetic radiation
Symbol α β γ
Charge 2+ 1-
Penetration
Low
Medium
High
Shielding
Paper, clothes
Metal foil
Lead, concrete (incomplete)