1. Matter Unit
Structure of an Atom

3. What is an atom?
Atom: the smallest unit of matter that retains the identity of the substance

4. The Evolution of atomic structure

5. 460 B.C. – Democritus
Democritus believed that all matter was composed of atoms that could be divided no further.

6. 1800 – John Dalton Dalton's Atomic Theory:
Matter is composed of small particles called atoms.
All atoms of an element are identical, but are different from those of any other element.
During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.
Atoms always combine in whole number multiples of each other. For example, 1:1, 1:2, 2:3 or 1:3.

7. Thomson’s Raisin in the Pudding model of the atom
1897- J.J. Thomson
Thomson’s Raisin in the Pudding model of the atom
He discovered the electron and proposed a model for the structure of the atom.
Positive Sphere
Negative Charges

8. Discovery of the Electron

9. 1910 – Ernst Rutherford

10. Rutherford’s Gold Foil Experiment

11. Rutherford’s Findings
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

12. 1912 – niels Bohr Bohr’s Finding: Quantized energy levels
Electron moves in a circular orbit
Electron jumps between levels by absorbing or emitting photon of a particular wavelength

13. How exactly are the particles arranged?
Bohr Model of the atom:
All of the protons and the neutrons
The 3rd ring can hold up to 8 e-
The 1st ring can hold up to 2 e-
The 4th ring and any after can hold up to 18 e-
The 2nd ring can hold up to 8 e-

14. What does carbon look like?
Mass # = 12 atomic # = 6
p+ = 6 no = 6 e- = 6
6 p and 6 n live in the nucleus

15. Discovered the Neutron.
James Chadwick
1932
Discovered the Neutron.

16. Quantum Mechanical Model
Electrons are located in specific energy levels.
There is no exact path around the nucleus.
The model estimates the probability of an electrons position

17. The Electron Cloud
The electron cloud contains the electrons.

19. Modern Atomic Theory
Element have a characteristic average mass which is unique to that element.
Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!
All matter is composed of atoms
Atoms of any one element differ in properties from atoms of another element

20. Atomic Structure Atoms are composed of 2 regions:
Nucleus: the center of the atom that contains the mass of the atom
Electron cloud: region that surrounds the nucleus that contains most of the space in the atom
Nucleus
Electron
Cloud

21. Atomic Structure The nucleus contains 2 of the 3 subatomic particles:
Protons: positively charged subatomic particles
Neutrons: neutrally charged subatomic particles
The 3rd subatomic particle resides outside of the nucleus in the electron cloud
Electron: the subatomic particle with a negative charge and relatively no mass

22. Atomic Particles e- p+ n0 Particle Charge Mass # Location symbol
Electron -1
Electron cloud e-
Proton +1 1
Nucleus p+
Neutron n0

23. How do these particles interact?
Protons and neutrons are in the tiny positively charged nucleus accounting for most of the mass of the atom
The negatively charged electrons are small and have a relatively small mass but occupy the larger volume of space outside the nucleus

24. How do the subatomic particles balance each other?
In an atom:
The # of protons = The # of electrons
If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral
The neutrons have no charge
they do not have to equal the number of protons or electrons

25. The number of protons identifies the atom.
An atom with 6 protons = Carbon
An atom with 8 protons = Oxygen
An atom with 20 protons = Calcium

26. He ATOMIC STRUCTURE 2 4 Atomic number Atomic mass
the number of protons in an atom 4
Atomic mass
the number of protons and
neutrons in an atom
# of electrons = # of protons
# of neutrons = Atomic mass – # of protons

27. Determine the number of subatomic particles in the following:
Cl has a mass # of 35 and an atomic # of 17
p+ = 17, no = 18, e- = 17
K has a mass # of 39 and an atomic # of 19
P+ = 19, no = 20 e- = 19