1. Atoms, Molecules, and Ions
Unit 2:
Atoms, Molecules, and Ions
Atomic Structure
Molecules and Molecular Compounds
Ions and Ionic Compounds
Names and Formulas of Compounds
Ionic

2. Atomic Structure Democritus (460-370 BC)
Theorized that all matter is composed of small indivisible particles called atomos (“not to cut”)
Ideas rejected by Greek thinkers of that time in favor of fire, earth, air, water, & heavenly ether as the basic elements
The idea that atoms are the fundamental building blocks of matter re-emerged in the 1800’s

3. Atomic Structure Dalton’s Atomic Theory (1803 – 1807)
1. Each element is composed of extremely small particles called atoms.
2. All atoms of a given element are chemically identical to each other;
Atoms of one element are different from the atoms of all other elements

4. Atomic Structure
3. During ordinary chemical reactions, atoms of one element cannot be changed into atoms of a different element.
Atoms are not created or destroyed.

5. Atomic Structure
4. Compounds are formed when atoms of more than one element combine.
A given compound contains atoms combined in definite whole number ratios.
Fe2O3 (rust)
H2O (water)
H2O2 (hydrogen peroxide)
These ratios are the same for all molecules of a particular pure substance.

6. Atomic Structure 100.0 g H2O Law of Conservation of Mass (or Matter)
Matter is neither created nor destroyed.
The mass of materials present after a chemical reaction is the same as the total mass before the reaction.
11.1 g H2
88.9 g O2
100.0 g H2O

7. Atomic Structure Law of Constant Composition
A given compound always contains the same relative numbers and kinds of atoms.
Methyl alcohol CH4O
Ethyl alcohol C2H6O
Isopropyl alcohol C3H8O
Methane always contains 25% H and 75% C

8. Atomic Structure
Dalton’s atomic theory was based on evidence gained in the macroscopic world (i.e. the lab).
Today, scientists can actually see individual atoms using special techniques.
See figure 2.2 of your text for a scanning tunneling micrograph of gallium arsenide

9. Atomic Structure Atoms are composed of subatomic particles Protons
positively charged subatomic particles found in the nucleus
Neutrons
neutral (uncharged) subatomic particles found in the nucleus
Electrons
very small, negatively charged subatomic particles found in a diffuse layer surrounding the nucleus

10. Atomic Structure Relative Particle Symbol Charge Mass
Proton p amu
Neutron n amu
Electron e x 10-4 amu
amu (atomic mass unit):
a unit used to express very small masses
x g

11. Atomic Structure Thompson’s “Plum Pudding” Model
Electrons are embedded in a positive sphere of matter just like raisins in plum pudding or blueberries in a blueberry muffin.

12. Atomic Structure
Rutherford (1909) disproved Thompson’s model with his famous scattering experiments.

13. Atomic Structure
Some alpha particles pass through the empty space in the atom.
Other alpha particles are deflected by the nucleus.
scattered

14. Atomic Structure Rutherford’s conclusions
Most of the mass and all of the positive charge of the atom are contained in a small space called the nucleus.
Most of the volume of the atom is empty space occupied by tiny negatively charged electrons.
There are as many negatively charged electrons outside the nucleus as units of positive charge inside the nucleus
the atom is electrically neutral

15. Atomic Structure Atoms can be visualized as a spherical particle with
tiny, positively charged, center (atomic nucleus) that contains most of the mass
diffuse outer layer that is negatively charged.

16. Atomic Structure Atomic nucleus
tiny, positively charged, center of an atom
contains protons and neutrons
contains most of the
mass of an atom

17. Atomic Structure
If all atoms are composed of protons, neutrons, and electrons, then why do different types of atoms have different properties?
The atoms of each element have a unique number of protons in the nucleus.
Atomic number: the number of protons in the nucleus
Carbon: 6 p (atomic # = 6)
Sulfur: 16 p (atomic # = 16)

18. Atomic Structure
Each atom contains the same number of protons and electrons.
atoms are electrically neutral
Helium: 2 p, 2 e-
Magnesium: 12 p, 12 e-
Iron: 26 p, 26 e-
The atomic number tells you both the number of protons and electrons in an atom.

19. Atomic Structure Mass Number = 1, 2, or 3 Mass Number = 16, 17, or 18
The number of neutrons found in atoms of the same element can vary.
Hydrogen atoms:
always 1 p and 1 e-
may have 0, 1, or 2 n
Oxygen atoms:
always 8 p and 8 e-
may have 8, 9, or 10 n
Mass number = # p + # n
NOTE: This is a calculated value! Not found on the periodic table!
Mass Number = 1, 2, or 3
Mass Number = 16, 17, or 18

20. Atomic Structure Isotopes
Atoms that have the same atomic number but different mass numbers
Atoms of the same element that have different numbers of neutrons
Examples
Carbon-12 (6 p, 6 n) most common
isotope
Carbon-14 (6 p, 8 n) used for carbon
dating

21. Atomic Structure
Atoms are small pieces of matter and therefore have mass.
Atomic weight (atomic mass)
The average atomic mass of an element expressed in amu
Calculated using a weighted average of each of the naturally occurring isotopes of an element

22. Ions
Atoms can gain or lose electrons to become charged particles called ions.
A chemical particle that has a positive or negative charge
Cations are positively charged ions.
Formed when an atom loses electrons
Anions are negatively charged ions.
Formed when an atom gains electrons

23. Ions 1 H+ 1 H Formation of a cation e- + Net charge = 01p e- +
Hydrogen atom
1p, 0 n, 1 e-
Hydrogen ion (cation)
1p, 0 n, 0 e-
1 H
1 H+
Net charge = 0
Net charge = +1

24. Ions 16 O2- 16 O Formation of an anion 8e- + 2e- 10e- Net charge = 08p 8n
8e- +
2e- 8p 8n
10e-
Oxygen atom
8p, 8 n, 8e-
Oxygen ion (anion)
8p, 8n, 10e-
16 O2-
16 O
Net charge = 0
Net charge = -2

25. Nuclear Symbols
Mass
Number Charge
Atomic
Number X
Charge = # p - # e-

26. O Nuclear Symbols Mass Number = Atomic Number = # protons =
Using nuclear symbols to determine the number of p, n, e, and total charge O 15 8
Mass Number =
Atomic Number =
# protons =
# neutrons =
# electrons =
Charge =

27. Ba Nuclear Symbols Mass Number = Atomic Number = # protons =
137 56 2+
Mass Number =
Atomic Number =
# protons =
# neutrons =
# electrons =
Charge =

28. Nuclear Symbols
Example: Write the nuclear symbol for an atom that contains:
1) 50 p, 70 n
2) 17 e-, 20 n

29. Nuclear Symbols
Example: Write the nuclear symbol for an ion containing:
1) 53 p, 74 n, 54 e-
2) 23 e-, 30 n, net charge = +3