1. Atomic Structure & Compounds (loosely based on Chapter 2 Sec 1 thru 4 of Jespersen 7th ed)
Dr. C. Yau
Spring 2015 1

2. Dalton's Atomic Theory (p.7)
Matter consists of tiny particles called atoms.
In any sample of a pure element, all the atoms are identical in mass and other properties.
The atoms of different elements differ in mass and other properties.
When atoms of different elements combine they form compounds with the elements in a fixed ratio by mass Reactions are merely rearrangement of atoms to form different compounds.
Atoms are indestructible In chemical reactions, the atoms rearrange but they do not themselves break apart.
(?)
Chemical
(?) 2

3. The 3 major subatomic particles
Know this well !
Name &
Symbol
Mass
Charge
Location
proton (p)
~ 1 u +1
inside nucleus
neutron (n)
electron (e-)
~ 0 u -1
outside nucleus
u (amu) = atomic mass unit
(a very small unit of mass)
1 gram = 602,000,000,000,000,000,000, 000 u 3 3

4. Atomic Mass
Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weight measured in u.
Atomic masses are based on a standard mass, that of an atom of C.
1 atom of carbon-12 weighs 12 u exactly.
Thus 1 u = 1/12 the mass of a carbon-12 atom. This is the definition of u.
Chem FAQ: What is an atomic mass unit?

5. Isotopes
Most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses.
For example: There are 3 isotopes of hydrogen, 4 isotopes of iron and 10 isotopes of tin.
Isotopes
are atoms of the same element with different masses.
are atoms with the same #p but with different #n.
Chemically, isotopes have virtually identical properties. (There are slight differences that we will note be concerned with at this level of chemistry.)
KNOW WELL
The relative proportions of the different isotopes are essentially constant

6. Atomic Notation
An element is a substance whose atoms all contain the identical number of protons, called the atomic number (Z)
Isotopes are distinguished by the
mass number (A):
Atomic number, Z = number of protons
Mass number, A = (# of protons) + (# of neutrons)
Note that for atoms, A is greater than Z: the symbol is "top-heavy."
For neutral atoms, the number of electrons and protons must be equal.

7. Example: uranium-235
This indicates the mass number.
235 U 92
Mass number, (protons + neutrons) 
Chemical Symbol 
Atomic number, (number of protons) 
From the nuclear symbol we can determine the # protons, electrons and neutrons:
# protons = 92
# electrons = 92 (since no charge is shown)
#neutrons = 235 – 92 = 143
Note that the number that follows the dash is the mass number.
Chem FAQ: How do I write symbols for isotopes?

8. Fill in the blanks:
symbol neutrons protons electrons
60Co
81Br 33 27 27 46 35 35

9. Trick question! You can’t tell! How many neutrons are there in 53Fe?
This is how Fe appears in the periodic table.
How many neutrons are
there in Fe?
Trick question! You can’t tell!
How many neutrons are there
in 53Fe?
A) 26
B) 55
C) 27
D) none of these
Ans. 53 – 26 = 27
mass number - # protons = # neutrons 26 Fe
55.85
This is NOT the atomic mass number.

10. protium deuterium tritium Hydrogen has 3 isotopes: 99.985% abundance
0.000 % abundnace
1 proton
no neutron
1 proton
1 neutron
1 proton
2 neutrons
approx. mass of each atom = 1 u
2 u
3 u 10 10

11. What we call "heavy water" is D2O.
(Regular water being H2O, with practically all the hydrogen atoms being protium.)
One molecule of H2O weighs 18 u.
One molecule of D2O weighs 20 u.
Hence D2O is known as “heavy water.”
Tritium is the only radioactive isotope of hydrogen. Such isotopes are referred to as a radioisotope.
NOTE: The term "isotope" does NOT mean that it is radioactive. 11 11

12. atomic mass = 35.4518 u (2 decimal places)
Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is u and that of chlorine-37 is u. Calculate the average atomic mass of chlorine.
x u = u (4 sig. fig.)
x u = u (4 sig. fig.)
atomic mass = u (2 decimal places)
Chem FAQs: How do I calculate an average atomic mass from isotopic masses and abundances?
How do I calculate isotopic masses or abundances using average atomic mass? 17 Cl
35.45
35.45 u
This is the atomic mass
or atomic weight.

13. Hint: Let x be the mass of the other isotope.
There are 2 isotopes of element Z. The first is 56.5% in abundance and has a mass of u. If the atomic mass is u, what is the mass of the other isotope?
Hint: Let x be the mass of the other isotope.
What % in abundance is this other isotope?
159.6 u = 160. u (3 sig.fig.)

14. Some Important Classifications:
A groups = representative elements
or main group elements
I A = alkali metals II A = alkaline earth metals
VII A = halogens VIII = noble (inert) gases
B groups = transition elements
Inner transition elements = elements 58 – 71 and 90 – 103
58 – 71 = lanthanide elements
90 – 103 = actinide elements
See figure 1.15 for graphical distinction of these groups.

15. Metals, Nonmetals, And Metalloids
FIG. 2.7 Distribution of metals, nonmetals, metalloids among the
in the periodic table.

16. Properties Of Metals Metals... reflect light (have metallic luster)
can be hammered or rolled into thin sheets (are malleable) and can be drawn into a wire (are ductile)
are solids at room temperature (except Hg)
conduct electricity and heat

17. Nonmetals And Metalloids
lack the properties of metals
tend to pulverize when struck with a hammer
Are non-conductors of electricity and heat
Many are gases, a few solids, and one liquid (Br2)
react with metals to form (ionic) compounds
Metalloids
have properties between metals and nonmetals
Chem FAQs: How do I classify elements as metals, nonmetals, or metalloids?
How do I classify an element as a metal or nonmetal based on its properties?

18. EXTREMELY IMPORTANT! nonmetal + nonmetal nonmetal + metal
covalent molecules
ionic compounds
alloys
Note: Alloys are not compounds. They are homogeneous mixtures of metals, sometimes with a small amount of nonmetals, such as C in steel.

19. Learning Check: Count The Atoms In A Chemical Formula3 2 1
Na2CO3
(NH4)2SO4
Mg3(PO4)2
CuSO4•5H2O
___Na, ___ C, ___ O
___N, ___H, ___S, ____O
___Mg, ___P, ____O
___Cu, ___S, ___O, ___H 2 8 1 4 3 2 8 1 9 10
What exactly does this dot tell you?
Chem FAQ: How do I count the atoms of elements in a chemical formula?

20. Covalent Molecules
The subscripts in the formula of covalent molecules tell you exactly how many atoms of each element is present.
They do not merely state a ratio.
e.g. CO2 tells us the molecule contains one C atom and two O atoms.

21. Ionic Compounds Positively charged ions are called cations
Negatively charged ions are called anions
Subscripts in the formula always specify the smallest whole-number ratio of the ions needed to make a neutral combination (formula unit, or f.u.)
It does not mean 2 Fe ions are bonded to 3 O ions.
Subscripts tell us the ions are in a ratio of 2Fe to 3 O.
Fe O
O2-
Fe3+ 3 2 +
See Chem FAQ: Ionic Compound Construction Kit

22. The subscripts in the formula of
covalent molecules tell you the exact number of the atoms of each element is present.
ionic compounds tell you the ratio of the atoms of each element that is present.

23. Overview: Molecules vs. Formula Units
Electrically neutral, discrete particles called molecules. (H2O)
Neutral groups of charged particles called formula units (NaCl)
FIG Molecular and ionic substances. (a) In water there are discrete molecules that each consist of one atom of oxygen and two atoms of hydrogen. Each particle has the formula H2O. (b) In sodium chloride, ions are packed in the most efficient way. Each Na is surrounded by six Cl, and each Cl is surrounded by six Na. Because individual molecules do not exist, we simply
specify the ratio of ions as NaCl.

24. Why are ions charged? How does Ca form Ca2+? # p+ = # e- if neutral
# p+ < # e- if negative
# p+ > # e- if positive
The number of p+ never changes when ions form.
Ions are formed when the atom gains or loses electrons. Protons are not affected.
How does Ca form Ca2+?
Ca lost 2 electrons
How is N3- formed?
N gained 3 electrons

25. Symbol neutrons protons electrons 60Co3+ 81Br-
Fill in the blanks:
Symbol neutrons protons electrons
60Co3+
81Br- 24 27 33 36 46 35
Chem FAQ: How do I count the number of electrons present in an atom or elemental ion?

26. Summary of Properties Hardness and brittleness
Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other.
Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions
Figure An ionic crystal shatters when struck

27. Melting Points
To melt the a solid, there must be sufficient kinetic energy to overcome the attractions between particles
Molecular compounds have weak attractions between particles and so tend to have low melting points.
Many molecular compounds are gases at room temperature.
Ionic compounds tend to have strong attractions so they have high melting points.
Nearly all ionic compounds are solids at room temperature.

28. Electrical Conductivity
Requires the movement of electrical charge
Ionic compounds:
Do not conduct electricity in the solid state
Do conduct electricity in the liquid and aqueous states-The ions are free to move.
Molecular compounds:
Do not conduct electricity in any state.
Molecules are comprised of uncharged particles.
Exception: Strong acids are molecular substances that become ionic when dissolved in water.
Figure 2.16
See video: electrical conductivity apparatus
See Chem FAQ: How can I distinguish ionic and molecular compounds given their properties?

29. Which of the following is likely true of NO2?
It conducts electricity well.
It has a low melting point.
It is likely a solid in its pure form.
None of these
The formula tells you it’s a molecular compound, so it is not expected to conduct electricity, should have a relatively low mp, and therefore not likely to be a solid in its pure form.
You should know how the formula tells you that it’s a molecular formula. See Slide 18!