1. Chapter 2 Atoms and Elements

2. Reacting Atoms
when elements undergo chemical reactions, the reacting elements do not turn into other elements
Dalton’s Atomic Theory
2H2(g) + O2(g)  2H2O(g)
since the number of protons determines the kind of element, the number of protons in the atom does not change in a chemical reaction
however, many reactions involve transferring electrons from one atom to another

3. Charged Atoms when atoms gain or lose electrons, they acquire a charge
charged particles are called ions
when atoms gain electrons, they become negatively charged ions, called anions (Cl-)
when atoms lose electrons, they become positively charged ions, called cations (Na+)
ions behave much differently than the neutral atom
e.g., The metal sodium, made of neutral Na atoms, is highly reactive and quite unstable. However, the sodium cations, Na+, found in table salt are very nonreactive and stable

4. Ionic Charge from Group Numbers
The charge of a positive ion is equal to its Group number.
Group 1A(1) = 1+
Group 2A(2) = 2+
Group 3A(3) = 3+
The charge of a negative ion is obtained by subtracting 8 or 18 from its Group number.
Group 6A(16) = = 2-
or = 2-

5. Atomic Structures of Ions
Nonmetals form anions
For each negative charge, the ion has 1 more electron than the neutral atom
F = 9 p+ and 9 e-, F ─ = 9 p+ and 10 e-
Metals form cations
For each positive charge, the ion has 1 less electron than the neutral atom
Na atom = 11 p+ and 11 e-, Na+ ion = 11 p+ and 10 e-

6. Ions and the Periodic Table

7. Examples
Predict the charge and identify the number of proton and electron from that ion
Mg  Al 
O  P 

8. Atomic Mass: The Average Mass of an Element’s Atoms
Atomic mass is sometimes called atomic weight or standard atomic weight (amu)
The atomic mass of each element is directly beneath the element’s symbol in the periodic table.
It represents the average mass of the isotopes that compose that element, weighted according to the natural abundance of each isotope.

9. Collection Terms
When we count large numbers of objects, we often use units such as
1 dozen objects = 12 objects.
1 gross objects = 144 objects.
A collection term states
a specific number of items.
In chemistry, how do chemists know the number of atoms in an element or compound?
As chemists, we often need to know the number of atoms in a sample of a given mass. Why? Because chemical processes happen between particles.
Therefore, if we want to know the number of atoms in anything of ordinary size, we count them by weighing.

10. The Mole: A Chemist’s “Dozen”
The chemist’s “dozen” is the mole (abbreviated mol). A mole is the measure of material containing × 1023 particles:
1 mole = × 1023 particles
This number is Avogadro’s number.
The second, and more fundamental, thing to understand about the mole is how it gets its specific value.
The value of the mole is equal to the number of atoms in exactly 12 grams of pure C-12.
12 g C = 1 mol C atoms = × 1023 C atoms

11. A Mole of Atoms
A mole is
a unit of measurement used in chemistry to express amounts of a chemical substance, the same number of particles as there are carbon atoms in 12.0 g of carbon.
a collection term “dozen”
1 mole = NA = x 1023 of anything
Avogadro’s Number = x 1023

12. Converting between Number of Moles and Number of Atoms
Converting between number of moles and number of atoms is similar to converting between dozens of eggs and number of eggs.
For atoms, you use the conversion factor 1 mol atoms = × 1023 atoms.
The conversion factors take the following forms:

13. Converting between Mass and Amount (Number of Moles)
To count atoms by weighing them, we need one other conversion factor—the mass of 1 mol of atoms.
The mass of 1 mol of atoms of an element is the molar mass.
An element’s molar mass in grams per mole is numerically equal to the element’s atomic mass in atomic mass units (amu).
The lighter the atom, the less mass in 1 mol of atoms.

14. Relationship Between Moles and Mass
The mass of one mole of atoms is called the molar mass
The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu

15. Examples Give the molar mass for each
A. 1 mole of Li atoms = ________ g
1 mole of Co atoms = ________g
1 mole of S atoms = ________g

16. Converting between mass, moles and atoms
g C
mol C
mol C
g C
g C
mol C
atoms

17. Examples
Assuming all pennies are pure copper and each has a mass of 2.5 g
Without doing calculation, determine the number atoms of copper present in 1 mole
How many pennies does it take to make a mole?

18. Examples In a 3.0 moles of O2 molecules
How many oxygen molecules are there ?
How many oxygen atoms are there?

19. Examples Calculate the moles of carbon in 0.0265 g of pencil lead
Calculate the mass (in grams) of moles of titanium