1. Molecules and Ions
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2. The Periodic Table Horizontal rows are referred to as periods
Vertical columns are referred to as groups or families
Special names on periodic table
Group 1: alkali metals
Group 2: alkaline earth metals
Group 17: halogens
Group 18: noble gases
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3. Molecule
Molecule – an aggregate of at least two atoms in a definite arrangement held together by chemical bonds
In a fixed ratio!
May include the same element or different
Compounds only have different elements
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4. Diatomic Molecules Remember the lucky seven?
These seven ALWAYS are diatomic in nature
Memorize these
H2, N2, O2, F2, Cl2, Br2, I2
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5. Ions Ion – has either GAINED or LOST electrons
Anion – negatively charge (gained electrons), ex: Cl-
Cation – positively charged (lost electrons), ex: Na+
Anion usually on the right, cations on the left
Must memorize these!
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6. Empirical vs. Molecular Formula
Empirical - Tells us which elements are present in the lowest-whole number ratio, NOT necessary how many atoms are present
Molecular – shows the EXACT composition of elements
Ex: hydrogen peroxide, used to kill bacteria, has the molecular formula of H2O2, there are 2 H and 2 O, however, the empirical formula is HO
Empirical – simplest, molecular - true

7. Empirical Formula
When analyzing an unknown compound, first step is to find the empirical formula
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8. Ex: Write empirical formulas for the following molecules
(a) C2H2 CH
(b) C6H12O6
CH2O
(c) N2O
Empirical formula is the same as the molecular formula
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