What is the oxidation state of chromium in K2Cr2O7, and what is the name of the compound?
- K is +1 and O is −2. Let each Cr be x.
- The total is zero: 2(+1) + 2x + 7(−2) = 0, so 2 + 2x − 14 = 0.
- 2x = 12, so x = +6.
Answer: +6; potassium dichromate(VI)
The periodic table: classification of elements
Chemistry · Structure 3: Classification of matter. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
The period number is the number of occupied main energy levels. The group tells you the number of outer electrons, and the block (s, p, d, f) is the sublevel being filled.
Trends are explained by the attraction between the nucleus and the outer electrons. Across a period the nuclear charge rises while the shielding stays almost the same, so the outer electrons are held more tightly. Down a group the outer electrons are further away and more shielded, so they are held less tightly.
Elements change from metals on the left to non-metals on the right. Their oxides change from basic to acidic. An oxidation state is the charge an atom would have if the compound were fully ionic.
Rules to remember
- Across a period: atomic radius decreases; first ionization energy and electronegativity increase. Down a group: radius increases; ionization energy and electronegativity decrease
- Ions with the same number of electrons: the more protons, the smaller the ion (N3− > O2− > F− > Na+)
- First electron affinity: X(g) + e− → X−(g). First ionization energy: X(g) → X+(g) + e−
- Group 1 with water: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g); more vigorous down the group
- A halogen higher in group 17 displaces a lower one from its halide: Cl2(aq) + 2Br−(aq) → 2Cl−(aq) + Br2(aq)
- Metal oxides are basic: Na2O + H2O → 2NaOH. Non-metal oxides are acidic: SO3 + H2O → H2SO4; CO2 + H2O ⇌ H2CO3. Al2O3 is amphoteric
- Oxidation states add up to zero in a compound and to the charge in an ion; O is usually −2 and H is usually +1
Common mistake
Explaining the decrease in radius across a period by saying there are more shells or more repulsion. The number of occupied levels is the same; the nuclear charge increases and pulls the electrons closer.
Worked example
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