CHEMISTRY
CHEMISTRY — KNOWT NOTES
PHYSICAL QUANTITIES, MEASUREMENT, ERRORS & PERIODIC TABLE
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PHYSICAL QUANTITIES & MEASUREMENT
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7 SI BASE QUANTITIES
Length → meter → m
Length formula → DVT
Mass → kilogram → kg
Mass formula → MDV
Time → second → s
Time formula → DVT
Temperature → kelvin → K
Temperature formula → K = °C + 273.15
Amount of substance → mole → mol
Mole formula → n = m/M
Electric current → ampere → A
Current formula → I = Q/t
Luminous intensity → candela → cd
No basic formula needed.
DVT:
D = distance
V = velocity
T = time
MDV:
M = mass
D = density
V = volume
n = moles
m = mass
M = molar mass
I = current
Q = charge
t = time
COMMON CONVERSIONS
1 kg = 1000 g
1 g = 1000 mg
1 L = 1000 mL
1 mL = 1 cm³
1 m = 100 cm
1 km = 1000 m
°C → K = add 273.15
K → °C = subtract 273.15
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SIGNIFICANT FIGURES
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Significant figures = meaningful digits in a measurement.
Nonzero digits are always significant.
Zeros between nonzero digits are significant.
Leading zeros are NOT significant.
Trailing zeros after a decimal are significant.
Trailing zeros in whole numbers without a decimal are ambiguous.
456 = 3 significant figures.
405 = 3 significant figures.
0.0045 = 2 significant figures.
0.03040 = 4 significant figures.
2.500 = 4 significant figures.
500 = generally 1 significant figure.
500.0 = 4 significant figures.
Memory:
Before first nonzero → NOT significant.
Between nonzero numbers → significant.
After decimal → significant.
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ACCURACY & PRECISION
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Accuracy = closeness to the TRUE or ACCEPTED VALUE.
Precision = closeness of measurements TO ONE ANOTHER.
Memory:
ACCURACY → TRUE
PRECISION → TOGETHER
High accuracy + high precision = close to true value AND close together.
High accuracy + low precision = close to true value but spread out.
Low accuracy + high precision = far from true value but close together.
Low accuracy + low precision = far from true value and spread out.
Important:
A measurement can be precise but inaccurate.
Exam clue:
Values close to EACH OTHER → precision.
Values close to the TRUE VALUE → accuracy.
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DENSITY
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Density = mass per unit volume.
Density is an intensive physical property.
Density does NOT depend on sample size.
Density triangle:
M on top
D and V on bottom
D = M/V
M = D×V
V = M/D
Asked for density → M ÷ V.
Asked for mass → D × V.
Asked for volume → M ÷ D.
Common density units:
g/cm³
g/mL
Important:
1 mL = 1 cm³.
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DENSITY & TEMPERATURE
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General rule:
Temperature increases → volume increases.
Volume increases → density decreases.
Temperature increases → density generally decreases.
Temperature decreases → density generally increases.
Memory:
HOTTER → BIGGER VOLUME → LOWER DENSITY
Water is an exception from 0°C to 4°C.
From 0°C to 4°C → water density increases as temperature increases.
Ice is less dense than liquid water.
Ice floats because it is less dense than liquid water.
Freezing water → open hydrogen-bond structure → volume increases → density decreases.
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PERCENT ERROR
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Accepted value = true/correct value from a reliable reference.
Experimental value = value measured during an experiment.
Error = difference between experimental and accepted values.
Error = Experimental − Accepted
Percent Error = |Experimental − Accepted| ÷ Accepted × 100
Memory:
SUBTRACT → ABSOLUTE → DIVIDE → ×100
Percent-error steps:
Experimental − Accepted.
Take absolute value.
Divide by Accepted.
Multiply by 100%.
Experimental value = accepted value → 0% error.
Higher accuracy → lower percent error.
Lower accuracy → higher percent error.
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TYPES OF ERROR
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Systematic error = consistent/repetitive error.
Systematic error has the same general magnitude and direction under the same conditions.
Memory:
SYSTEMATIC → SAME
Systematic error is also called cumulative error.
Systematic error mainly affects accuracy.
Examples of systematic error:
Incorrect calibration.
Balance always reads 0.50 g too high.
Thermometer always reads 2°C too low.
Random error = unpredictable variation between measurements.
Memory:
RANDOM → CHANGES
Random error is different from one measurement to another.
Random error mainly affects precision.
Examples of random error:
Different viewing angles.
Changing wind conditions.
Slight posture differences.
Air currents affecting a balance.
Random error can be reduced by repeating measurements.
Averaging repeated measurements helps reduce random error.
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SOURCES OF ERROR
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Natural error = caused by environmental conditions.
Memory:
NATURAL → ENVIRONMENT
Examples:
Wind.
Air temperature.
Atmospheric pressure.
Humidity.
Gravity.
Earth curvature.
Atmospheric refraction.
Instrument error = caused by imperfect, incorrectly adjusted, or poorly calibrated equipment.
Memory:
INSTRUMENT → EQUIPMENT
Examples:
Poor calibration.
Imperfect equipment.
Incorrect adjustment.
Human error = caused by physical limitations or inconsistent human actions.
Memory:
HUMAN → OBSERVER
Examples:
Misreading a scale.
Misalignment.
Improper setup.
Incorrect observation.
DO NOT CONFUSE:
Types of errors:
Systematic + Random
Sources of errors:
Natural + Instrument + Human
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HISTORY OF THE PERIODIC TABLE
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1808 → DALTON
Dalton assigned atomic weights to early elements.
Dalton → atomic weights.
1817 → DÖBEREINER
Dobereiner grouped elements with similar properties into groups of THREE.
Three elements → TRIADS.
Dobereiner → TRIADS.
1864 → NEWLANDS
Newlands noticed repeating properties every EIGHTH element.
Eight → OCTAVES.
Newlands → OCTAVES.
1869 → MENDELEEV
Mendeleev arranged elements using relative atomic mass and chemical/physical properties.
Mendeleev left GAPS for undiscovered elements.
Mendeleev predicted properties of undiscovered elements.
Mendeleev → Father of the Periodic Table.
1913 → MOSELEY
Moseley arranged elements by ATOMIC NUMBER.
Moseley → atomic number.
MEMORY:
Dalton → WEIGHTS
Dobereiner → THREE
Newlands → EIGHT
Mendeleev → GAPS
Moseley → NUMBER
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PERIODIC TABLE ORGANIZATION
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Atomic number = number of PROTONS.
Neutral atom:
Protons = electrons.
Atomic mass = average mass of naturally occurring isotopes.
Isotope = atoms of the same element with the SAME number of protons but DIFFERENT numbers of neutrons.
Memory:
ISOTOPE → SAME PROTONS, DIFFERENT NEUTRONS
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PERIODS
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Periods = HORIZONTAL ROWS.
Periodic table has 7 periods.
Atomic number increases LEFT → RIGHT across a period.
Elements in the same period have the same number of electron shells.
Memory:
PERIOD → ACROSS
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GROUPS / FAMILIES
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Groups = VERTICAL COLUMNS.
Periodic table has 18 groups.
Groups are also called families.
Elements in the same group generally have similar chemical properties.
Similar properties are related to similar electron configurations/valence-electron patterns.
Memory:
GROUP → DOWN
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METALS, NONMETALS & METALLOIDS
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Metals → LEFT + CENTER.
Nonmetals → UPPER-RIGHT.
Metalloids → STAIR-STEP LINE.
Metalloids have properties of both metals and nonmetals.
Nonmetals are generally dull.
Nonmetals are generally brittle when solid.
Nonmetals are generally poor conductors.
Nonmetals tend to GAIN electrons.
Memory:
LEFT/CENTER → METALS
STAIR-STEP → METALLOIDS
UPPER-RIGHT → NONMETALS
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PERIODIC TRENDS — MASTER MAP
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UPPER-RIGHT ↗
Electronegativity ↑
Ionization energy ↑
Electron affinity ↑
Nonmetal reactivity ↑
BOTTOM-LEFT ↙
Atomic radius ↑
Metal reactivity ↑
MASTER MEMORY:
UPPER-RIGHT → PULLS/WANTS ELECTRONS.
BOTTOM-LEFT → BIGGER ATOMS + MORE REACTIVE METALS.
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ELECTRONEGATIVITY
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Electronegativity = ability of an atom to ATTRACT electrons in a chemical bond.
Electronegativity increases toward the UPPER-RIGHT.
Highest electronegativity = FLUORINE (F).
Memory:
EN → ATTRACT
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IONIZATION ENERGY
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Ionization energy = energy required to REMOVE an electron from a neutral gaseous atom.
Ionization energy increases toward the UPPER-RIGHT.
High ionization energy = harder to remove an electron.
Memory:
IE → REMOVE
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ELECTRON AFFINITY
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Electron affinity = tendency of a neutral atom to GAIN an electron.
Electron affinity generally increases toward the UPPER-RIGHT.
Electron gain generally releases energy.
Important exception:
CHLORINE (Cl) has the highest electron affinity, NOT fluorine.
Memory:
EA → GAIN
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ATOMIC RADIUS
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Atomic radius = size/distance from nucleus to outermost electron shell.
Atomic radius increases toward the BOTTOM-LEFT.
Largest atomic radius = CESIUM (Cs).
Memory:
RADIUS → SIZE
BOTTOM-LEFT → BIGGEST
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REACTIVITY
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Reactivity = ability to undergo a chemical reaction.
Metal reactivity increases toward the BOTTOM-LEFT.
Nonmetal reactivity increases toward the UPPER-RIGHT.
Memory:
METALS → BOTTOM-LEFT
NONMETALS → UPPER-RIGHT
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PERIODIC TREND EXAM SHORTCUT
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Highest electronegativity → UPPER-RIGHT.
Highest ionization energy → UPPER-RIGHT.
Highest electron affinity → generally UPPER-RIGHT.
Electron affinity exception → CHLORINE.
Largest atomic radius → BOTTOM-LEFT.
Most reactive metal → BOTTOM-LEFT.
Most reactive nonmetal → UPPER-RIGHT.
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FASTEST FINAL MEMORY SHEET
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BIG NUMBER → LEFT → +
SMALL NUMBER → RIGHT → −
ACCURACY → TRUE
PRECISION → TOGETHER
DENSITY → M/V
MASS → D×V
VOLUME → M/D
KELVIN → °C + 273.15
CELSIUS → K − 273.15
% ERROR → |E−A|/A ×100
SYSTEMATIC → SAME
RANDOM → CHANGES
NATURAL → ENVIRONMENT
INSTRUMENT → EQUIPMENT
HUMAN → OBSERVER
PERIOD → HORIZONTAL
GROUP → VERTICAL
ATOMIC NUMBER → PROTONS
ISOTOPE → SAME PROTONS, DIFFERENT NEUTRONS
METALS → LEFT/CENTER
METALLOIDS → STAIR-STEP
NONMETALS → UPPER-RIGHT
EN → ATTRACT
IE → REMOVE
EA → GAIN
RADIUS → SIZE
UPPER-RIGHT → EN, IE, EA, NONMETAL REACTIVITY
BOTTOM-LEFT → RADIUS, METAL REACTIVITY
EXCEPTIONS:
Water 0–4°C → density increases as temperature increases.
Ice → less dense than liquid water.
Chlorine → highest electron affinity.