Chemistry 2: Scientific Chemistry 2: Scientific MeasurementMeasurement

• Quantitative measurements give results in a definite form, usually as numbers

• Qualitative measurements give results in a descriptive nonnumeric form

Accuracy and PrecisionAccuracy and Precision• Accuracy is how close a measurement

comes to the actual dimension or true value of whatever is measured

• Precision is concerned with the reproducibility of the measurement

Significant Figures in Significant Figures in MeasurementsMeasurements

• The significant figures in a measurement include all the digits than can be known precisely plus a last digit that must be estimated.

• 1. Every nonzero digit is significant.• 2. Zeros appearing between nonzero digits are

significant.• 3. Zeros appearing in front of all nonzero digits are

not significant.• 4. Zeros at the end of a number and to the right of

a decimal point are significant.• 5. Use scientific notation whenever possible.

The Metric SystemThe Metric System• Length meter m• Mass kilogram kg• Time second s• Electric currentampere A• Temperature kelvin K• Amt of substance mole mol• Light intensity candela cd• Pressure pascal pa• Energy joule J• Force newton N• Capacitance farad F

Units of Length, meterUnits of Length, meter• kilo k 1000 103

• deci d 1/10 10-1

• centi c 1/100 10-2

• milli m 1/1000 10-3

• micro 10-6

• Nano n 10-9

• Pico p 10–12

Units of Volume, literUnits of Volume, liter• A liter is the volume of a cube that is 10

cm on each edge (10cmx10cmx10cm=1000cm3=1 L

Units of Mass, kilogramUnits of Mass, kilogram• A kilogram is the mass of 1 L of

water at 4 oC.• A gram is defined as the mass of 1

cm3 of water at 4 oC.

Density and Specific GravityDensity and Specific Gravity• Density is the ratio of the mass of an

object to its volume.• Density = mass / volume• Specific gravity is a comparison of the

density of a substance to the density of a reference substance, usually at the same temperature.

• Specific gravity = density of substance (g/cm3) /density of water (g/cm3)

Measuring TemperatureMeasuring Temperature• Temperature is the degree of hotness or coldness

of an object.• Heat transfer occurs when two objects at different

temperatures contact each other.• Swedish astronomer Anders Celsius (1701-1744)—

the Celsius scale takes the freezing point of water as 0 oC and boiling point at 100 oC at one atmosphere pressure.

• Scottish physicist Lord Kelvin (1824-1907)—the Kelvin scale, the freezing point of water is 273 K, and the boiling point is 373 K.

• The zero point (0 K) on the Kelvin scale is absolute zero. It is –273 oC.

Measuring HeatMeasuring Heat• English physicist James Joule (1818-

1889) formulated heat conversion.• 1 J = 0.239 cal and 1 cal = 4.18 J• 1 calorie is the quantity of heat that

raises the temperature of 1 g of pure water 1 oC.

• 1 kcal = 1000 cal = 1 Cal.

Specific Heat CapacitySpecific Heat Capacity• The quantity of heat required to

change an object’s temperature by exactly 1 oC is the heat capacity of that object.

• The specific heat capacity, or simply the specific heat, of a substance is the quantity of heat required to raise the temperature of 1 g of the substance 1 oC.

Matter is made up of atomsMatter is made up of atoms• Joseph Proust, in 1799, observed that

water consists of 11% hydrogen and 89% oxygen: in a ratio of 1:8 by mass

• Antoine Lavoisier, in 1774, discovered the law of conservation of matter

• John Dalton in 1800s proposed the atomic theory

Dalton’s Atomic TheoryDalton’s Atomic Theory• 1. All matter is made up of atoms• 2. Atoms are indestructible and

cannot be divided into smaller particles (not true anymore)

• 3. All atoms of one element are exactly alike, but they are different from atoms of other elements

The Methods of ScienceThe Methods of Science• Repeated observation give rise to

hypothesis, which is tested by experiments

• Repeated experiments either confirm the hypothesis or revise the hypothesis

• After a hypothesis has been verified my other scientists, it becomes a theory

• More experiments will give rise to a revised theory

• When the theory is firmly established without exceptions, it becomes a law

The Discovery of Atomic The Discovery of Atomic StructureStructure

• J.J. Thomson, in 1897, discovered the electron using the cathode-ray tube, and that electrons are negatively charged

• Rutherford’s gold foil experiments showed that atoms have positively charged nuclei

• Thomson’s atomic model: electrons embedded in a ball of positive charge

• Nagaoka’s atomic model: electrons orbit around the positive nucleus like planets round the sun

Atomic Numbers and Atomic Numbers and MassesMasses

• Atomic number is the number of protons in the nucleus of an element

• Mass number is the sum of protons and neutrons in the nucleus. It is the average of all isotopes of an element

• Isotopes are different forms of an element having the same number of protons but different number of neutrons, hence their masses are different

• Atomic mass, 1 u = 1/12 the mass of C-12

Common Particles of an Common Particles of an AtomAtom

• Particle, symbol, charge, Z, mass in u• Proton p+ 1+ 1 1.01• Neutron n0 0 1 1.01• Electron e- 1- 0 0.00055

Electrons in MotionElectrons in Motion• Niels Bohr proposed that electrons have

energies to move round the nucleus• Now electrons are considered as particles

and waves• Electrons move round the nucleus near the

speed of light• Electrons cannot be accurately located,

only the probability of finding it

The electromagnetic The electromagnetic spectrumspectrum

• All forms of radiant energy can be placed in the electromagnetic spectrum, from radio waves, AM, FM, microwaves, infrared, visible spectrum, UV, X-rays, to gamma rays, which are very energetic and have very short wave-lengths

Electron and LightElectron and Light• Hydrogen can be shown to consist of

many energy levels, which are similar to the rungs on a ladder

• The electron cloud model of an atom suggests that energy levels are concentric spherical regions of space around the nucleus

• Electrons in the outermost energy level are called the valence electrons

Lewis Dot DiagramsLewis Dot Diagrams• A Lewis dot diagram illustrates

valence electrons as dots around the chemical symbol of an element. Each dot represent one valence electron

• Chemical changes only involve the outermost electrons of an element