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Chapter 4 - The particle model and density
Key points Checklist Developing models Density Solids, liquids and gases Determining density of objects experiment
Atomic models through time
Everyday matter is made up of particles called atoms. The Greek word 'atomos' means 'indivisible' - in the past, atoms were thought to be the smallest particles possible. Developing models Dalton's model (1803) John Dalton imagined atoms as tiny solid balls. Dalton's model included these ideas: atoms cannot be broken down into anything simpler the atoms of a given element are identical to each other the atoms of different elements are different from one another during chemical reactions atoms rearrange to make different substances In Dalton's time, it was not possible to investigate the structure of atoms. Thomson's model (1897) In 1897, J J Thomson discovered the electrons, which are negatively charged subatomic particles that are smaller than atoms. Atoms are neutral overall, so in Thomson's 'plum pudding model': atoms are spheres of positive charge electrons are dotted around inside Rutherford's model and the Geiger- Marsden experiment (1909 - 1911) Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively charged alpha particles at very thin
gold foil. According to the plum pudding model, these particles should have passed straight through. However, many of them changed direction instead. Ernest Rutherford explained these results in his 'planetary model': atoms have a central, positively charged nucleus with most of the mass electrons orbit the nucleus, like planets around a star Bohr's model (1913) There is a problem with Rutherford's model - the electrons would eventually fall into the nucleus because they are negatively charged and so attracted to the positive nucleus. Niels Bohr improved Rutherford's model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus. Summary 圖圖圖 Dalton's model (1803) A:it is no charged atom atoms as tiny solid balls
- atoms cannot be broken down into anything simpler
- all atoms of an element (substance) are identical
- different elements (substance) contain different types of atom Thomson's model (1897) 100 years later, The beams of cathode rays (-ve) changed direction A:neutral overall Plum pudding model Discovered the electrons +ve nucleus and -ve electron neutral overall Rutherford's model (1909 -
The beams of +ve alpha particles changed direction A:opposite charges attract each other planetary model discovered the nucleus atoms have a central, positively charged nucleus with most of the mass electrons orbit the nucleus, like planets around a star
Density
Density is a measure of the mass in a given volume of a substance. It can be calculated using the equation: Power can be calculated using: Example 1 A piece of aluminium has a mass of 200 kg and a volume of 0.074 m^3. Calculate its density in kg/m^3 , to two significant figures.
d =
m
v
d =
¿ 2,700 kg / m
3 The units for density The kilogram (kg) is the standard unit for mass. The cubic metre (m^3 ) is the standard unit for volume. However, in many laboratory situations mass will be measured in grams (g) and volume in cubic centimetres (cm^3 ). Converting between units kg/m3 to g/cm3, divide by 1, g/cm3 to kg/m3, multiply by 1, Formula:
density =
mass
volume
d =
m
v
Units: density is measured in kilograms per metre cubed (kg/m³) mass is measured in kilograms (kg) volume is measured in metres cubed (m³)
Example 2 The density of air at 20 °C is 0.00120 g/cm3. Give its density in kg/m3. density = 0.00120 × 1, = 1.20 kg/m Solids, liquids and gases All matter consists of particles including atoms and molecules. The arrangement and movement of these particles differ between the different states of matter: solids - particles are close together in a regular structure and vibrate about fixed positions liquids - particles are close together but free to move past each other randomly gases - particles are far apart and move randomly in all directions The density of a substance changes when it changes state. Its mass does not change. This is because its particles do not disappear - they are just rearranged and occupy a different volume. Solids and liquids A substance melts when it changes from the solid state to the liquid state. Its particles remain close together, so there is usually a relatively small change in volume and density. For example: the density of solid iron is about 7,870 kg/m the density of liquid iron is about 6,980 kg/m Liquids and gases A substance evaporates or boils when it changes from the liquid state to the gas state. Its particles move so there is a large change in volume and density. For example: the density of liquid oxygen is about 1,140 kg/m the density of gaseous oxygen is about 1.42 kg/m
Results Record the results in a suitable table. The one below gives some example measurements. Object Mass (g) Volume (cm3) Density (g/cm^3 ) density = mass ÷ volume Density (kg/m^3 ) Convert to kg/m^3 by (x1000) Steel cuboid 468 60 468/60 = 7.8 g/cm^3 7.8 × 1,000 = 7,800 kg/m^3 Stone 356 68 357/68 = 5.25 g/cm^3 5.25 × 1,000 = 5, kg/m^3 Water 50 50 50/50 = 1.0 g/cm^3 1.0 × 1,000 = 1,000 kg/m^3 Evaluation Example Describe two sources of error in the experiment. For each one, suggest an improvement. The top pan balance is only precise to ±1 g. A balance that reads to 1 decimal place (±0.1 g) or to 2 decimal places (±0.01 g) would be more precise. The displacement can may not have been set up correctly each time, so another added drop of water would cause some to dribble out of the spout before use. Check that no more water comes out before placing the measuring cylinder under the spout. Hazards and control measures Hazard Consequence Control measures Water spilled from displacement can Slip and fall Work next to a sink and have paper towels ready to mop up any spills
