Measuring Matter
Chapter 1
Sections 2 and 3
Mass/Weight
• Your weight is a measure of the force of gravity on you.
• The mass of an object is the measurement of how much matter an object has.
• The mass of an object is the measurement of how much matter it contains.
• Unlike its weight, an object’s mass will not change if the force of gravity on it changes.
Units of Mass
• To measure the properties of matter, scientists use a system of units called the International System of Units (SI)
• For mass the SI unit is kilogram.• If you weigh 90 pounds on Earth, then
your mass is approximately 40 kilograms.
Volume
• The amount of space that matter occupies is called its volume.
• It’s easy to see the volume of a liquid but gases have volume, too.
• Think about a balloon as you blow it up.• For rectangular objects such as a block of
wood, the volume is found by multiplying length x width x height.
• Volume is measured in cubic centimeters, meters, millimeters, etc.
Density
• Density relates the mass and volume of an object or material.
• To calculate the density of a sample, divide its mass by its volume.
• Density = mass
volume
Particles of Matter
Chapter 1
Section 3
Atoms
• An atom is the smallest particle of an element.
• A molecule is a group of atoms that are joined together and act as a single unit.
• The force that holds two atoms together is called a chemical bond.
Democritus
• One of the first people known to have thought that matter is formed of small pieces was Democritus a Greek philosopher who lived about 440 B.C.
• He thought that there were smallest possible “pieces” of everything, and that you could chop matter into ever smaller pieces until you got to its smallest piece.
• Democritus called this smallest piece atomos, which is Greek for “uncuttable”
Dalton’s Ideas• In 1802, an atomic theory - a theory about atoms -
was proposed by a British schoolteacher named John Dalton.
• His main conclusions were that atoms can’t be broken into smaller pieces.
• Dalton thought atoms were like tiny marbles, or rigid spheres that are impossible to break.
• In any element all the atoms are exactly alike.• Atoms of different elements are different.• Atoms of each element have a unique mass.• The masses of the elements in a compound are
always in a constant ratio.
Dalton’s Ideas cont.
• Today, scientists have identified some important exceptions to Dalton’s statements.
• However, Dalton’s ideas form the basis of understanding atoms.
Atoms and Molecules
• A molecule is group of atoms that are joined together and act a single unit.
• The force that holds two atoms together is called a chemical bond.
• Molecules can contain as many as a billion atoms or as few as two atoms.
• How small are atoms? • There are about 2,000 billion billion atoms of
oxygen in one drop of water.
How we can see atoms
• The machine used to magnify things so that we can capture images of atoms is called a scanning tunneling microscope.
• Look on page 33, Figure 15, to see the picture of a model of an atom.
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