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.