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.