3.4 A Universe of Energy and Matter
Life, like everything else in the universe, is at its most basic level an interplay of the things that we call matter and energy.
Atoms come in different types, and each type corresponds to a different chemical element. Atoms are made of particles that we call protons,
neutrons, and electrons.
Protons and neutrons are found in the tiny nucleus at the center of the atom. The rest of the atom’s volume contains electrons, which surround
The electrons in an atom form a kind of “smeared out” cloud that surrounds the nucleus and gives the atom its apparent size.
The structure of a typical atom. Notice that atoms are
extremely tiny: The atom shown in the middle is magnified
to about one billion times its actual size, and the nucleus
on the right is magnified to about 100 trillion times its
Most of an atom's size is its nucleus, because protons and neutrons are each about 2000 times as massive as an electron. The properties of an atom depend mainly on the amount of electrical charge in its nucleus; an object’s electrical charge is a mea sure of how
strongly it will interact with other charged particles.
We de fine the electrical charge of a proton as the basic unit of positive charge, which we write as +1. The electron has an electrical charge that is
precisely opposite that of a proton, so we say it has negative charge (1). Neutrons are electrically neutral, meaning that they have no charge.
Oppositely charged particles attract one another, and similarly charged particles repel one another. An atom is held together by the attraction
between the positively charged protons in the nucleus and the negatively charged electrons that surround the nucleus.*
Most of the atoms in and around you contain the same number of electrons as protons, making them electrically neutral overall. However, atoms
often lose or gain electrons, in which case they obtain a net electrical charge. We call such atoms ions.
A positive ion is an atom that has lost one or more electrons so that it has more positive than negative charge overall; a negative ion is an
atom that has gained one or more electrons, giving it a net negative charge.
Since the nucleus is buried so deeply inside an atom, interactions between atoms are almost exclusively interactions between their electrons.
atomic number = number of protonsatomic mass number = number of protons + neutrons
(A neutral atom has the same number of electrons as protons.)
Different isotopes of a given element contain the same number of protons, but different numbers of neutrons.
The complete set of the more than 100 known elements is listed in the periodic table of the elements
Organic molecules contain carbon (and usually also contain hydrogen).
Compounds are molecules made from atoms of two or more different elements.
Molecules consist of two or more atoms.
Chemical bond is the name we give to the interactions between electrons that hold the atoms in a molecule together.
Gas Phase = Atoms or molecules move essentially unconstrained. Liquid Phase = Atoms or molecules remain together but move relatively freely.
Solid Phase = Atoms or molecules are held tightly in place.
The melting point (0°C at sea level on Earth) is the temperature at which the water molecules finally break the solid bonds of ice. Above the
boiling point (100°C at sea level), all the bonds between adjacent molecules are broken so that the water can exist only as a gas.
The process by which molecules escape from a solid is called sublimation, and the process by which molecules escape from a liquid is called
evaporation. Higher temperatures lead to higher rates of subli mation or evaporation.
In essence, energy is what makes matter move. We can classify all these various types of energy into just three major categories.
Energy of motion, or kinetic energy: falling rocks, orbiting planets, and the molecules moving in the air around us are all examples of objects with
Energy carried by light, or radiative energy. All light carries energy, which is why light can cause changes in matter.
Potential energy is stored energy. This can be converted into kinetic or radiative energy. For example, a rock perched on a ledge has gravitational
potential energy, and gasoline's chemical potential energy can be converted into kinetic energy of the moving car.
Einstein discovered that mass itself is a form of stored energy, sometimes called mass energy, described by his formula E = MC