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Biology
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2-1 The Nature of Matter
Photo Credit: © John Conrad/CORBIS
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Learning Objectives Identify three subatomic particles found in atoms Explain how all of the isotopes of an element are similar Explain what chemical compounds are Describe the two main types of chemical bonds
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Atoms
Atoms
The study of chemistry begins with the basic unit of matter, the atom.
The Greek philosopher Democritus called the smallest fragment of matter the atom, from the Greek word atomos.
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Atoms
Placed side by side, 100 million atoms would make a row only about 1 centimeter long.
Atoms contain subatomic particles that are even smaller.
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Atoms
What three subatomic particles make up atoms?
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Atoms
The subatomic particles that make up atoms are
protons
neutrons
electrons
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Atoms
The subatomic particles in a helium atom.
Helium atoms contain protons, neutrons, and electrons. The positively charged protons and uncharged neutrons are bound together in the dense nucleus, while the negatively charged electrons move in the space around the nucleus.
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Atoms
Protons and neutrons have about the same mass.
Protons are positively charged particles (+).
Neutrons carry no charge.
Strong forces bind protons and neutrons together to form the nucleus, which is at the center of the atom.
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Atoms
The electron is a negatively charged particle (−) with 1/1840 the mass of a proton.
Electrons are in constant motion in the space surrounding the nucleus.
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Atoms
Electrons are attracted to the positively charged
nucleus but remain outside the nucleus because
of the energy of their motion.
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Atoms
Because atoms have equal numbers of electrons
and protons, and because these subatomic particles
have equal but opposite charges, atoms are neutral.
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Elements and Isotopes
Elements and Isotopes
A chemical element is a pure substance that consists entirely of one type of atom.
Elements are represented by a one- or two-letter symbol.
C stands for carbon.
Na stands for sodium.
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Elements and Isotopes
The number of protons in an atom of an element is the element's atomic number.
Carbon has 6 protons, so its atomic number is 6.
More than 100 elements are known, but only about two dozen are commonly found in living organisms.
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Elements and Isotopes
Isotopes
Atoms of the same element that differ in the number of neutrons they contain are known as isotopes.
The sum of the protons and neutrons in the nucleus of an atom is called its mass number.
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Elements and Isotopes
Isotopes are identified by their mass numbers.
For example, carbon has three isotopes—carbon-12, carbon-13, and carbon-14. Each isotope has a different number of neutrons.
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Elements and Isotopes
How are all of the isotopes of an element similar?
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Elements and Isotopes
Because they have the same number of electrons, all isotopes of an element have the same chemical properties.
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Elements and Isotopes
Isotopes of Carbon
Because they have the same number of electrons, these isotopes of carbon have the same chemical properties. The difference among the isotopes is the number of neutrons in their nuclei.
6 electrons
6 protons
7 neutrons
6 electrons
6 protons
6 neutrons
6 electrons
6 protons
8 neutrons
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Elements and Isotopes
Radioactive Isotopes
Some isotopes are radioactive, meaning that their nuclei are unstable and break down at a constant rate over time
Although the radiation these isotopes give off can be dangerous, they have important scientific and practical uses.
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Elements and Isotopes
Radioactive isotopes can be used:
to determine the ages of rocks and fossils.
to treat cancer.
to kill bacteria that cause food to spoil.
as labels or “tracers” to follow the movement of substances within an organism.
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Chemical Compounds
Chemical Compounds
In nature, most elements are found combined with other elements in compounds.
A chemical compound is a substance formed by the chemical combination of two or more elements in definite proportions.
The physical and chemical properties of a compound are different from the elements from which it is formed.
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Chemical Compounds
Scientists show the composition of compounds by a kind of shorthand known as a chemical formula.
Water, H2O, contains two atoms of hydrogen for each atom of oxygen.
The formula for table salt, NaCl, indicates that sodium and chlorine combine in a 1 : 1 ratio.
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Chemical Bonds
What are the two main types of chemical bonds?
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Chemical Bonds
Chemical Bonds
The atoms in compounds are held together by chemical bonds.
Bond formation involves the electrons that surround each atomic nucleus.
The electrons that are available to form bonds are called valence electrons.
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Chemical Bonds
The main types of chemical bonds are:
ionic bonds
covalent bonds
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Chemical Bonds
Ionic Bonds
An ionic bond is formed when one or more electrons are transferred from one atom to another.
An atom that loses electrons has a positive charge. 
An atom that gains electrons has a negative charge.
These positively and negatively charged atoms are known as ions.
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Chemical Bonds
Sodium atom (Na)
Sodium ion (Cl-)
Sodium ion (Na+)
Sodium atom (Cl)
The chemical bond in which electrons are transferred from one atom to another is called an ionic bond. The compound sodium chloride forms when sodium loses its valence electron to chlorine.
Protons
Electrons
Charge
Protons
Electrons
Charge
Protons
Electrons
Charge
Protons
Electrons
Charge
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Chemical Bonds
Covalent Bonds
Sometimes electrons are shared by atoms instead of being transferred.
Sharing electrons means that the moving electrons actually travel in the orbitals of both atoms.
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Chemical Bonds
A covalent bond forms when electrons are shared between atoms.
When the atoms share two electrons, the bond is called a single covalent bond.
When atoms share four electrons it is called a double bond.
When atoms share six electrons it is called a triple bond.
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Chemical Bonds
The structure that results when atoms are joined together by covalent bonds is called a molecule.
A molecule is the smallest unit of most compounds.
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Chemical Bonds
In a water molecule, each hydrogen atom forms a single covalent bond with the oxygen atom.
The chemical bond in which electrons are shared between atoms is called a covalent bond. In a water molecule, each hydrogen atom shares two electrons with the oxygen atom.
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Chemical Bonds
Van der Waals Forces
When molecules are close together, a slight attraction can develop between the oppositely charged regions of nearby molecules.
Chemists call such intermolecular forces of attraction van der Waals forces, after the scientist who discovered them.
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Chemical Bonds
Although van der Waals forces are not as strong
as ionic bonds or covalent bonds, they can hold
molecules together, especially when the molecules
are large.
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Chemical Bonds
For example, van der Waals forces form between the molecules on the surface of a gecko’s foot and the molecules on the surface of the wall.
The combined strength of all the van der Waals forces allows the gecko to grip the wall.
Photo Credit: top: Kellar Autumn & Ed Florance; bottom: © Mark Moffett/Minden Pictures, Inc.
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2-1
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2-1
The particles that move around the nucleus of an atom are called
neutrons.
protons.
electrons.
isotopes.
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2-1
The atomic number of a carbon atom is 6. How many neutrons does the isotope carbon-14 have? 6 8 12 14
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2-1
Which of the following statements about the three isotopes of carbon is true?
They are all radioactive.
They have different numbers of electrons.
They have the same chemical properties but differ in atomic mass.
They have the same number of protons and neutrons.
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2-1
A chemical compound consists of
Electrons mixed with neutrons.
two or more elements combined in a definite proportion.
two or more elements combined in any proportion.
at least three elements combined by ionic or covalent bonds.
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2-1
Van der Waals forces are the result of
unequal sharing of electrons.
ionic bonds.
the bonding of different isotopes.
the chemical combination of sodium and chlorine.
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