Biology Notes for Ch. 6 Sect. 1
Atoms, Elements, and Compounds
Main Idea: Matter is composed of tiny particles called atoms.
Objectives:
1. Identify the particles that make up atoms.
2. Diagram the particles that make up an atom.
3. Compare covalent bonds and ionic bonds.
4. Describe van der Waals forces.
1. Atoms-are the building blocks of matter
A. Structure of Atoms
a. Atoms are made up of protons, neutrons, and electrons.
b. Nucleus includes the protons and neutrons.
i. Protonsii. Neutronsc. Electrons are located outside the nucleus (e-) and are constantly
moving around an atoms’s nucleus in energy levels.
(Draw the structure of the atom here and label it.)
2. Elements- are pure substances that cannot be broken down by physical or
chemical means.
A. Elements
a. Made of only 1 type of atom.
b. There are over 100 known elements, 92 of which occur naturally.
B. Periodic Table of Elements-is organized into horizontal rows, called
periods, and vertical columns, called groups.
a. Table is called periodic because elements in the same group have
similar chemical and physical properties. This organization even
allows scientists to predict elements that have not yet been
discovered or isolated.
C. Isotopes- are atoms or elements with a different number of neutrons.
a. Identification of isotopes is done by adding the number of
protons and neutrons in the nucleus. (e.g. the most abundant
form of carbon, carbon—12 has 6 protons and 6 neutrons in its
nucleus.)
b. Radioactive isotopes are isotopes that give off radiation.
i. When the number of neutrons is changed the stability of
the nucleus is lost leading it to decay or break apart. When
the nucleus decays it gives off radiation that can be
detected and used for many applications.
ii. When scientists know the half life of a radioactive isotope,
they can calculate the age of an object by how much of the
isotope is left in the object.
3. Compounds- is a pure substance formed when two or more different
elements combine.
a. Chemical formula-made up of the chemical symbols from the periodic
table. (e.g. H2O)
b. Characteristics of Compounds
i. Compounds are always formed from a specific combination of
elements in a fixed ratio.
ii. Compounds is that they cannot be broken down into simpler
compounds or elements by physical means, such as tearing or
crushing.
4. Chemical Bonds- the force that holds the substances together such as water,
salt, and methane.
a. Covalent Bonds- is the chemical bond that forms when electrons are
shared
i. Molecules- is a compound in which the atoms are held together
by covalent bonds.
b. Ionic Bonds- is and electrical attraction between two oppositely
charged atoms or groups of atoms called ions.
i. Ions- an atom that has lost or gained one or more electrons.
ii. Examples of Ions: NaCl, potassium, calcium, chloride and
carbonate ions
c. Characteristics of Ionic Materials
i. Some atoms tend to donate or accept electrons more easily than
other atoms.
ii. The elements identified as metals tend to donate electrons.
iii. The elements identified as nonmetals tend to accept electrons.
iv. Most ionic compounds are crystalline at room temperature and
have higher melting points than molecular compounds formed by
covalent bonds.
5. Van der Waals Forces- When molecules come close together, the attractive
forces between slightly positive and negative regions pull on the molecules
and hold them together.
a. Named for the Dutch physicist Johannes van der Waals.
b. The strength of the attraction depends on the size of the molecule, its
shape, and its ability to attract electrons.
c. Not as strong as covalent and ionic bonds, but they play a key role in
biological processes such as geckos climbing a smooth surface.
d. Water wouldn’t form water droplets, and water droplets wouldn’t form
a surface of water.
**It is important to understand that van der Waals forces are the
attractive forces between the water molecules, not the forces between
the atoms that make up water.