Download Ch. 4: Atoms and the Periodic Table – Study Guide

Ch. 4: Atoms and the Periodic Table – Study Guide
The first person who suggested that matter was made up of atoms was the Greek philosopher Democritus. The word
atom comes from the Greek word that means “unable to be divided.”
Dalton’s atomic theory stated that every element was made of atoms that could not be subdivided, atoms of the same
element are exactly alike, and atoms can join to form molecules.
According to Bohr’s model of the atom, electrons behave like planets orbiting the sun.
According to the modern atomic theory, the exact location of an electron is uncertain. The periodic law states that
when elements are listed in order of atomic number, similarities in their properties will emerge in a regular pattern.
The nucleus of an atom is made of protons and neutrons and has a positive charge. Atoms have no electric charge
because they an equal number of electrons and protons.
The order of the elements in the periodic table is based on number of protons in the nucleus. Oxygen’s atomic number
is 8. This means that an oxygen atom has eight protons in its nucleus. Atoms of elements that are in the same group the
same number of valence electrons.
An element’s average atomic mass is a weighted average, so common isotopes have greater effect than uncommon
ones. An atomic mass unit is equal to one-twelfth the mass of a carbon-12 atom. An atom’s mass number equals the
number of protons plus the number of neutrons.
Ionization refers to the process of losing or gaining electrons. The valence electron of a lithium atom is easily removed
to form a lithium ion with a charge of 1+. A lithium ion is much less reactive than a lithium atom because it has a full
outermost energy level.
Isotopes of an element have the same atomic number but different atomic mass.
Group 1 of the periodic table consists of the alkali metals, a highly reactive group of elements. Alkali metals are located
in the left-most column of the periodic table. They are extremely reactive because they have one valence electron that
is easily removed to form a positive ion. Some elements are highly reactive because their outermost energy levels are
only partially filled.
Noble gases exist as single atoms rather than as molecules, because they are inert. Noble gases make up Group 18, and
are inert because their outermost energy level is full. The halogens are reactive elements in Group 17 of the periodic
table. Most halogens form compounds by gaining an electron to form negative ions. Transition metals such as copper
or tungsten form compounds by losing electrons to form positive ions. Transition metals are located in center of the
periodic table.