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Chapter 12 – Atoms and Elements
Essential Question: What is Matter Composed of?
An element is a substance made up of only one kind of atom. Examples of
elements are hydrogen, oxygen, helium, gold, etc.
The Atom
An atom is the smallest unit of an element that can still be identified as that
element. After studying atoms scientist believe that a single atom is made up of even
smaller pieces known as subatomic particles. Each atom also as a center or nucleus.
There are three types of subatomic particles:
Proton - positive electric charge. The nucleus of an atom contains at least one proton.
Neutron - NO electric charge and the same mass as a proton. Most atoms nucleuses
contains one or more neutrons.
Electron - negative electric charge. Orbit the nucleus at great speeds.
Even though elements contain the same subatomic particles they are different
because they contain a different number of subatomic particles. The most important
subatomic particle is the number of protons. The number of protons in an atom is its
atomic number. Each element has its own atomic number meaning it contains a
different number of protons.
Isotopes
All atoms of one element have the same number of protons, but do not all have the
same mass. Some are heavier because they have a different number of neutrons in their
nuclei.
Atoms that have the same number of protons but different numbers of neutrons
are called isotopes. An isotope can be identified by using a value called an atomic
mass. Atomic mass is the sum of the number of neutrons and the number of protons in
the nucleus.
Essential Question: What Are Elements and Compounds?
Elements
An element is a substance that contains only one kind of atom. All atoms of an
element have the same number of protons also their atomic number.
At this time we know more than 110 elements, but only 90 of these occur in
nature. Some familiar natural elements are : gold, silver, lead, iron, etc.
Periodic Table
The periodic table is used to organize the elements by their atomic number.
Each element has a specific place in the table. The box where the element can be
found contains the elements name, atomic number, and symbol. The element’s symbol
is one or more letters that stands for the element. Some are easy to remember such as
H for hydrogen, while others are not such as Au for gold.
One of the most useful tools of the periodic table is that it arranges the columns
(up and down) of elements into a family. Each column or family of elements has similar
properties.
How Compounds Are Formed
A compound is a substance made of atoms of two or more elements that are
chemically combined. The chemical formula for a compound shows the elements that
are in the compound. The chemical formula also compares the number of atoms of the
different elements within the compound.
Compounds do not have the same properties as the elements that form them.
Water is a familiar compound. Water is 2 hydrogen atoms combined with 1 oxygen
atom. Water’s chemical formula is H 2O .
A compound holds together because atoms bond to one another.
Compounds in Nature and in Industry
Elements are not often found in their pure form. Generally our world is made up
of compounds. Removing an element from a compound is not an easy process.
Essential Question: What Are the States of Matter?
States of Matter
Matter has 3 different states:
Solid: a definite shape and a definite volume. Solids have their own shape.
Liquid: a definite volume but NO definite shape. A liquid takes the shape of the
container that holds it.
Gas: NO definite shape or NO definite volume. Gas expands to fill whatever space is
available. (think of a balloon)
Matter can exists in three different states solid, liquid, or gas because of the
different amounts of the energy, or motion of their atoms or molecules.
Changes of State
The atoms and molecules of all elements and compounds can exist in each state
of matter: solid, liquid, or gas. Each changes state at certain temperature depending
on the substance. Adding or removing heat cause changes in state. The increase or
decrease of decrease on a substance can also cause changes in states of matter.
Melting Point: temperature at which a substance changes from a solid to a liquid.
Boiling Point: temperature at which a substance changes from a liquid to a gas.
When a gas is cooled below the boiling point it becomes a liquid. When this
liquid is cooled below the melting point it becomes a solid.
A substance that can change from a solid directly to a gas without first becoming
a liquid goes through a process called sublimation.
Plasma
Plasma is a mixture made up of charged atoms, uncharged atoms, and
electrons. Plasma is the fourth state of matter. Is rare on Earth, but makes up 99% of
all matter in the universe. stars, interstellar clouds, and comets are mostly plasma.