Notes

Of the known elements, only eight make up most of Earth’s continental crust. These eight elements are listed in the table. Notice that six of the eight elements in the table are classified as metals.

Metals have specific properties such as the ability to be shaped and drawn into wire. Metals are also good conductors of heat and electricity. They combine in thousands of ways to form compounds, the building blocks of most Earth materials. To understand how elements form compounds we need to review their building blocks which are atoms.
Element Approx. Percentage by Weight
O (Oxygen) 46.6
Si (Silicon) 27.7
Al (Aluminum) 8.1
Fe (Iron) 5.0
Ca (Calcium) 3.6
Na (Sodium) 2.8
K (Potassium) 2.6
Mg (Magnesium) 2.1
All others 1.7

A proton has about the same mass as a neutron. Hydrogen atoms have only a single proton in their nuclei. Other atoms contain more than 100 protons. The number of protons in the nucleus of an atom is called the atomic number. All atoms with six protons, for example, are carbon atoms. The atomic number of carbon is 6. Likewise, every atom with eight protons is an oxygen atom. The atomic number of oxygen is 8.

Atoms have the same number of protons and electrons. Carbon atoms have six protons and therefore six electrons. Oxygen atoms have eight protons in their nuclei and have eight electrons surrounding the nucleus.

Atoms of the same element always have the same number of protons. For example, every carbon atom has 6 protons. Carbon is element number 6 on the periodic table. But the number of neutrons for atoms of the same element can vary.

Atoms with the same number of protons but different numbers of neutrons are isotopes of an element. Isotopes of the same element are labeled using a convention called the mass number and with the element’s name or symbol.

The mass number of an atom is the total mass of the atom (protons plus neutrons) expressed in atomic mass units. The proton and the neutron each have a mass that is slightly larger than the atomic mass unit. The mass of an electron is so small that the number of electrons has no effect on the mass number of an atom.

Carbon has 15 different isotopes. Models for three of these are shown in the figure.

Carbon-12 makes up almost 99 percent of all carbon on Earth. Carbon-12 has 6 protons and 6 neutrons. Carbon-13 makes up much of the remaining naturally occurring carbon on Earth. Carbon-13 has 6 protons and 7 neutrons.

Though only traces of carbon-14 are found in nature, the presence of this isotope is used to determine the age of once-living things. Carbon-14 has 6 protons and 8 neutrons.

The nuclei of most atoms are stable. However, many elements have atoms whose nuclei are unstable. Such atoms disintegrate through a process called radioactive decay. Radioactive decay occurs because the forces that hold the nucleus together are not strong enough.

During radioactive decay, unstable atoms radiate energy and particles. Some of this energy powers the movements of Earth’s crust and upper mantle. The rates at which unstable atoms decay are measurable. Therefore, certain radioactive atoms can be used to determine the ages of fossils, rocks, and minerals.

Most elements exist combined with other elements to form substances with properties that are different from the elements themselves. Sodium is often found combined with the element chlorine as the mineral halite. Lead is found in the mineral galena, which is made up of the element lead combined with the element sulfur.

Chemical combinations of the atoms of elements are called compounds. A compound is a substance that consists of two or more elements that are chemically combined in specific proportions.

Ionic Bonds
An atom that gains electrons becomes negatively charged. This happens because the atom now has more electrons than protons. An atom that loses electrons becomes positively charged. This happens because the atom now has more protons than electrons.

An atom that has an electrical charge because of a gain or loss of one or more electrons is called an ion. Oppositely charged ions attract each other to form crystalline compounds. Ionic bonds form between positive and negative ions. Oppositely charged ions attract each other to form crystalline compounds. Ionic bonds form between positive and negative ions.

Some common compounds on Earth have both a chemical name and a mineral name. For example, table salt has a chemical name, sodium chloride, and a mineral name, halite. Salt forms when sodium (Na) reacts with chlorine (Cl) as shown in the figure.

Sodium is very unstable and reactive. Sodium atoms lose one electron and become positive ions. Chlorine atoms gain one electron and become negative ions. These oppositely charged ions are attracted to each other and form the compound sodium chloride.

The properties of a compound are different from the properties of the elements in the compound. Sodium is a soft, silvery metal that reacts vigorously with water. If you held it in your hand, sodium could burn your skin. Chlorine is a green poisonous gas. When chemically combined, these atoms produce table salt, the familiar crystalline solid that is essential to health.

Compounds that contain ionic bonds are called ionic compounds. The figure shows calcium fluoride, a common ionic compound.

How does an atom become positively charged?

It loses an electron. All atoms have equal numbers protons and electrons. If an atom loses an electron, it gains a positive charge.

Ionic compounds are rigid solids with high melting and boiling points. These compounds are poor conductors of electricity in their solid states. When melted, however, many ionic compounds are good conductors of electricity.

Most ionic compounds consist of elements from groups 1 and 2 on the periodic table reacting with elements from groups 16 and 17 of the table.

Covalent bonds form when atoms share electrons. Compounds with covalent bonds are called covalent compounds.

The figure shows quartz crystals, which form when one silicon atom and two oxygen atoms share electrons in their outermost energy levels. Quartz is also known as silicon dioxide and is one of the most common covalent compounds on Earth.

The bonding in covalent compounds results in properties that differ from those of ionic compounds. Unlike ionic compounds, many covalent compounds have low melting and boiling points.

For example, water, a covalent compound, boils at 100°C at standard pressure. Sodium chloride, an ionic compound, boils at 1413°C at standard pressure. Covalent compounds also are poor conductors of electricity, even when melted.

How do covalent bonds form?

Covalent bonds form when atoms share electrons.

The smallest particle of a covalent compound that shows the properties of that compound is a molecule. A molecule is a neutral group of atoms joined by one or more covalent bonds.

Water, for example, consists of molecules. These molecules are made of two hydrogen atoms covalently-bonded to one oxygen atom. The many gases that make up Earth’s atmosphere, including hydrogen, oxygen, nitrogen, and carbon dioxide, also consist of molecules.

Metals are malleable, which means that they can be easily shaped. You’ve observed this property when you wrapped aluminum foil around food or crushed an aluminum can. Metals are also ductile, meaning that they can be drawn into thin wires without breaking. The wiring in your school or home is probably made of the metal copper. Metals are excellent conductors of electricity.

Metallic bonds form when electrons are shared by metal ions. The sharing of an electron pool gives metals their characteristic properties. An electrical current is easily carried through a pool of electrons. Some metals are classified as minerals.