Scientists have discovered a lot about God’s universe and the matter that forms it. All matter can be divided into three groups: elements, compounds, and mixtures. What you learn in this section will help you classify particular items, such as sugar or iron, as elements, compounds, or mixtures.

ELEMENTS

Defining elements. Elements are substances composed of only one kind of atom. Each element’s atoms are distinct from those of other elements. Elements are sometimes called the building blocks of the universe. There are over one hundred elements. Many substances we use regularly are elements. These elements include iron, tin, oxygen, and aluminum. Gold, silver, and mercury are also elements.

The elements carbon, hydrogen, oxygen, and nitrogen are plentiful in living things. Food, wood, paper, and most fuels are rich in carbon and hydrogen. Your body is made of carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and calcium with traces of several other elements.

Elements exist as solids, liquids, and gases. Iron and lead are examples of elements that are solids. Mercury, the silver-colored metal often used in thermometers, is a liquid at room temperature. The air you breathe is made up of several elements which are gases.

Classifying elements. Scientists have found that elements differ from one another. Oxygen is different from lead. Iron is different from sodium. They not only look different, but they react differently with other elements.

Although every element has special features, scientists have found that certain elements have some things in common. As the atomic number of elements increases, certain chemical properties reappear periodically. This discovery resulted in a classification system called the Periodic Table of the Elements.

As of 2004, there are 116 known elements. Ninety-one of these elements occur naturally. The rest are synthetic (made in a laboratory) and radioactive. The first synthetic element, called Technetium, was discovered in 1937. The most recent synthetic element, Ununhexium, was discovered in 2004.

The Periodic Table is an orderly grouping chemical elements known to scientists. This table includes the name of each element, its symbol, atomic weight, and electron distribution. The first letter of each chemical symbol is capitalized.

Study the Periodic Table of the Elements in the resource center. Notice that the elements are arranged in both horizontal (sideways) rows and also in vertical (up and down) columns. The horizontal rows are called periods. Elements in the same period all have the same number of electron shells. The vertical columns divide the elements into groups. All the elements of the same group have similar chemical properties. Some groups have special names. The elements in group 18 are called noble gases. The elements in group 1, except for hydrogen, are called alkali metals.

You can learn to read the table by looking at an entry for one of the elements. Study the entry for calcium below. The atomic number is 20. This means that the calcium atom has 20 protons in its nucleus. The atomic weight is 40 and the electron distribution is two, eight, eight, two.

With this information, you could diagram an atom of calcium. An example diagram is shown below. Notice that there are four shells around the nucleus. Each shell is identified with a number, n = 1, 2, 3, and 4. The n = 1 shell has two electrons, the n = 2 and n = 3 shells each have eight electrons, and the n = 4 shell has two electrons. Many elements of the Periodic Table have the electron distribution number listed for each element. If four numerals are listed for an element, you can assume that the element has shells n = 1, 2, 3, and 4. The sequence of numerals tells you the total number of electrons in each shell.

In an atom, nearly all of the mass is in the nucleus. Electrons have almost no mass. The particles in the nucleus which give it its mass are the protons and neutrons. Since the atomic number of an element tells you the total number of protons in each atom of a specific element, the atomic weight of an atom is obtained by adding neutrons. For example, the Periodic Table gives the atomic mass of calcium (Ca) as 40. To get a total of 40 particles in the center, you need 20 neutrons (n) plus 20 protons (p) to have the necessary 40 particles in the nucleus.

Consider the element Beryllium. It has an atomic number of 4. This tells us that there are four protons in the nucleus. Since the atomic weight is 9, the difference must be made up by the neutrons. Five neutrons are in the nucleus.