Elements and the Periodic Table

Science, Grade 6


Table Of Contents: Elements and the Periodic Table

1. Introduction to Elements

2.1. Elements
An element is a pure substance that cannot be separated into simpler substances. This means an element is a substance made entirely of one type of atom. For example, every atom in a piece of copper is the same. Scientists have identified 90 elements that occur naturally in our universe. Some elements you might recognize include oxygen, iron, and calcium. There are also over 20 man-made elements, such as einsteinium. Man-made elements are radioactive and unstable and decay over time into lighter elements.
2.2. Compounds
Most living and nonliving things are made up of a combination of elements. Elements chemically combine in a variety of ways to form compounds. A compound is made of two or more different elements bonded together in an exact ratio according to their masses. For example, water is a compound made of hydrogen and oxygen with a mass ratio of 1 to 8. However, if the mass ratio of hydrogen to oxygen is 1 to 16, the compound formed would be hydrogen peroxide. While water and hydrogen peroxide are both made of hydrogen and oxygen, they are different compounds with distinct properties.
2.3. Atomic Number
Although atoms are amazingly small, scientists have discovered they are made up of even smaller particles - protons, neutrons, and electrons. The nucleus of the atom contains neutrons and positively charged protons. Neutrons have no charge, but are thought to help to keep the nucleus together. The number of protons in the nucleus of an atom is the atomic number of that atom. All atoms of an element have the same atomic number. For example, all atoms of the oxygen element have 8 protons in their nucleus. Therefore, oxygen has an atomic number of 8. The atomic number represents the most important property of an element.
2.4. Isotopes
Each element has a limited number of naturally occurring isotopes. Isotopes are different forms of the same atom. An isotope of an element has the same number of protons but different numbers of neutrons. Because atomic mass is determined by the number of protons and neutrons in an atom, isotopes of the same element have different masses. For example, while the most common isotope of oxygen has 8 neutrons, other isotopes of oxygen have 9 or 10 neutrons. While some isotopes are stable, others are radioactive and unstable. They decay to form other isotopes by releasing radiation in the form of particles or electromagnetic waves.
2.5. Atomic Mass
Scientists express the mass of an atom in atomic mass units (amu). The mass of a proton, the positively charged particle in the nucleus, is about 1 amu. The mass of a neutron, the particle in the nucleus with no charge, although a bit more massive than a proton is still considered to be 1amu. Each isotope of an element is identified by its mass number. The mass number is the sum of the number of protons and neutrons in an atom. The mass of an electron is so small that it is not used in calculating atomic mass. Because most elements contain a mixture of two or more isotopes, the atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element.
2.6. Electron Configuration
The specific way the electrons are arranged in an atom is called the electron configuration. Electrons play an important role in how elements interact with each other and form compounds. Electrons are negatively charged particles that surround the nucleus in the form of a cloud. The atomic number of an atom, which is equivalent to its number of protons, also represents the number of electrons in that atom. The electrons are distributed among orbital shells or energy levels (1, 2, 3 and so on) that are different distances from the nucleus. The larger the number of the energy level, the farther it is from the nucleus. Electrons that are in the highest or outmost energy level are called valence electrons. The valence electrons are the ones that are lost, gained or shared during chemical bonding. In the electron configuration for oxygen which has a total of 8 electrons, the first orbital or energy level closest to the nucleus is filled with two electrons. The second energy level can hold up to eight electrons. It begins to fill with the remaining six electrons when the first level is full.
2.7. Electrons and Bonding
The outermost orbital shell, called the valence shell, is most often involved in chemical bonding. Oxygens valence shell has six electrons but it can hold up to eight electrons. Therefore, oxygen will often combine with other elements that allow it to share or lose two electrons. Atoms with four or more electrons in the valence shell like to gain electrons to fill the shell. Atoms with three or less electrons in their valence shell like to lose electrons to reveal the full inner shell. Atoms with full valence shells will not combine with other elements.

2. Pause and Interact

3.1. Review
Use the whiteboard text tool to complete the graphic organizer. Write one similarity and one difference between the pairs of terms.
3.2. Identifying Elements
Use the atomic number, number of protons, to identify the element.

3. Classes of Elements

4.1. Classes of Elements
Many elements have similar properties. Scientists find it useful to group elements with similar properties into classes. The classes of elements are metals, nonmetals, and metalloids. The elements within each class have similar physical properties such as appearance, density, and state of matter. They also have similar chemical properties such as the number of electrons in their outermost orbital, which determines an elements reactivity, or how likely the element is to react with other elements.
4.2. Metals
The largest class of elements in the periodic table is metals. Examples of metals include calcium, iron, gold, silver and mercury. Metals have some distinct properties. Due to their high melting points, most metals are solid at room temperature, with the exception of mercury which is a liquid. Most metals are malleable and ductile. Malleability is the ability of a metal to be hammered into shapes. Ductility is the ability of a metal to be stretched into wire. They are often shiny in appearance and are good conductors of heat and electricity. Atoms of most metals have 1-3 electrons in the outer shell and easily lose valence electrons. Metals combine with other metals, and select non-metals, to form a vast number of alloys that enhance the properties of metals in specific applications. For example combining iron, nickel and chromium produces a series of stainless steel alloys that are commonly used. Metals are often found combined with nonmetals as salts. Table salt is produced when the metal sodium combines with the nonmetal chlorine.
4.3. Nonmetals
Nonmetals make up another class of elements. Some examples of nonmetals include oxygen, nitrogen, and fluorine. Due to their low melting points, most nonmetals are gases at room temperature. Some nonmetals are solids and one element, bromine, is a liquid at room temperature. Unlike metals, nonmetals have low densities, are brittle, and do not conduct heat and electricity. Nonmetals do not conduct electricity because their atoms are not likely to give up or lose valence electrons. Nonmetals usually have four to eight electrons in their outer orbital and most of them readily gain or share valence electrons. Although nonmetals can form compounds with metals, they can also form compounds with other nonmetals. For example, oxygen combines with sulfur to form sulfur dioxide. Carbon is a very important nonmetal because it is part of all compounds found in living things. Some nonmetals such as neon and helium are called inert or nonreactive because their valence shell is full and do not react with other elements to form compounds.
4.4. Metalloids
The smallest class of elements has properties of both metals and nonmetals. These elements are called metalloids. Examples of metalloids include silicon, germanium, and boron. All metalloids are solids and can be shiny or dull in appearance. Some metalloids are malleable but some are brittle. These elements have 3-7 electrons in their outer shell and can form bonds in similar ways as metals and nonmetals. Metalloids conduct heat and electricity, but not as well as metals. Semiconductors, made of such metalloids as silicon, can conduct electricity under certain conditions. Semiconductors are used to make electronic components like computer chips. Metalloids are also useful when combined with metals to form alloys. Pewter is an alloy that contains the metalloid antimony.

4. Pause and Interact

5.1. Review
Use the whiteboard text tools to fill in the table.
5.2. Metals, Nonmetals and Metalloids
Select the best answer(s) and then click on the check button.

5. The Periodic Table of Elements

6.1. Arranging the Elements
In 1869 a scientist named Dmitri Mendeleev developed a periodic table to help organize information about the 63 elements known at that time. He arranged the elements in order of increasing atomic mass. In doing so, he noticed that those that had similar properties such as density, appearance and melting point occurred in a repeating or periodic pattern. He found that these properties of elements repeated every seven elements. Because of this pattern his table became known as the periodic table of the elements. His first attempt at arranging the known elements left some gaps in the pattern of his periodic table. Mendeleev made a prediction that elements yet to be discovered would fill these gaps. When scientists discovered these missing elements they found many of Mendeleevs predictions were correct.
6.2. Changing the Arrangement of the Periodic Table
In 1914, Henry Moseley, a British scientist discovered how to determine the number of protons in an atom. Moseley's discovery of the atomic number (number of protons) resulted in a more accurate arrangement of elements in the Periodic Table. It became apparent that atomic mass was not the significant player in the periodic law as Mendeleev had proposed, but rather, the properties of the elements varied periodically with atomic number. When atoms were arranged according to increasing atomic number, the few problems with Mendeleev's periodic table disappeared. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements.
6.3. The Modern Periodic Table
There are over 110 elements in the modern periodic table arranged by increasing atomic number and similarities. These similarities are found in the rows and columns. The 18 vertical columns are called groups. The seven horizontal rows are called periods. In order to make the table more compact, two rows of elements are displayed below the main part of the table instead of in rows six and seven. When the elements are organized by period and group, elements in the same class are grouped together. Metallic elements range from the far right, across the middle and onto the left side. Metalloids form stair steps, starting at Group 13. The far right hand elements are nonmetals. Most of the radioactive elements are in the bottom rows.
6.4. Element Key
Each square in the periodic table represents an element. The square may display information about the elements symbol, atomic number, atomic mass, state of matter, and class of element. The symbol is a short hand way to represent each element. The first letter of the symbol is always uppercase. If the symbol has a second letter, it is always lowercase. Some symbols are easy to remember, such as O for oxygen or C for carbon. While other symbols such as Fe for iron do not share any common letters. Some periodic tables display the symbols in different colors depending on their state of matter - solid, liquid, or gas at room temperature. The color of the square may also be used to show whether the element is a metal, nonmetal, or metalloid.
6.5. Periods of the Periodic Table
The rows of the periodic table are called periods. Periods are numbered one through seven on the left-hand side of the table. The elements in a period increase in atomic number from left to right. Elements in the same period also have the same number of orbitals. For example, all the elements in Period 2 have two orbitals while all the elements in Period 4 have four orbitals. At this time, the maximum number of electron orbitals or electron shells for any element is seven. The physical and chemical properties of elements in the same period are not all that similar but they follow a repeated pattern as you move across a period. For example properties such as conductivity and reactivity steadily change from left to right in each period.
6.6. Groups of the Periodic Table
The vertical columns on the periodic table are called groups. Elements in a group have similar properties, because the atoms of the elements have the same number of valence electrons in their outer shell. For example, atoms in Group 1 have one valence electron while atoms in Group 2 have two valence electrons. The valence electrons are often involved in chemical bonding, making certain groups more likely to react with other elements. An element from Group 1 will combine with an element from Group 17 in a one to one ratio. For instance sodium (Na) combined with chlorine (Cl) forms sodium chloride, NaCl.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to answer the questions.
7.2. Review
Use the whiteboard tools to answer the questions.

7. Groups on the Periodic Table

8.1. Group 1- Alkali Metals
Group 1 elements, which include lithium, sodium, potassium, rubidium, cesium and francium, are alkali metals. The metals in this group are so soft that they can be cut with a knife. They are also shiny, and have low density. Alkali metals are the most reactive metals and are never found in elemental form. They are found combined only with other elements. In fact these metals will explode when they come in contact with water and need to be stored in oil. Alkali metals are very reactive because they can easily give away or lose their single valence electron. Some important compounds formed with alkali metals include sodium chloride (table salt), borax (sodium tetraborate), potash (potassium carbonate), and washing soda (sodium carbonate) among others.
8.2. Group 2- Alkaline-Earth Metals
Group 2 elements, which include beryllium, magnesium, calcium, strontium, barium, and radium, are called alkaline-earth metals. These metals are fairly hard, shiny, and good conductors of electricity. Although alkaline-earth metals are not as reactive as alkali metals, they are still not found in elemental form, but only in compound form. They have two electrons in their valence shell and like alkali metals react readily with halogens to form salts, such as calcium chloride.
8.3. Groups 3 to 12 - Transition Metals
The 38 elements in Groups 3 through 12 of the periodic table are called "transition metals". Iron, copper, zinc, silver, and gold are all transition metals. As with all metals, transition metals are ductile and malleable, shiny and good conductors of heat and electricity. Because the atoms of transition metals do not give away their electrons as easily as atoms of Group 1 and 2, they are less reactive than alkali metals and alkaline-earth metals. The transition metals are unique in that their valence electrons, or the electrons they use to combine with other elements, are present in more than one shell. Also, unlike other elements, they don't always use the same number of valence electrons in chemical reactions. Iron (Fe), for example, sometimes likes to give away two electrons, and sometimes three, forming different compounds. Transition metals can be mixed with other metals to form alloys such as steel, or with calcium to form compounds found in cement.
8.4. Transition Metals - Lanthanides and Actinides
The lanthanides and actinides, the two rows below the main part of the periodic table, are part of the transition metals group. The first row includes the lanthanides. At one time, the lanthanides were called the rare-earth elements, although they are not particularly rare. The lanthanides include the metal neodymium used as a component to make high-strength, powerful neodymium magnets. The elements in the second row are radioactive or unstable and are called actinides. Only two of the actinides, thorium and uranium, occur in nature, the others are man-made.
8.5. Group 13- Boron Group
The Boron Group, which includes boron, aluminum, gallium, indium, and thallium, is the first group of mixed elements. The group contains one metalloid (boron) and four metals. The elements in this group have three electrons in the valence shell and are very reactive. Aluminum is the most important metal in the Boron Group. It is used to make light weight alloys used in automotive parts, foil, cans and other products. The metalloid boron is used to make the compound boric acid. Boric acid is used as a cleaning agent and is also added to Pyrex glass to make it heat resistant.
8.6. Group 14- Carbon Group
The Carbon Group includes carbon, silicon, germanium, tin, lead and flerovium (temporary name ununquadium). The properties of the Carbon Group vary greatly. In their elemental solid state, Group 14 metalloids, silicon and germanium, act as electrical semiconductors, although silicon is mainly non-metallic. Tin and lead are metals, while flerovium, also known as Element 114, is radioactive. Carbon, the fourth most abundant element on Earth, is a very important nonmetal that forms compounds in a variety of ways. When combined with oxygen, hydrogen, and trace amounts of other elements, carbon forms all the organic compounds that make up living organisms.
8.7. Group 15- Nitrogen Group
The Nitrogen Group includes two nonmetals: nitrogen and phosphorous; two metalloids: arsenic and antimony; and one metal: bismuth. The elements in this group have five valence electrons and generally gain three electrons when bonding with other elements. Nitrogen is a gas that makes up 80% of Earths atmosphere. Natural sources of nitrogen can be found in animal and plant proteins and in fossilized remains of ancient plant life. Nitrogen from the atmosphere can be combined with hydrogen to form ammonia, used in fertilizer production. Unlike nitrogen, phosphorus is extremely reactive and only found in compounds with other elements.
8.8. Group 16- Oxygen Group
The Oxygen Group includes three nonmetals: oxygen, sulfur, and selenium; and two metalloids: tellurium, and polonium. These can be found in nature in both free and combined states. All elements of the Oxygen Group have six electrons in their outermost shell which makes this group reactive. Oxygen is the most abundant element on Earth, found in the atmosphere and dissolved in water. It is essential for the survival of most living organisms.
8.9. Group 17- Halogens
The Halogen Group includes four nonmetals: fluorine, chlorine, bromine, and iodine; and one metalloid: astatine. The elements in this group have seven valence electrons and are very reactive because they only need to gain one more electron to complete their outer shell. In their pure form, halogens form diatomic molecules, such as chlorine (Cl₂). Halogens combine with metals to form salts. Most halogens, such as chlorine or iodine, can be used as disinfectants.
8.10. Group 18- Noble Gases
Group 18, which includes elements helium, neon, argon, krypton, xenon, and radon, are all colorless, odorless gases at room temperature. These gases are inert, or nonreactive and do not react with other elements to form compounds because their valence shell is full. These elements can be used in gaseous form. Neon is used to make lighted signs. Helium, because its less dense than air, is used to make blimps and weather balloons float.
8.11. Hydrogen
Hydrogen is a special element because it is not like any other group of elements. It is placed above Group 1 in the periodic table because it has one electron in its valence shell but does not belong in the alkali metals group. Hydrogen is found in stars and it is the most abundant element in the universe. However, it only makes up one percent of the Earths mass. On Earth, most hydrogen is combined with oxygen in the form of water.

8. Vocabulary Review

9.1. Elements and the Periodic Table Vocabulary Matching Review

9. Virtual Investigation

10.1. Flame Test
How are elements identified by using a flame test? A metal salt is a compound of a metal and a nonmetal. When dissolved in water, the metal and nonmetal atoms separate into charged particles called ions. As the metal ion is heated by the flame, the electrons gain energy and move to outer orbitals. When the metal ion cools, the electrons go back to their original orbital and release energy in the form of light. Are these colors a characteristic of the metal? In this virtual investigation you will burn a small amount of various metal salt solutions and record the colors of the flames. From your results, you will draw conclusions about the colors released from heated metal ions. You will also test an unknown metal salt and determine what metal is present.

10. Assessment

11.1. Elements and the Periodic Table