Elements and the Periodic Table Organizing the Elements

Looking for Patterns in the Elements Matter is made up of about 100 different elements that have a wide variety of properties. Some elements are very reactive - they form compounds readily with other elements. Other elements are less reactive. Others do not form compounds at all.

Mendeleev In the 1800’s, scientists began to suspect that the elements could be organized in a useful way. By 1869, one Russian scientist, Dmitri Mendeleev recognized a hidden pattern in the elements. Mendeleev studied the evidence, considered each clue, and looked for patterns.

Mendeleev’s Observations One of Mendeleev’s first observations was that some elements have similar chemical and physical properties. For example, Flourine and chlorine, are both gases that irritate your lungs if you breathe them. Silver and copper are both shiny metals that gradually tarnish if exposed to air. Mendeleev believed that these similarities were important clues to the pattern he was looking for. To help him find the pattern, Mendeleev wrote facts about the elements on individual paper cards. He kept track of all the properties he knew about an element including melting point, density, and color.

Atomic Mass Mendeleev also recorded two other important properties: Atomic mass and bonding power. Atomic mass of an element is the average mass of one atom of the element. Atomic masses were determined by comparing elements to hydrogen, the lightest element. Bonding power refers to the number of chemical bonds an element can form. Chemical bond was determined by studying how each element formed compounds with oxygen.

The First Periodic Table Mendeleev tried arranging his cards on elements in various ways. He noticed that patters appeared when the elements were arranged in order of increasing atomic mass. He also discovered that the bonding power of the elements from lithium to flourine change in an orderly way.

Arranging the Table Mendeleev discovered that arranging the elements by increasing atomic mass does not produce a perfect table. He moved cards to positions where they fit best. This left three blank spaces. Mendeleev proposed that the blank spaces would be filled by elements that had not yet been discovered. He even predicted their properties.

Periodic In 1869, Mendeleev published the first periodic table of elements. The word periodic means a regular, repeated pattern. In the modern periodic table, the properties of the elements repeat in each row or period of the table. Within 16 years, chemists discovered all three of the missing elements and named them scandium, gallium, and germanium.

Inside an Atom Deep within every atom is a core called a nucleus. The model of the atom used today shows the nucleus as containing smaller particles called protons and neutrons. Outside the nucleus are other particles, called electrons.

Measuring an Atom Because atoms are so small, we cannot measure them with everyday units of mass. Scientists created the atomic mass unit to measure the particles in atoms. The mass of a proton or a neutron is about one atomic mass unit. A proton has almost two thousand times the mass of an electron, which means that most of an atom’s mass is in its nucleus. Thus an atom that contains 3 protons, 4 neutrons, and 3 electrons has the mass of about 7 atomic mass units.

Electrical Charges Protons and electrons also carry electrical charges. Neutrons, as the name implies, are neutral and carry no electrical charge. Protons carry a positive electrical charge. Electrons carry a negative electrical charge. Electrons also move constantly and rapidly in the space around the nucleus. This fact is very important.

Atomic Number Every atom of a particular element contains the same number of protons. For example, every carbon atom contains 6 protons. Thus an element’s atomic number (the number of protons in its nucleus) is a unique property that identifies that element. However, the atoms of an element may vary in the number of neutrons they contain. For example a carbon atom, may have 5, 6, 7, or 8 neutrons. This means that the mass of an atom may vary. For this reason, chemists now organize the periodic table according to atomic number instead of atomic mass.

The Periodic Table of Today Although the periodic table is now arranged according to atomic number, the modern version of the table is much like Mendeleev’s table in many ways. It now contains more than 100 elements. Let’s look at the table on page 80 together.

Reading the Periodic Table Each square of the periodic table usually includes the element’s atomic number, chemical symbol, name, and atomic mass. Chemical Symbol for an element usually contains either one or two letters. The last entry in the square is the atomic mass or the average mass of an element’s atoms. Despite the different masses of each atom, all iron atoms, for example, react the same way chemically.

Organization of the Periodic Table An element’s properties can be predicted from its location in the periodic table. As you look at elements across a row or down a column, the elements’ properties change in a predictable way. This predictability is the reason why the periodic table is so useful to chemists.

Groups The main body of the periodic table is arranged into 18 vertical columns and seven horizontal rows. The elements in a column are called a group. Groups are also known as families. Notice that each group is numbered, from Group 1 on the left of the table to Group 18 on the right. The group is given a family name based on the first element in the column. The elements in each group, or family, have similar characteristics. For example, in Group 1 all of the elements are metals that react violently with water.

Periods Each horizontal row across the table is called a period. A period contains a series of different types of elements from different families, just as a week on a calendar has a series of different days. Unlike the elements in a family, elements in each period are not alike in properties but there is a pattern as you move across each period. Let’s look at the fourth row on the periodic table. In this row the elements range from being very reactive metals to relatively un-reactive metals to an inactive gas.

Valence Electrons It is the electrons that explain bonding power because electrons can be shared between or transferred to other atoms. This is not true for all electrons. When an atom has two or more electrons, they may be different distances from the nucleus. Only the electrons farthest out can be shared or transferred. Electrons involved in transfer or sharing are called valence electrons.

Bonding Power Elements may have different numbers of valence electrons. The number of valence electrons determines whether the electron gives up, shares, or accepts electrons. The number of valence electrons an element has increases from left to right across a period.

Valence Electrons in Groups Atoms that have the same number and arrangement of valence electrons have similar properties, which is why the elements fall into a periodic pattern. The elements in each group of the periodic table have the same number and arrangement of valence electrons. For example, each of the elements in Group 1, have atoms with 1 valence electron. In Group 18, each element except helium has atoms with 8 valence electrons. In the next sections we will be studying the groups of elements and their properties.