A Guided Tour of The Periodic Table Chapter-3 Section 3.2

When you are in a store, chances are you know where to look for your favorite items because they are not placed randomly on the shelves. Similar items are usually grouped together, so that you can find what you need quickly. The elements are also arranged in a certain order. The order is based on the number of protons an atom of that element has in its nucleus. A hydrogen atom has one proton, so hydrogen is the first element listed in the periodic table.

A hydrogen atom has one proton, so hydrogen is the first element listed in the periodic table A helium atom has two protons and is the second element listed, and so on. Elements are listed in this order in the periodic table because the periodic law states that when elements are arranged this way, similarities in their properties will occur in a regular pattern. Periodic law-properties of elements tend to change in a regular pattern when elements are arranged in order of increasing atomic number, or number of protons in their atom.

Period- a horizontal row of elements in the periodic table Period- a horizontal row of elements in the periodic table. Just as the number of protons an atom has increases by one as you move from left to right across a period, so does its number of electrons. You can determine how an atom’s electrons are arranged if you know where the corresponding element is located in the periodic table. Group (family)- a vertical column of elements in the periodic table. Valence electrons determine the chemical properties of atoms.

Atoms of elements in the same group, or column, have the same number of valence electrons, so these elements have similar properties. Atoms of Group l are reactive because their outermost energy levels are only partially filled. Atoms that do not have filled outermost energy levels may undergo a process called Tionization. Ionization- the process of adding electrons to or removing electrons from an atom or group of atoms.

That is, they may gain or lose valence electrons so that they have a full outermost energy level. If an atom gains or loses electrons, it no longer has the same number of electrons as it does protons. Because the charges do not cancel completely as they did before, the ion that forms has a net electric charge. Ion- an atom or group of atoms that has lost or gained one or more electrons and therefore has a net electric charge. Lithium is located in Group 1 of the periodic table. It is so reactive that it even reacts with air. A lithium atom has three electrons.

Two of these electrons occupy the first energy level, but only one electron occupies the second energy level. This single valence electron makes lithium very reactive. Removing this electron forms a positive ion, or cation. A lithium ion, written as Li+, is much less reactive than a lithium atom because it has a full outermost energy level. Cation- An ion with a positive charge. Anion- An ion with a negative charge.

The most common hydrogen isotope, protium, has only a proton in its nucleus. Hydrogen is present on both the sun and on Earth. Seventy-five percent of the mass of the sun is hydrogen. If you know the atomic number and mass number of an atom, you can calculate the number of neutrons it has. The atomic mass unit (amu)- a quantity equal to one-twelfth of the mass of a carbon-12 atom. Average atomic mass- the weighted average of the masses of all naturally occurring isotopes of an element.