A period in the table is a horizontal row. Hydrogen and Helium make up one period. In all, as you can see, there are 7 periods. (sorry, female agents, we know how you must feel.)
A group is a vertical row, which has different names (see below)
Before we go on, let's have a quick recap:
Element - made of itself and nothing else; cannot be broken down into smaller parts without loosing its identity (maybe not now, but in the future.....)
Atom - smallest piece of any element. (also a comic character in days gone by - for all you Jeopardy buffs)
Now to recap the strucutre of an atom. Protons are small particles that have a positive charge and rotate around in the nucleus. Electrons are negatively charged particles that rotate around the nucleus in orbits. The nuetrons share space with the protons in the nucleus and they are nuetral; no charge. Here is a picture of all this:
Note that each element box has several aspects to it.
First, the top number indicates the atomic number - which is the number of protons in the nucleus.
The symbol of course is a shortened name of the element, which is usually indicated by 1, 2, or 3 letters.
The number below indicates the atomic mass, or mass number. This is the weight of the particular element. It represents the weight of the neutrons and protons in the nucleus. (Neutrons are like protons with similar weight but it has a neutral charge. Electrons float out of the nucleus in the atom. These have a negative charge.) Now you may be wondering (but I doubt it) why electrons are not included in the atomic mass. After all, it is the weight of the whole atom, right? That's true but what distinguishes electrons from any of the other particles is their weight. Electrons have no weight. (Actually they do but their weight is sooo sooo sooo small that it is practically equal to 0. Also note that what keeps an atom together is the attraction between the protons in the nucleus to the electrons outside of the nucleus. This creates a circular field around the electrons which keeps an atom together.
Normally an atom is electrically neutral. The number of protons equals the number of electrons. BUT, when an atom gains or loses electrons it becomes unstable. The new resulting atom is called an ion. When an atom gains an electron there are more negative particles than positive ones, so the atom becomes negative. This new type of atom is called anion. If the atom loses an electron, it becomes positively charged and is called a cation.
Isotopes are atoms with the same atomic number but differ in the amount of neutrons. For example Carbon can occur as carbon -12 or carbon 14. The number after carbon indicates the atomic mass. If it gets higher, that means that more neutrons are there since the number of protons stays the same.
Chemical reactions involve interactions that take place between the electrons of atoms. So to understand how and why atoms react we need to know something about electron configurations - which describe how electrons are arranged in atoms.
First of all electrons do not occupy true orbits but instead live in things called orbitals. An orbital is a region in an atom where an electron may be found (a probability). It doesnít have to be circular but just a region of any shape. There are four types of shapes. Orbitals that have the same shape in a given energy shell comprise a subshell. An s subshell consists of one sphere like orbital. A p orbital consists of three dumbshell shaped orbitals. A d and f subshell contain 5 and 7 oddly shaped orbitals, respectively. Any orbital regardless of shape and size can hold a maximum of two electrons.
Basically to find the electron configuration of an element, one must first count the number of protons in the element. Letís take krypton for example. It has an atomic number of 36. It's electron configuration is
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6. If you count up the last numbers, you will get 36, that is, 36 electrons total.
You can see that each number before the subshells is the order. 1 contains only s, then 2 contains s and p, etc.
But you have to note that around 4 the order changes. Here is the complete ordering.
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s2 4d10 5p6 4f14 5d10 ....and on.
Here is a graphical view of the ordering:
Practice on more elements and you'll get the hang of it.
Now that we are on the topic, one more thing to know. Valence electrons are electrons in the outer shells of an element. If you do configurations of all elements you will see this:
The frist group has 1 valence electron. For example Lithium has atomic number of 3. That means its electron configuration is 1s2 2s1. That 1 in the last shell is its valance electron.
The second group has 2 valence electrons and it goes on till the noble gases which are completely filled with 8 valance electrons. See for your self.
Note that transition elements are not counted in the groups. Just the main 8 groups.
Now let's go over some periodic trends.
1. Ionization energy: the amount of energy needed to remove an electron from an atom.
2. Electronegativity: the amount of pull that an atom's nucleus exerts on another atom's electrons when it gets itself involved in a bond. Moving from left to right in the periodic table, electronegativity increases.
3. Atomic radius: The distance from the center of an atom to the edge of it. The larger the atom, the more radius it has.
TYPES OF ELEMENTS
Now let's look at types of elements. This section is very comprehensive, so you do not have to view all of it. Just get the jist of each type of element. The uses and histories of the types are just extra info.
By far the largest category of elements on the Periodic Chart is the metal elements. Metals share a set of properties that are not as universal to them as the inert gases. Metal elements usually have the following properties: They have one, two, or three electrons on the outside electron shell. The outside electrons make it more likely that the metal will lose electrons, making positive ions. The ions of metals are usually plus one, plus two, or plus three in charge. Metals tend to lose electrons to become stable. They will attach to other elements with ionic bonds almost exclusively. When metal atoms are together in a group, there is a swarm of semi-loose electrons around the atoms. These electrons move about freely among the metal atoms making what is called an electron gas.
The properties of non-metals are not as universal to them as the metals; there is a great deal of variation among this group. Non-metals have the following properties: Non-metals usually have four, five, six, or seven electrons in the outer shell. When they join with other elements non-metals can either share electrons in a covalent bond or gain electrons to become a negative ion and make an ionic bond. When non-metal elements join by covalent bonds, it is usually to other non-metals. Non-metals can attach together with covalent bonds to make a group of (usually non-metal) elements with a common charge called a radical or polyatomic ion. Elemental non-metals often have a dull appearance. They are more likely to be brittle, or shatter when struck. Although not a constant rule, non- metals tend to have lower melting and boiling points than metals and the solids tend to be less dense. Non-metals are not as cohesive as metals and certainly not ductile. Non-metals are not usually good conductors of heat or electricity. Many non-metals form diatomic or polyatomic molecules with other atoms of the same element. Many non-metals have more than one form of the free element, called allotropes, that appear in different conditions. (The word free here means that the element is unattached to other types of atom, not that it has a monetary value of zero.)
Semi Metals (Metalloids)
We have pretended that there is a sharp dividing line between the metals and non-metals. This is not the case. The staircase-shaped line between metals and non-metals has several elements on or near it that have properties somewhere between the two categories. By having three electrons in the outside shell, boron should be a metal element. It is not. Boron is more likely to form covalent bonds like a non-metal than donate electrons like aluminum, the next element down the chart in the same group. Aluminum is definitely a metal in most of its traits, but it has its own idiosyncrasy. Aluminum is amphoteric; it reacts with both acids and bases. Silicon, germanium, arsenic, antimony, and tellurium are on the line between metals and non-metals and exhibit some of the qualities of both. These elements do not really comprise a clear-cut category, but, due to the mix of properties they show, they are often lumped into a classification called semi-metals. Many of the elements on the line are semiconductors of electricity, meaning that they have the ability to conduct electricity somewhere between almost none and full conduction. This property is useful in the electronics industry.
We have failed to include hydrogen in any of the categories, for good reasons. Hydrogen just does not match anything else. More than ninety-nine-point-nine percent of hydrogen is just one proton and one electron. A very small proportion (one atom in several thousand) of hydrogen is deuterium, one proton, one neutron, and one electron. An even smaller portion (one hundred atoms per million billion) of hydrogen is tritium, one proton, two neutrons, and one electron. When a hydrogen atom gains an electron, it becomes a negative ion. The negative hydrogen ion, called hydride ion, can be attached to metals, but it is not seen in nature because it is not stable in water. The positive hydrogen ion is what is responsible for acids. There really is no such thing as a (positive) hydrogen ion. Having only a proton and an electron, hydrogen becomes only a proton if it loses its electron. Loose protons attach themselves to a water molecule to make H3O+ ion, a hydronium ion. This hydronium is the real chemical that produces the properties of acids. Elemental hydrogen is a diatomic gas. Except for having a valence of +1, hydrogen has few other similarities with the Group 1 elements. Hydrogen makes covalent bonds between other hydrogen atoms or other non-metals.
Group 1 elements, lithium, sodium, potassium, rubidium, cesium, and francium, are also called the alkali metal elements. They are all very soft metals that are not found free in nature because they react with water. In the element form they must be stored under kerosene to keep them from reacting with the humidity in the air. They all have a valence of plus one because they have one and only one electron in the outside shell.
Group 2 elements, beryllium, magnesium, calcium, strontium, barium, and radium, all have two electrons in the outside ring, and so have a valence of two. Also called the alkaline earth metals, Group 2 elements in the free form are slightly soft metals. Magnesium and calcium are common in the earth's crust.
Group 3 elements, boron, aluminum, gallium, indium, and thallium, are a mixed group. Boron has mostly non- metal properties . Boron will bond covalently by preference. The rest of the group are metals. Aluminum is the only one common in the earth's crust. Group 3 elements have three electrons in the outer shell, but the larger three elements have valences of both one and three.
Group 4 elements, carbon, silicon, germanium, tin, and lead, are not a coherent group either. Carbon and silicon bond almost exclusively with four covalent bonds. They both are common in the earth's crust. Germanium is a rare semi-metal. Tin and lead are definitely metals, even though they have four electrons in the outside shell. Tin and lead have some differences in their properties from metal elements that suggest the short distance from the line between metals and non-metals (semi-metal weirdness). They both have more than one valence and are both somewhat common in the earth's crust.
Group 5 is also split between metals and non-metals. All of the Group 5 elements have five electrons in the outer shell. For the smaller elements it is easier to complete the shell to become stable, so they are non-metals. The larger elements in the group, antimony and bismuth, tend to be metals because it is easier for them to donate the five electrons than to attract three more.
Group 6 elements, oxygen, sulfur, selenium, and tellurium, have six electrons in the outside shell.
Fluorine, chlorine, bromine, and iodine make up Group 6, the halogens. Halogens have a valence of negative one when they make ions because they have seven electrons in the outer shell. They are all diatomic gases as free elements near room temperature. They are choking poisonous gases. Fluorine and chlorine are yellow-green, bromine is reddish, and iodine is purple as a gas. All can be found attached to organic molecules. Chlorine is common in the earth's crust. Fluorine is the most active of them, and the activity decreases as the size of the halogen increases.
The inert gases or noble gases (well, the serfs did all the work, remember) all have a complete outside shell of electrons. Helium is the only one that has only an 's' subshell filled, having only two electrons in the outer and only shell. All the others, neon, argon, krypton, xenon, and radon, have eight electrons in the outer shell. Since the electron configuration is most stable in this shape, the inert gases do not form natural compounds with other elements.
Transition elements are all metals. Very few of the transition elements have any non-metal properties. Within the transition elements many charts subdivide the elements into groups, but other than three horizontal groups, it is difficult to make meaningful distinctions among them. The horizontal groups are: iron, cobalt, and nickel; ruthenium, rhodium, and palladium; and osmium, iridium, and platinum.
Lanthanides, elements 57 through 70, are also called the rare earth elements.
NOTE: All this must be new and overwhelming. Just remember the names rather than their bonding properties. We will cover this more extensively in later areas.
The last section in this chapter is radioactivity. Remember before we said that when an atom loses electrons, it becomes unstable. Radioactive atoms try always to become more stable. Generally everything in the universe tries to become more stable and orderly. Radioactive atoms have special names, at least the three we're going to talk about.
First is alpha decay. An alpha particle consists of 2 protons and 2 neutrons. When a nucleus gives off an alpha particle, it reduces its atomic number by 2 and it reduces its mass number by 4. Since the atom changes its atomic number, it basically a different element.
Second is beta decay. During beta decay, a neutron in the nucleus is converted into a proton. So that means that the atomic number goes up by 1. BUT the mass number remains the same. Weird huh? Well that's beta for ya.
Last is gamma decay. Gamma rays are basically electromagnetic radiation. Radioactive nuclei often emit gamma rays together with alpha particles or beta particles.