Understanding The Periodic Table
THE PERIODIC TABLE
The list of all the elements in the world, in order from the lightest to the heaviest, is written down on the Periodic Table. Students could be given their copy of the Periodic Table, and asked to find hydrogen on it.
(Teacher’s note: The more updated definition of Atomic Number is that it is equal to the number of protons in an atom’s nucleus. However, the original PT was constructed as a ranking of elements in order of their masses and it remains consistent with the more recent definition. It is a more concrete and conceptually easier way of introducing the Periodic Table to students. The other definition will be explored in the Atomic Structure section.)
Rows and Groups
When scientists laid out all the elements in one long horizontal row, they realised that some of them had remarkably similar properties. For example, helium (2), neon (10), argon (18), krypton (36), xenon (54) and radon (86) were all unreactive, colourless gases. (“Unreactive” here means that they do not join to other atoms.) Scientists call this group of elements the Noble Gases, because they were a bit “aloof” in their avoidance of chemical bonding. To show that these elements shared striking similarities, they broke the long list into smaller segments, and lined up theses similar Noble Gases into the same vertical column (See Group 18 on the Periodic Table). They became a family of elements.
They also found that they could line up other groups of elements into various vertical columns, too. The elements with an Atomic Number one less than the Noble Gases were all similar, and were called the Halogens and aligned as the vertical column, Group 17 on the Periodic Table. Elements with Atomic Number one more than the Noble gases were all soft metals that exploded in water, and were called the Alkali Metals (Group 1). Elements with Atomic Numbers one more than the Alkali Metals were called the Alkaline Earth Metals (Group 2). These four named groups show the strongest consistency of similar properties than the other groups on the Periodic Table, but all elements in the same vertical group share similarities, especially those close to each other.
The splitting all the elements into segments and rearranging them in vertical columns with similar elements automatically created 7 horizontal rows. Each horizontal Row, also called a Period, begin with metals on the left, and gradually move to non-metals on the right. For example, Row 3 begins with the metal sodium on the left side, then part way across is semi-metallic silicon, then at the far right side is non-metallic argon.
Going down from one Row to the next increases the size of the atoms i.e Row 2 elements are bigger than Row 1 element, and so on. They are also, of course, heavier, because they have higher Atomic Numbers.
The two “rows” at the bottom of the PT, called the Lanthanides and Actinides, are not really extra rows, but belong to Rows 6 and 7 respectively. If inserted into these positions, the PT would be too wide to conveniently fit onto a single page, so these Row fragments are extracted and written separately.
Metals and Non-metals: The grey diagonal band
The grey diagonal band across the PT divides it into the metals on the left, and non-metals on the right. Metals have a cluster of properties including shininess (unless ‘tarnished’), good electrical conductivity, good heat conductivity, malleability, ductility etc. Non-metals have the opposite properties including dullness, electrical non-conductivity, poor heat conductivity, brittleness etc. The elements within the grey diagonal band are called semi-conductors, semi-metals, or metalloids. They have intermediate properties, for example, they conduct electricity a little bit. This property makes silicon ideal for use in computer chips. The position of the grey diagonal means that elements of a particular Row start out on the left as non-metals and progressively become more non-metallic to the right. Each consecutive Row repeats this same metal-to-non-metal pattern, which is why it is called the Periodic Table. Elements within a particular group start out as being more non-metallic on the top and progressively become more metallic going down. Both Groups 14 and 15 start as non-metals at the top, and become metals at the bottom. Group 1 elements are all metals, but the elements at the bottom are more metallic than those above i.e. potassium is more malleable than sodium, which is more malleable than lithium.
Hydrogen is an exception to the rule, in that it is to the left of the grey diagonal band, yet is a non-metal. It appears to belong to Group 1, but it isn’t really an alkali metal. Some PTs write hydrogen further to the right (in the non-metal section) to reflect this.
Nearly ¾ of the elements on the PT are metals. All the metals are solids, except liquid mercury. Many of the non-metals are gases, but as one goes down a non-metal group, they become progressively more ‘condensed’. In Group 17 for example, fluorine and chorine are gases, but bromine is a liquid, and iodine and astatine are both solids. Mercury and bromine are the only two liquids on the PT. (Of course, if we heated the solid elements they would melt and become liquid. However, the solid, liquid or gaseous states refer to those at normal temperatures and pressures.)