Atomic Structure Part I:
The Periodic Table of Elements

Elements and Atoms

Atoms are the building blocks of all matter. In one sense they are the smallest pieces of ordinary matter that there are. The idea of a ‘smallest piece of matter’ has been around for at least 2,600 years. Around 600 B.C. a Greek philosopher by the name of Democritus proposed the existence of atoms. He suggested that if you were to cut something in half, then cut it in half again and again and again you would eventually find that there was an end to the process. You would come to a point where you could not cut what you have left in half anymore. He called what was left an atom. The word ‘atom’ comes from Greek for ‘uncuttable’.

Another Greek philosopher by the name of Aristotle disagreed. He claimed that there was no such thing as a smallest piece of matter. Aristotle believed that you could go on cutting things in half forever. The two philosophers never tried to do any experiments to find out who was right and the world waited until the middle of the 18^th century to find out (around 24 centuries!). The ancient Greeks did not believe in doing experiments to find out about the world. Instead, they tried to make discoveries simply by thinking carefully about things.

As it turns out, neither of them was exactly right or exactly wrong. Democritus’s idea of the atom is a very useful one but he was wrong about one thing: an atom is made up of smaller parts. So Aristotle was right about something, too: atoms are not the smallest pieces of matter. Atoms have parts called protons (p⁺), neutrons (n⁰) and electrons (e^-). Modern scientists have discovered that even the parts of atoms have parts but that is a discussion for another day.

The modern definition of the atom is the smallest particle of an element. Atoms come in different varieties called elements. Elements are categories for atoms: they are not physical objects. Atoms can be different elements in the same way that candy comes in different flavors. Sour-apple-flavored candy you can eat; sour-apple flavor is a description, not something you can eat. The elements were first discovered as elements during the 17^th and 18^th centuries. At that time scientists were making the first experiments that would break down everyday substances into the elements they are made up of. In fact, one definition of the word ‘element’ is: essential part. The things that these early chemists could not break down into simpler substances they called elements. This is the definition of ‘element’ that is important in chemistry: an element is a type of substance which cannot be broken down into simpler chemicals.

One of the defining features of an element was discovered as a result of the work of Dmitri Mendeleev, the inventor of the periodic table. He first made a version of the periodic table by putting elements with similar attributes in the same column. He put them in order by atomic mass (defined below) and found that a regular pattern resulted. Mendeleev had stumbled acoss the Periodic Law: the idea that the physical and chemical attributes of the elements recur periodically if the elements are organized by the atomic number. Take a look at your copy of the periodic table. Notice the large number at the top of each sqaure in the table. This is called the atomic number and it refers to the number of protons (p⁺) and electrons (e^-) in an atom of that element.

Protons (p⁺) and electrons (e^-) are two of the three particles that are put together to make atoms of all the elements. Atoms of each element are defined by the number of protons they have. The number of electrons exactly balances the number of protons so that the atomic number tells you how many protons an element has and how many electrons it has. The atomic number is referred to by chemists by the letter Z. For example, Carbon (C) is element number 6 so for C Z = 6. Protons are particles that are found in the nucleus of an atom: the tiny, massive center of an atom. They all have an electric charge of +1. Electrons orbit around the nucleus, are much smaller, and have an electric charge of –1.

In addition to an atomic number, every element has a unique atomic symbol. Normally, the symbol is derived from the name: Carbon is C, Oxygen is O, Boron is B, Uranium is U, etc. Some symbols are derived from ancient names for elements, especially from Latin: Silver (Ag) gets its symbol from the Latin word for silver: Argentum. Gold’s symbol (Au) is from Latin, too: Aurum. For every symbol the case of the letters is important: Na (Sodium) is never NA or na or nA.

A neutron is another particle found inside atoms. They, like protons, are found in the nucleus but they have an electrical charge of 0. Together, protons and neutrons make up more than 99% of the mass of each type of atom (element). The atomic mass is the number found directly below the atomic symbol in each square of the periodic table. The mass given is an average mass for each element but the mass number of an element is defined as the sum of the number of protons and neutrons. The definition of atomic mass is that it is the sum of p⁺ plus n⁰. Chemists symbolize atomic mass using the letter A. For example, Carbon’s atomic mass is 12 so A = 12 for ¹²C. The units of atomic mass are atomic mass units. An atomic mass unit is defined as ¹/₁₂ of the mass of a carbon-12 atom. 1 proton (p⁺) has the same mass as 1 neutron (n⁰): one atomic mass unit. The mass of an electron (e^-) is about 5.4 × 10^-4 atomic mass units. The atomic mass unit is abbreviated by amu.

Each element has only its defined number of protons but there is some leeway as to how many neutrons an atom can have. For example, carbon atoms all have 6 protons but there are three common types of carbon atom which have different numbers of neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Carbon’s isotopes are carbon-12, carbon-13, and carbon-14. The number at the end of each name is the atomic mass number for the isotope. Carbon-12 has 6 protons and 6 neutrons, carbon-13 has 6 protons and 7 neutrons, and carbon-14 has 6 protons and 8 neutrons. The mass of carbon-12 atoms is 12 amu; carbon-13, 13 amu; carbon-14, 14 amu. Because each element can have two or more common isotopes, the atomic mass in the periodic table is an average of the diffent masses. Isotopes of an element have nearly the same chemical and physical properties, they only differ in their atomic mass numbers.

Understanding the Structure of the Periodic Table

Reading the periodic table:

You will color your Periodic Table On the web this links to a page showing the coloring that is taught with this lesson. according to a color scheme shown by your teacher. And available here. The periodic table is divided up into groups and periods. The groups are columns and are numbered from 1 to 18. The power of the periodic table comes from the fact that elements in each group all have similar properties. Take the Noble Gases, group 18. All of these elements share the common trait that they have practically no chemical reactivity at all. That means that they do not combine with other atoms to form compounds.

The periods are the rows of the periodic table. The periods do not have any predictive power about the physical and chemical properties of the elements. The rows below the main table that start with lanthanum (La) and actinium (Ac) belong to periods six and seven. Notice that at element 56 (Barium, Ba) it goes straight to element 72 (Hafnium, Hf). Similarly, there is a gap in the numbering at element 88 (Radium, Ra). The missing elements (from 57 - 71 and from 89 - 103) are found below the main table because the table becomes inconveniently wide when you put them in their proper places.

The block on the right side of the table consists of metals, semi-metals, and non-metals. Metals (mostly groups 1 - 12 but including some elements in groups 13 - 15) are shiny, conduct electricity and heat well, and are easily bent and shaped. Non-metals (groups 14 - 18) are dull-colored, do not conduct electricity or heat, and are brittle (or are gases). Semi-metals (the stair-step pattern) have traits of both metals and non-metals. The halogens (group 17) are very reactive elements. The noble gases (group 18) are very unreactive elements. The alkali metals (group 1) are very reactive metals and the alkali earth metals are similar, but less reactive metals. The rare earth metals (elements 57 - 70 and 89 - 102) are all quite similar to each other and are largely metallic in character. Hydrogen (H) falls into its own special group because it is not like any other element.

Z = 42 & A = 96 Z = the number of protons Z = the atomic number

A = the atomic mass A = the atomic mass number Z + n0 = A or A - Z = n0

Questions