Most of us know the three fundamental particles of which all atoms are composed. The electron, the proton and the neutron. Protons and neutrons are together and make up the nucleus of an atom. The electron is located outside the nucleus.

Electron

This was the first atomic particle discovered by J.J. Thomson in 1897. He characterized the properties of cathode rays, as a stream of negatively charged particles or electrons. Thomson found the particle to be negatively charged. He was also able to measure the charge-to-mass ratio of the cathode rays. The value he obtained was independent of the gas used in the cathode ray tube.

It was not until 12 years later that Robert Millikan was able to determine the charge of an electron. He experimentally measured a value of -1.6022 x 10^-19 Coulombs. Using Thomson's charge-to-mass ratio the mass of an electron had a value of 9.109 x 10^-31 kg.

Proton

Thomson experimentally determined the existence of positively charged particles in the cathode ray tube, but he was unable to characterize these particles further. In 1919 Ernest Rutherford characterized the proton as a particle with a charge equal in magnitude to that of the electron but with the opposite sign. The mass was measured as 1.673 x 10^-27 kg.

Neutron

The neutron was characterized by James Chadwick in 1932. The neutron has almost the same mass as the proton and no charge.

Structure of the Atom

Our current view of the structure of the atom was described as a result of experiments performed under the direction of Ernest Rutherford. In his experiment alpha particles (which he had characterized by 1908) were 'shot' at a thin piece of gold foil. The behavior of the scattered particles lead Rutherford to postulate a new model of the atom. His model, which we currently hold, locates almost all of the mass of the atom in the nucleus with the electron located outside the nucleus.

Knowing that all atoms contain electrons, protons and neutrons we might ask how many of each of these particles are found in an atom? The answer to that question depends on the element. Elements have only one kind of atom. It turns out that information in the periodic table tells us the number of electrons, protons and neutrons in each atom of an element. We will not go into the details of how it was discovered, but a scientist named Moseley discovered a correlation between the X-rays released by an atom and the number of protons in the atom. Moseley suggested that the elements in the periodic table could be ordered using an 'atomic number', a whole number representing the number of protons in the atom. So the number above the symbol of each element in the periodic table is called the atomic number and equals the number of protons in that element.

The number of electrons equal the number of protons for a neutral atom.

To obtain the number of neutrons in an atom we need to first discuss the atomic mass number.

The atomic mass number is located below the symbol of the element in the periodic table. For hydrogen the atomic mass is 1.00797. When we hear the term atomic mass we think of the mass of an atom. For example the mass of a hydrogen atom is 1.6736 x 10^-24 grams. Now this is interesting...? The two numbers are different! The first thing we notice is the number below the symbol does not have any units. It turns out the atomic mass number, the number written below the symbol on the periodic table, is a 'relative' mass. Relative to what you might ask? The atomic mass is relative to a particular carbon atom. The carbon atom selected is the carbon atom with 6 protons, 6 electrons and 6 neutrons. This particular atom has a mass of 1.99268 x 10^-23 grams. By definition an atomic mass unit (u), also called a Dalton, is equal to 1/12 of the mass of the carbon atom with 6 protons, 6 electrons and 6 neutrons.

1/12 * 1.99268 x 10^-23 grams = 1.66057 x 10^-24 grams = 1 atomic mass unit

Now we can calculate the mass of a hydrogen atom in atomic mass units (u).

So the mass of the hydrogen atom when expressed in atomic mass units is 1.0078 u. But the atomic mass in the periodic table is 1.00797 u. Why the difference...a calculation error? No. The reason for the difference is because a sample of any element is not composed of the exact same type of atom. Each of the atoms in a pure sample of an element have the same number of protons, but the atoms of the sample have different numbers of neutrons. Because of this the atoms in a pure sample of an element have different masses. The term isotope is used to describe the atoms of an element that differ only by the number of neutrons.

Isotopes of an element are atoms with the same number of protons, but a different number of neutrons. For example, a sample of the element hydrogen has two different isotopes, one of the isotopes has 1 proton and 0 neutrons while the other isotope has 1 proton and 1 neutron. This second isotope of hydrogen is called deuterium. Carbon has two stable isotopes ¹²C and ¹³C. It has many more unstable isotopes. ¹⁴C is one of the most common radioactive isotopes of carbon.

The atomic masses found in the periodic table take into account all of the stable isotopes and their masses to determine a weighted average atomic mass. The 'weighted average' means that we must account for the difference in abundance of each isotope of an element. Hydrogen has two isotopes, as shown below;

So a pure sample of hydrogen contains mostly hydrogen with a small amount of deuterium. To calculate the weighted average atomic mass for the element hydrogen we must account for the different abundances of the two isotopes. We can not just add the two masses together and divide by two because doing that would assume the two isotopes were equally abundant. (Go here for an example of what I mean.)

To calculate the weighted average of a sample we must use the following equation;

weighted average = (atomic mass * fractional abundance)₁ + (atomic mass * fractional abundance)₂ +....+(atomic mass * fractional abundance)_n

Let's go ahead and use this approach to calculate the weight average of the element hydrogen.

Here is another example that we did in class.

So the atomic mass number below the symbol of the element in the periodic table is called the relative, weighted average atomic mass. The units are atomic mass units (u). Finally if we round the atomic mass to the nearest whole number that value is equal to the sum of the protons and the neutrons in the element.

Before we forget try these problems to be sure you know how to determine the protons, electrons and neutrons in an atom.

The periodic table is a very important tool which contains a very large quantity of information. We learned some of the information the periodic table reveals to us. The symbol for each elemetn is included in the periodic table. The formula is not shown, that is information we need to recall from memory. We know the physical state of all of the elements in the periodic table. We know the reactivity of the alkali metals with water, increases as you go down the group.

The elements in the periodic table can be classified into three groups based on their physical properties; metals, nonmetals and metalloids.

The metals are the largest group of elements. They are;

• all solids (except mercury);

• have a lustrous (shiny) appearance;

• are malleable and ductile;

• are good conductors of heat and electricity.

The next largest group are the nonmetals. The nonmetals;

• are found in gas, liquid or solid state;

• lack the remaining properties of the metals, i.e. are not malleable or ductile and are generally poor conductors (graphite is a very good electrical conductor).

The metalloids are a small collection of elements that lie between the metals and the nonmetals in the periodic table and share some of the properties of metals and nonmetals.

In the periodic table below the metals, nonmetals and metalloids are color coded for easy identification. The metals are blue and nonmetals are yellow.

Recall that isotopes are atoms of the same element which differ in the number of neutrons in the nucleus. What happens when we change the number of electrons on an atom? Since electrons have a negative charge changing the number of electrons will cause a neutral atom to acquire charge. Lose or gain of electrons produce species with different charge. A charged atom is called an ion. There are two types of ions; cations and anions. A cation has fewer electrons than protons and occurs as a result of the loss of electrons. Cations are positively charged. An anion has more electrons than protons and is produced when a neutral atom gains electrons. Anions are negatively charged. Cations ae attracted to anions due to the opposite charges.

How do we represent ions using elemental symbols? Do atoms prefer to gain or lose electrons? Or do atoms do both?

To help understand why atoms gain or lose electrons we need to look at a group of elements in the periodic table that prefer NOT to gain or lose electrons. This group is the noble gases. These elements are the least reactive of all the elements in the periodic table. When we investigate the behavior of elements around the noble gases we discover they gain or lose electrons so that they form ions with the same number of electrons as the nearest nobel gas.

Let's consider some examples to show this happening.

The first example we'll talk about is sodium. The symbol for sodium is Na. If we consider the most common isotope of sodium, ²³Na, there are 11 protons, 11 electrons and 12 neutrons. Sodium has one more electron than the noble gas neon, which has ten electrons. Sodium will lose one electron to have the same number as neon. A sodium atom with 10 electrons and 11 protons will have one extra positive charge. The symbol for the sodium cation is Na⁺. In fact all of the elements in the alkali metals lose one electron to form the 1+ cation. That is why the Group number is IA. The 'I' stands for the number of electrons these metals lose when they combine with other elements. The alkaline earth metals are in Group IIA. These metals lose two electrons, forming 2+ cations. Aluminum in Group IIIA loses three electrons to form a 3+ cation. Metals prefer to lose electrons to form cations. While the Group 1A, IIA, and IIIA elemetns lose those same number of electrons, the transition metals elements can lose two, three and sometimes more electrons. For our purposes we'll assume transition metals will lose either two, or three electrons. There are however, some exceptions to this statement. Silver prefers to lose only one electron, and copper and zinc prefer to lose only two electrons. We'll cover this issue again in Chapter 7.

Chemists classify compounds into two broad groups; ionic compounds and covalent compounds. Although there are many properties which distinguish the members of these two groups, at this point in CHEM 1314 we will only mention a few. First, all of the ionic compounds are solid at room temperature. Ionic compounds contain ions, so for those ionic compounds soluble in water, ions are formed. In terms of the elements, ionic compoudns are composed of at least one metallic element and at least one nonmetallic element. We've discussed the elements which fall into these groups.

Covalent compounds are found as solids, liquids or gases at room temperature. The covalent compounds are not made of ions. In many cases, covalent compounds do not form ions when added to water. However, there are some exceptions to this statement. When looking at the formula of a covalent compound we note the elements are only nonmetals.