Atomic Number

The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of the same element have the same number of protons, meaning they all have the same atomic number; atoms of different elements have different numbers of protons in its atoms, meaning their atomic numbers will also be different. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. However, for carbon atoms always have 6 protons, providing for the atomic number of 6.

Since atomic numbers are unique to each element, we can use them to identify elements if we have a periodic table of elements. An example of the periodic table of elements is below. While we will learn more about it, for now, we are going to focus on some simple details.

For our purposes in this activity, we will be using the periodic table to help use identify the atomic number, atomic mass, the quantity of subatomic particles (protons, electrons, and neutrons), and to identify each element. Each element is contained in its own box on the periodic table; currently there are 118 boxes on the periodic table, meaning that there are 118 elements known to exist.

Each box details the atomic number (at the top of each box), the element symbol (one capital letter or two letters - one capital letter and one lowercase letter), element name (usually), and the atomic mass (below the element abbreviation) for each element. Notice the atomic number is a whole (non-decimal) number, while the atomic mass often contains decimals. Since it represents the number of protons in an atom of an element, the atomic number must be a whole number, as an atom cannot contain part of a proton. Also, for a single atom, the number of protons cannot change, to do so would change the identity of the element.

The figure below displays the element box for hydrogen (H) from the above periodic table. Notice the atomic number is above-left of the element symbol. In this case, it is a '1' meaning that hydrogen atoms have 1 proton.

Atomic Mass (Mass Number)

The vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number is defined as the total number of protons and neutrons in an atom. It can be calculated by adding the number of neutrons and the number of protons (atomic number) together.

Mass number = atomic number + number of neutrons

Contrary to the atomic number, atomic mass is often reported as a decimal number. This is because, while the number of protons for an atom will not change, the number of neutrons can. For example, there three different "versions" of hydrogen - protium, deuterium, and tritium - and the difference between them all is the number of neutrons in the nucleus. What this means is the atomic mass is often reported as the average atomic mass. While it is true that most forms of hydrogen only have 1 proton and 0 neutrons, there are others that have 1 or 2 neutrons - this is why the atomic mass in the element box for hydrogen (below) is 1.008 instead of merely 1.

While the periodic table does not report how many neutrons an element normally has, we can use the mass number along with the atomic number to determine this. However, we must round the mass number to the nearest whole number.

number of neutrons = mass number - atomic number

For hydrogen → 1.008 = 1; neutrons = 1 - 1 = 0

Thus, most forms of hydrogen do not have any neutrons.

Atomic Charge Calculation

Definition of Atomic Charge:
Atomic charge is determined by the balance of protons and electrons in an atom. Protons have a positive charge, while electrons have a negative charge. Neutrons are neutral and do not contribute to the charge.

Calculation:

• Atomic Charge = Number of Protons - Number of Electrons

• Example:

  • A sodium atom has 11 protons and 10 electrons.

  • atomic charge = 11 - 10 = +1

Particles Involved:

• Protons: Positively charged particles found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus.

Particles Not Involved:

• Neutrons: These particles are neutral and do not affect the overall charge of the atom.

Different Types of Charged Atoms:

• Ions: Atoms that have gained or lost electrons, resulting in a net charge.

  • Cations: Positively charged ions (more protons than electrons).

  • Anions: Negatively charged ions (more electrons than protons).

Atomic Charge Simulation

Directions:

1. Review the data table below the simulation; across the top, are element abbreviations for five different atoms; underneath each are the quantity of electrons and protons for each atom.

2. Select the needed atom at the top of the simulation (begin with hydrogen - H), then make the necessary adjustment to the protons/electrons for that atom by use the red -/+ button in the simulation.

3. In the simulation, read the charge for each atom and report it in the corresponding place in the data table (bottom box for each atom).

4. You will use the information collected on the data table to answer questions.