When you did the card sort, could you predict which piece was missing and what it would look like? If you said “orange moon with 30 as the top number and a number between 54 and 56 at the bottom” you would be correct! Just like you, Scientists use the way the elements are arranged to discover new elements! Although there are many ways the table is organized, you will learn about three specifically. Metals/Non-Metals, Atomic number and atomic mass.
Instructions: Watch the video below and read the article as well. Take focus notes as you watch and read. If you need an electronic template you can download one here:
TOPIC: Periodic Table
ESSENTIAL QUESTION: What are three ways the Periodic Table is sorted?
Watch the view below to learn how the table can be sorted by metal and non-metal elements.
Read the article below to learn what Atomic Number and Atomic Mass are and how they are used to organize the Periodic Table of Elements.
Athletes wearing the same-colored jerseys are all on the same team. In addition, each player’s jersey has a unique number to distinguish him from his teammates. Imagine how confusing it would be if members of both teams wore the same-colored jerseys or all the members of a team had the same number on their jerseys. How could you tell the athletes apart?
Telling Atoms Apart
It’s often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. It’s important to be able to distinguish atoms of one element (Links to an external site.) from atoms of other elements. Elements are pure substances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples in the Figure below (Links to an external site.).
Q: The table shown above is called the periodic table of the elements. Each symbol stands for a different element (Links to an external site.). What do you think the symbol K stands for?
A: The symbol K stands for the element (Links to an external site.) potassium. The symbol comes from the Latin name for potassium, which is kalium.
The symbols in the table above would be more useful if they revealed more information about the atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. That’s where atomic number and mass number (Links to an external site.) come in.
Atomic Number
The number of protons in an atom (Links to an external site.) is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. 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. The atomic number of an element is usually written in front of and slightly below the element’s symbol, like in the Figure below (Links to an external site.) for helium.
Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom (Links to an external site.) also tells you how many electrons the atom has. This, in turn, determines many of the atom’s properties.
Mass Number (Links to an external site.)
There is another number in the box above for helium. That number is the mass number (Links to an external site.), which is the mass of the atom (Links to an external site.) in a unit called the atomic mass unit (Links to an external site.) (amu). One atomic mass unit (Links to an external site.) is the mass of a proton (Links to an external site.), or about 1.67 × 10-27 kilograms, which is an extremely small mass.
A neutron (Links to an external site.) has just a tiny bit more mass than a proton (Links to an external site.), so its mass is often assumed to be one atomic mass unit (Links to an external site.) as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units.
Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the box above.
Q: What is the mass number of a helium atom that has three neutrons?
A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5.
Q: How would you represent this isotope (Links to an external site.) of helium to show its atomic number and mass number (Links to an external site.)?
A: You would represent it by the element’s symbol and both numbers, with the mass number on top and the atomic number on the bottom:
Summary
- Elements are pure substances that make up all matter, so each one is given a unique name. The names of elements are also represented by unique one-, two-, or three- letter symbols.
- The number of protons in an atom is called its atomic number. This is also unique for each element.
- An atom’s mass number is its mass in atomic mass units (amu), which is about equal to the total number of protons and neutrons in the atom. Different isotopes of an element have different mass numbers because they have different numbers of neutrons.