AT_Other Contributions to Early Atomic Theory Lesson

Subatomic Particles

Located in an extremely dense region of space in the center of the atom, called the nucleus, are the protons. Protons are the identifiable characteristic of atoms, defining which element it is. All atoms of the same element have the same number of protons. For example, all atoms with 6 protons are carbon atoms. The proton was found to have a positive charge and a mass of approximately 1 u, a unit of mass used to measure extremely small particles such as this.

Also located in the nucleus is the neutron. It has no charge and has a mass of also approximately 1 u. Electrons are located outside of the nucleus. The area of space they occupy is responsible for the vast majority of the volume of the atom. Electrons have a negative charge and a mass that is about 1/1800 that of a proton or neutron. This mass is so small that it is often neglected (meaning rounded to zero). The function of the electron is extremely important, as this is where bonding occurs.

In the diagram to the right, protons and neutrons are shown in red and purple in the nucleus. The electrons are in the cloud, shown in this diagram in the shaded are around the nucleus.

Subatomic Particle

Mass (g)

Mass (u)

Electrical Charge

proton

1.672631x10-24

1.007276470

1+

neutron

1.6749286x10-24

1.008664904

0

electron

9.1093897x10-28

0.000548579903

1-

Writing Symbols

Atom.pngThe number of protons is often called the atomic number, and given the symbol Z. The number of protons and neutrons in a given atom is known as the mass number, A. We can use these numbers, along with the element symbol, to create a shorthand way of identifying elements.

ATOMIC NOTATION A - mass number Z - atomic number X - generic element symbol

Look at the symbol for uranium-235.

Example 235, U 92 Element Symbol: U Element Name: uranium Mass number (A): 235 Atomic number (Z): 92 By knowing the atomic number, we can identify the number of protons. Protons = atomic number = 92 By knowing the mass number, we can identify the number of neutrons. Neutrons = mass number - atomic number = 143 By knowing that atoms are, by definition, neutral, we can identify the number of electrons. In an atom, Electrons = protons = atomic number = 92

Watch the following video to review subatomic particles.

Recall from your first year chemistry course that information about the subatomic particles of each element can be found easily on the periodic table. The whole number within each element box is the atomic number (meaning, the number of PROTONS. Since the periodic table shows only atoms, we know that the number of protons equals the number of electrons . The larger number in the box that is not a whole number is known as atomic mass. This is similar to mass number, but not exactly the same. We will discuss this after you check your understanding with the interactive table below. You will see questions similar to this on the module assessments.

Atomic Mass

Now let's talk about the difference in mass number and atomic mass (found on the periodic table). Unlike the number of protons, the number of neutrons in different atoms of the same element can differ. These are called isotopes. If you take a natural random sample of an element from anywhere in the world, you will find the same percentages of the different isotopes present. This is known as relative abundance and is used to determine the average mass for that element.

Understanding the Difference in Mass Number and Atomic Mass 25% of the students made an 84 Let's say that this is a breakdown of grades on a test: 75% of the students made a 96 The individual grades made were 84 and 96. This is like the mass number, representing just one atom. If you just averaged these two numbers, you would get a mass of 90. (84+96) 2 = 90 However, that is not how the average atomic mass is reported on the periodic table. The mass is a weighted average. The weighted average class grade on this test would be found like this (84x.25)+(96x.75)=93 So, 93 is like the atomic mass on the periodic table. It represents an average of the sample.

Watch this video for a recap about elements and isotopes, and then try the problem below.

If you were confused by the significant figures mentioned in the explanation above, you should review and practice this skill. Remember to always express answers in the correct number of significant figures!

Recall Dalton's Atomic Theory

So, let's go back and recall the points of Dalton's atomic theory. See if you remember them on your own before you look below!

As you read through each one, think about what else you have learned. Some of the information discussed in this content contradicts certain aspects of Dalton's theories.

Can you find the failures of Dalton's atomic theory? • Matter is made of tiny particles called atoms. • Atoms of the same element are identical in size, mass, and other properties. • Atoms of different elements are different in size, mass, and other properties. • When atoms of different elements combine to form compounds, they do so in a fixed numerical ratio. • Atoms are indivisible. They can rearrange, but do not themselves break apart.

image of Dalton holding an "I'm not perfect" signThe second statement is not correct.  Atoms of the same element CAN differ in mass because of the number of neutrons.  These atoms are called what? The fifth statement is not correct.  Atoms are made up of subatomic particles and so are divisible.

We now know that this theory is not perfect. Atoms can be broken into smaller pieces. Most elements exist with isotopes, which are elements that have different atomic masses due to different numbers of neutrons in their nucleus. But, this fundamental theory explained the laws that govern how chemicals combined with each other quite successfully. 

Test your learning by doing the following problems:

Remember to work on the module practice problems as you complete each section of content.

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