Lecture 2.1: Atoms, Ions, and Isotopes

Lecture 2.1: Atoms, Ions, and Isotopes

By Joshua Baltzell

Atoms are made of smaller particles called protons, neutrons, and electrons.

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What do atoms look like?

​What atoms are made of...

The Subatomic Particles

Protons

Protons are positively charged subatomic particles that are located in the nucleus of the atom. Each proton has an atomic mass of 1 amu. We use the symbol p⁺. The number of protons is very important to atoms, as it is also the atomic number. This is how we identify elements. If the number of protons changes, the element changes.

Consider what happens to similarly charged magnets when they're held together. Why don't protons do this if they're packed in the nucleus?

The Subatomic Particles

Neutrons

Neutrons are neutrally charged particles that are also in the nucleus of the atom which also have a mass of 1 amu. They create a buffer between the protons preventing the protons from repelling each other. They use a strong nuclear force to hold one another together. The symbol we use is n⁰.​

Consider what happens to similarly charged magnets when they're held together. Why don't protons do this if they're packed in the nucleus?

The Subatomic Particles

Electrons

Electrons are negatively charged particles​ that reside outside of the nucleus in the electron cloud. The electrons have nearly no mass (it's considered negligeable because of how miniscule it is). When calculating atomic mass, the electrons are considered to have a mass of 0 amu. The symbol we use for electrons is e^-.

Why don't electrons just fly away from the atom? What is holding them in the cloud? Consider how magnets work and the structure of atoms.

​Picture of an atom...

​This is a photo of an atom of strontium. Strontium is one of the larger atoms, but notice how tiny it is even zoomed in.

Atomic Models

Since atoms are really small and difficult to see, we use models to represent them while we study them. Models are visual representations of something that is either too big or too small to be seen clearly. In chemistry, there are several models used in the study of atoms. Each model is used to explain a concept in chemistry.

Which model do you think most accurately describes the appearance of atoms?

The Planetary Model of atoms is the most commonly recognized model. However it is the least accurate among the models regarding structure.

Planetary Model

Modeling the Structure of Atoms

Why do you think the planetary model is considered the least accurate?

The Bohr model is used for describing not only the general location of subatomic particles, but it shows the energy levels of electrons.

Bohr Model

Measuring Energy with Models

Which electrons do you think have the most energy? The next model may help with this question.

This model depicts the random nature of electrons while showing the probable location of them.

Electron Cloud Model

Measuring Electron Locations

Using the model above, what happens to the energy of electrons as they move further away from the atom?

Open Ended

Using the previous two models, what do you think happens to the energy of electrons as they move away from the nucleus of the atom?

Adding Energy to Atoms

As energy is added to atoms, the energy can be absorbed by any of the subatomic particles. When electrons absorb energy, they become excited. Excited electrons will move away from the nucleus to their excited state.

What would this do to the size of the atom?

Ions

What would happen if enough energy were added to remove an electron?

If electrons become excited enough, they can actually be removed from atoms. This creates an ion. If we remove an electron, it will actually change the charge of the atom.

Example: Sodium (Na) with 11p⁺ and 11e^- become Na⁺ with 11p⁺ and 10e^-.​

Can the number of protons ever change?

Formation of Ions

Not only can electrons be removed from atoms, atoms can actually steal electrons to gain them. This would increase the negative charge of the atom.

Example: Fluorine (F) has ​9p⁺ and 9e^-. It steals an electron becoming fluoride (F^-) which has 9p⁺ and 10e^-.

What happened to the name of the element when it became more negative?

Fill in the Blank

Calcium (Ca) has a habit of losing two electrons when it gets too close to certain elements. What would the charge of this new ion be?

Fill in the Blank

Nitrogen (N) is known to steal three electrons from other elements becoming nitride. What would the charge of this new ion be?

Open Ended

Considering the formation of ions, explain how atoms become charged. Discuss where new electrons come from or go to, what this does to the charge of the atom, and see if you can explain name changes r

Naming Ions

When an ion is formed that develops a negative charge, we change the name of that ion to signify that it has developed a charge. We do this for negatively charged ions because they typically require some kind of external force to develop the charge.

The name negatively charged ions, we take the root of the element's name and at -ide to the end of it.​

What would be the name of a negatively charged atom of Oxygen?

Naming Ions Examples

Fluorine → Fluoride

Oxygen → Oxide​

Nitrogen → Nitride​

Iodine → Iodide​

Sulfur → Sulfide​

Isotopes

Are all atoms of the same element exactly the same?

The answer is no! We know that the number of protons cannot change without changing the element, but the electrons can fluctuate while remaining the same element. This just forms ions.

Which subatomic particles can be changed while remaining the same element?

Changes to Neutrons

Neutrons, like electrons, don't have to be the same between different atoms. We can alter the number of neutrons which alters the mass of the atom. Every atom of an element has a slightly different mass depending on the number of neutrons contained in the nucleus.

​Remember that the mass of an atom is the sum of the protons and neutrons because they are the particles with mass.​ The different masses of elements are called isotopes.

Subject | Subject

How do we calculate the mass of an atom?

Different flavors of atoms

Ice cream is a common dessert with multiple flavors. Just because one ice cream is chocolate and the other is vanilla doesn't mean they are no longer ice cream. Similarly, carbon can have a mass of 12 or a mass of 14, but still be carbon. This is because of the change in neutrons.​

Does the difference in neutrons affect the charge of the atom?

​Carbon Isotope	​Protons (p+)	​Neutrons (n0)	​Mass (amu)
​Carbon-12	​6	​6	​12
Carbon-14​	6​	​8	14​

Fill in the Blank

How many neutrons would an atom of nitrogen (N) have if it had a mass of 15 amu?

Fill in the Blank

What is the mass of an atom with 12p⁺, 11n⁰, and 13e^-?

Looking at the Entire Element

Average Atomic Mass

It's not enough for us to look at individual atoms. We need to have numbers we can use to measure entire elements. For this, we calculate the average atomic mass of different elements. This is the weighted average of different isotopes.

It's important to recognize that some isotopes are more common than others.​

How is an average calculated? Can we use that kind of average for atomic masses?

Mixtures of Atoms

Within a sample of an element, there are going to be many different isotopes of the same element. This means that you may have a sample of carbon that contains Carbon-12, Carbon-13, and Carbon-14.

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Because of this, a standard average won't be sufficient for finding the average atomic mass. We need a weighted average.​

Where have you heard of weighted averages before?

Weighted Averages

Your grade is a weighted average. This means that a value only counts towards a total based on the weight it holds in the form of a percent. For example, let's say your grade is based on two tests, both worth 50% of your grade. However, the first test is worth 10 points, and the second is worth 1,000 points.

If you scored a 1/10 on the first test and a 999/1,000 on the second, what grade did you get in the class?​

Subject | Subject

Some text here about the topic of discussion

What does your grade end up being?

Your final grade ends up being about a 55%. This may seem weird, because the test you failed was worth 10 points, and the one you aced was 1,000, but remember the tests are weighted the same!

Application to Atomic Mass

We discussed how isotopes are not all equally common. Some isotopes of an element are more common than others. We refer to this as the isotopes abundance. For example, 98.9% of the carbon on earth is Carbon-12. Carbon-13 only makes up 1.1% of the abundance.

If we found the mathematical average of this element, it would be 12.5 amu's.​

Would using a normal mathematical average work for finding the average?

Average Atomic Mass Equation

In order to solve average atomic mass, we use the following equation...

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For example, Magnesium has 3 isotopes with the following abundances: ​²⁴Mg (79.0%), ²⁵Mg (10.0%), and ²⁶Mg (11.0%).

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Fill in the Blank

What is the average atomic mass for an element that has the following isotopes:

Mass [% abundance]

87 amu [12%]

88 amu [8%]

89 amu [80%]

(Round your answer to 4 significant figures)