2-2: The Bohr-Rutherford Model

In the previous section we examined how we got to the Bohr-Rutherford model of the atom, which is the model of the atom we will be examining here in more detail. Now we will examine the parts of the atom in more detail.

Subatomic Particles

A submarine is a boat that goes under the surface of the water.

A subscript is text which is set below the rest of the text around it, like this.

So, whenever you see the prefix sub- on something, that usually means it is under, or below, something else.

A subatomic particle is a particle which is below, or smaller than, the size of an atom. Remember, neither Dalton nor Democritus thought there could be pieces of matter smaller than atoms, so all of this would've been a surprise to them. In the Bohr-Rutherford model, there are three subatomic particles we need:

• Protons are positively charged, are located in the nucleus, and are relatively heavy.

• Electrons are negatively charged, are located on the outside of the atom, and are relatively light.

• Neutrons do not have an electric charge, are located in the nucleus, and are relatively heavy.

We can see all three of these particles in the same Bohr-Rutherford diagram we saw before:

Sometimes we also call protons and neutrons nucleons, because they exist in the nucleus of the atom, which is in the centre. The diagram above doesn't give a sense of the scale of the atom, but the analogy below can. (One femtometre (fm) is one millionth of one billionth of a metre.)

Needless to say, the atom is mostly empty space.

Atomic Number vs. Mass Number

We will be looking at the Periodic Table of the Elements a little later -- there is a lot of information packed into it -- but if we examine one part of it, there are two numbers we need to know. Not every Periodic Table is going to look exactly like this, but most of them will have all the same information on it, even if it's in a slightly different place.

If we examine one small part of the Table, we can see the two numbers that we need to understand.

• Atomic Number is always a whole number, and it will always be the smaller of the two numbers in each box.

• Mass Number (also called Atomic Mass) is always a decimal, and it will always be the larger of the two numbers.

But, what do each of these represent?

• Atomic Number is the number of protons in the nucleus of all atoms of that element. Since you can only have 3 protons, or 11 protons, or 95 protons -- but never 94.7 protons -- that's why it's always a whole number.

• Mass Number is the (number of protons) + (number of neutrons), on average, for an element. For now, just round to the nearest whole number.

Looking at the example above, for the element beryllium...

• The atomic number is 4, so there are 4 protons in the nucleus of a beryllium atom.

• The mass number rounds to 9, so (protons + neutrons) must be 9, on average. Out of those 9, 4 are protons, so there must be 9 - 4 = 5 neutrons in the nucleus.

Isotopes

The mass number of beryllium above is 9.0122. Previously we just rounded this down to 9, and most of the time we will. But what does this actually mean?

Beryllium, like every other element, exists in a variety of different isotopes: atoms with the same number of protons (and are therefore the same element), but different numbers of neutrons. The mass number, 9.0122, is an average of all the mass numbers of all the beryllium atoms we've studied. The following is a simplification, but it gives you a good idea about what's going on.

Most beryllium exists as the isotope beryllium-9, which has 4 protons (p⁺) and 5 neutrons (n⁰), for a mass number of 9. However, a few beryllium atoms have 6 neutrons, which means beryllium-10 has a mass number of 10. If you take the average weight of all the atoms of beryllium there are, that average will be a little bit above 9.

Chemically, beryllium-9 and beryllium-10 behave very similarly. They have similar physical and chemical properties, the main difference being that a sample of beryllium-10 will have a slightly higher density than beryllium-9. Otherwise, they are identical and other atoms in compounds can't tell the difference.

A molecule of normal water (or just "water") contains two atoms of hydrogen and one atom of oxygen. Hydrogen usually only contains one proton in its nucleus, but an isotope of hydrogen, called deuterium (or hydrogen-2) has one proton and one neutron in its nucleus. Chemically, heavy water acts much like water, but it has a slightly different melting and boiling point, and is about 10% more dense. In high concentrations it can cause chemical processes in some living things to not work as efficiently. However, a small percentage of the water you drink, and the water in you, is already heavy water, so you're fine to have it in small concentrations.

Here is a video showing water-ice and heavy water-ice cubes being placed in water.

Drawing Bohr-Rutherford Diagrams

In Grade 9 Science, we will assume that all atoms are electrically neutral: they contain the same number of protons (positive charges) in the nucleus as they have electrons (negative charges) in the shells on the outside.

Remember, Rutherford's contribution to the model focused on the nucleus in the centre, and Bohr worked out how the electrons on the outside are arranged. When we put both of these together, we can draw a Bohr-Rutherford Diagram of a neutral atom.

Let's take a neutral atom of beryllium-9 above as an example. What do we know about it?

• Its atomic number is 4, so there are 4 protons in the nucleus.

• Its mass number is 9, so there are 9 - 4 = 5 neutrons in the nucleus.

• Since it's neutral, there must also be 4 electrons in the electron shells.

Bohr's part, the electron shells, can tell us a lot of information about how the element will act in chemical reactions with other elements. So it's important that we arrange these electrons properly.

Think about a parking lot at a shopping mall.

• Vehicles are only allowed to park in the spots painted on the pavement.

• The best parking spots will fill up first.

• These "best" spots are the ones closest to the doors.

Similarly, with electrons surrounding a nucleus:

• Electrons are only allowed to exist in spaces that the Bohr model allows.

• The best spaces for electrons will fill up first.

• These "best" spaces are the ones closest to the nucleus.

So, what does this "electron parking lot" look like? (We will only be examining the first 20 elements; after that things get a little strange and it's a topic you will examine in Grade 11 Chemistry.)

The empty circles in the above diagram are possible spaces for electrons to fit into, and the dashed circles around the nucleus are electron shells, like rows in the parking lot. (There are various reasons for why the spaces exist as pairs, but they're not important right now.) Notice that the number of available spaces follows the Periodic Table, if you know how to read it:

• Row #1 has 2 elements in it, and the first shell of the Bohr-Rutherford orbits also has 2 spaces.

• Row #2 has 8 elements, and the second shell has 8 spaces. Same for row #3.

• For row #4, we only go up to element #20 (calcium) in this course, so we will only look at the 2 spaces in the fourth row.

Now let's fill in the electrons for beryllium-9, which has 4 electrons to park.

1. Fill in the information about the nucleus (protons and neutrons).

2. Place the first two electrons in the first shell.

3. Since there are no more spaces for electrons in the first shell, the third electron needs to go in the second shell. (It wants to be as close to the nucleus as it can get, which is why it doesn't go in the third or fourth shell instead.) By convention, we put it at the top of the second shell, at the 12 o'clock position.

4. It would be tempting to put the fourth and final electron beside the third. But electrons don't want to be right beside each other, so the second electron goes somewhere else. We can put it over on the right side, at the 3 o'clock position, or at the bottom, it really doesn't matter. To keep things easy, just place electrons clockwise, starting at the 12 o'clock position.

Following the same steps, the Bohr-Rutherford diagram for a neutral atom of oxygen-16...

• atomic number 8, so there are 8 protons in the nucleus

• mass number 16, so there must be 16 - 8 = 8 neutrons in the nucleus

• it's a neutral atom, so there must be 8 electrons (same as the number of protons)

• of the 8 electrons that need to be placed, 2 go in the first, then the remaining 6 are placed in the second shell, starting at the 12 o'clock position and going clockwise

Standard Atomic Notation

There is a standard way to write out isotopes, in a way that can be recognized around the world. To use the example of beryllium-9:

To the left of the short-form symbol for beryllium (Be), there are two numbers stacked on top of each other. The top number is the mass number (9), and the bottom number is the atomic number (4). Writing things out this way is called stanard atomic notation.

For oxygen-16:

Practice

The Basics

1. Which subatomic particle(s) is/are located in the nucleus?

2. How many electrons can fit in the second shell? And how does this relate to the Periodic Table?

3. Determine the number of protons, neutrons and electrons are present in a neutral atom of sodium-23.

4. Draw a Bohr-Rutherford diagram for sodium-23.

5. Write out the standard atomic notation for sodium-23.

Extensions

1. If a neutral atom of sodium lost one of its electrons, it would no longer be electrically neutral. What overall charge would this atom be, positive or negative? How can you tell?

2. Draw Bohr-Rutherford diagrams for neon-20 and argon-40. What do you notice about their outermost electron shells which contain any electrons?