Atomic Structure

Build an Atom Simulation Phet

Even though electrons, protons, and neutrons are all types of subatomic particles, they are not all the same size. When you compare the masses of these particles, you find will find that the electrons have an extremely small mass compared to the masses of protons and neutrons. On the other hand, the masses of protons and neutrons are fairly similar, although technically, with the mass of a neutron being slightly greater than the mass of a proton. Because protons and neutrons are so much more massive than electrons, almost all of the atomic mass in any atom comes from the nucleus, which contains all the neutrons and protons.

Table above gives the properties and locations of electrons, protons, and neutrons. The second row shows the masses of the three subatomic particles in grams. The third row, however, shows the masses of the three subatomic particles in atomic mass units. An atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom. A carbon-12 atom has exactly 6 protons, 6 electrons, and 6 neutrons. Atomic mass units are useful, because, as you can see, the mass of a proton and the mass of a neutron are almost exactly 1.0 in this unit system.

Electrons are much smaller than protons or neutrons. How much smaller? If an electron was the mass of a penny, a proton or a neutron would have the mass of a large bowling ball!

In addition to mass, another important property of subatomic particles is the electric charge. You already know that neutrons are neutral and thus have no charge at all. Therefore, we say that neutrons have a charge of zero. What about electrons and protons? You know that electrons are negatively charged and protons are positively charged, but what’s amazing is that the positive charge on a proton is exactly equal in magnitude (magnitude means “absolute value” and ignores positive and negative signs) to the negative charge on an electron. The third row in Table above shows the charges of the three subatomic particles. Notice that the charge on the proton and the charge on the electron have the same magnitude. Previously, you learned that negative and positive charges of equal magnitude cancel each other out. This means that the negative charge on an electron perfectly balances the positive charge on the proton. In other words, a neutral atom must have exactly one electron for every proton. If a neutral atom has 1 proton, it must have 1 electron. If a neutral atom has 2 protons, it must have 2 electrons. If a neutral atom has 10 protons, it must have 10 electrons. Do you get the idea? In order to be neutral, an atom must have the same number of electrons and protons.

Atomic Number and Mass Number

Scientists can distinguish between different elements by counting the number of protons. If an atom has only one proton, we know it’s a hydrogen atom. An atom with two protons is always a helium atom. If scientists count four protons in an atom, they know it’s a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom... the list goes on (see Figure below for more examples).

It is difficult to find qualities that will distinguish one element from another. Each element, however, does have a unique number of protons. Sulfur always has 16 protons, silicon always has 14 protons, and gold always has 79 protons.

Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, scientists are always interested in this number and give this number a special name. An element’s atomic number is equal to the number of protons in the nucleus of any of its atoms. An element's atomic number can be found in the periodic table, which is a convenient way to summarize information about the different elements. We will come examine periodic tables in greater details in a later chapter. When looking at the periodic table, the atomic number will usually be written as a whole number above the chemical symbol of each element. The atomic number for hydrogen is 1 because every hydrogen atom has 1 proton. Similarly, the atomic number for helium is 2 because every helium atom has 2 protons. What is the atomic number of carbon? (Answer: Carbon has 6 protons, so the atomic number for carbon is 6.)

Of course, since neutral atoms have to have one electron for every proton, the atomic number also tells you how many electrons are in a neutral atom. For example, hydrogen has an atomic number of 1. This means that a neutral hydrogen atom has one proton and one electron. Gold, on the other hand, has an atomic number of 79, which means that an atom of gold has 79 protons and, if it’s neutral, 79 electrons.

The mass number of an atom is the total number of protons and neutrons in its nucleus. Why do you think that the “mass number” includes protons and neutrons, but not electrons? Recall that most of the mass of an atom is concentrated in its nucleus. You have already learned that the mass of an electron is very, very small compared to the mass of either a proton or a neutron (like the mass of a penny compared to the mass of a bowling ball shown in Figure above). Counting the number of protons and neutrons tells scientists about the total mass of an atom.

An atom’s mass number is a very easy to calculate once you know the number of protons and neutrons in the atom. Notice that the mass number is not the same as the mass of the atom. You can easily relate the mass number to the mass by recalling that both protons and neutrons have a relative mass of 1 amu.

Example

What is the mass number of an atom of helium that contains 2 neutrons?

Solution

Scientists will also sometimes refer to a specific isotope by adding the mass number after the elemental name. For example, a helium atom with two neutrons is sometimes written as helium-4. Recall earlier that we defined 1 amu as one-twelfth the mass of a carbon-12 atom. Since carbon has 6 protons, a carbon-12 atom will have 6 protons and 6 neutrons in the nucleus.

The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium nucleus with 146 neutrons.

In the nickel nucleus represented above, the atomic number 28 indicates the nucleus contains 28 protons, which means that it must contain 31 neutrons in order to have a mass number of 59. The uranium nucleus has 92 protons, as do all uranium nuclei, and this particular uranium nucleus has 146 neutrons. The other way of representing these nuclei would be nickel-59 and uranium-238, where 59 and 238 are the mass numbers of the two atoms, respectively.

We can now use what we know about atomic number and mass number to find the number of protons, neutrons, and electrons in any given atom or isotope. Consider the following example problems.

Example

How many protons, electrons, and neutrons are in an atom of ?

Solution

Finding the number of protons is simple. The atomic number, or the number of protons, is listed in the bottom right corner.

number of protons

For all atoms with no charge, the number of electrons is equal to the number of protons.

number of electrons

The mass number, 40, is the sum of the protons and the neutrons. To find the number of neutrons, subtract the number of protons from the mass number.

number of neutrons

Example

How many protons, electrons, and neutrons in an atom of zinc-65?

Solution

Finding the number of protons is simple. The atomic number, or the number of protons, is found on the periodic table. All zinc atoms have 30 protons.

number of protons

For all atoms with no charge, the number of electrons is equal to the number of protons.

number of electrons

The mass number, 65, is the sum of the protons and the neutrons. To find the number of neutrons, subtract the number of protons from the mass number.

number of neutrons

Atomic Weight

This whole discussion of isotopes brings us back to Dalton’s atomic theory. According to Dalton, atoms of a given element are identical. But if atoms of a given element can have different numbers of neutrons, then they can have different masses! How did Dalton miss this? It turns out that elements found in nature exist as uniform mixtures with a constant ratio of their naturally occurring isotopes. In other words, a piece of lithium always contains both types of naturally occurring lithium (the type with 3 neutrons and the type with 4 neutrons). Moreover, it always contains the two types in the same relative amounts (or “relative abundances”). In a chunk of lithium that could contain more than hundreds of lithium atoms, 93% of the lithium atoms will always have 4 neutrons, while the remaining 7% will always have 3 neutrons.

Unfortunately, Dalton always experimented with large chunks of an element – chunks that contained all of the naturally occurring isotopes of that element. As a result, when he performed his measurements, he was actually observing the averaged properties of all the different isotopes in the sample. Luckily, aside from having different masses, most other properties of different isotopes are similar.

Knowing about the different isotopes is important when it comes to calculating atomic weight. The atomic weight (sometimes referred to as the relative atomic mass) of an element is the weighted average mass of all the atoms in a naturally occurring sample of the element. Atomic weight is typically reported in atomic mass units. You can calculate the atomic weight of an element if you know the relative abundances the element’s naturally occurring isotopes and the masses of those different isotopes. The examples below show how this is done.

Example

Boron has two naturally occurring isotopes. In a sample of boron, 20% of the atoms are B-10, which is an isotope of boron with 5 neutrons and a mass of 10 amu. The other 80% of the atoms are B-11, which is an isotope of boron with 6 neutrons and a mass of 11 amu. What is the atomic weight of boron?

Solution

To do this problem, we will calculate 20% of the mass of B-10, which is how much the B-10 isotope contributes to the “average boron atom.” We will also calculate 80% of the mass of B-11, which is how much the B-11 isotope contributes to the “average boron atom.”

Step One: Convert the percentages given in the question into their decimal forms by dividing each percentage by 100%:

Total mass of average atom

The mass of an average boron atom, and thus boron’s atomic weight, is 10.8 amu.

Example

Neon has three naturally occurring isotopes. In a sample of neon, 90.48% of the atoms are Ne-20, which is an isotope of neon with 10 neutrons and a mass of 19.99 amu. Another 0.27% of the atoms are Ne-21, which is an isotope of neon with 11 neutrons and a mass of 20.99 amu. The final 9.25% of the atoms are Ne-22, which is an isotope of neon with 12 neutrons and a mass of 21.99 amu. What is the atomic weight of neon?

Solution

To do this problem, we will calculate 90.48% of the mass of Ne-20, which is how much Ne-20 contributes to the “average neon atom.” We will also calculate 0.27% of the mass of Ne-21 and 9.25% of the mass of Ne-22, which are how much the Ne-21 and the Ne-22 isotopes contribute to the “average neon atom” respectively.

Step One: Convert the percentages given in the question into their decimal forms by dividing each percentage by 100%:

Total mass of average atom

The mass of an average neon atom, and thus neon’s atomic weight, is 20.20 amu.

The periodic table gives the atomic weight of each element. The atomic weight is a number that usually appears below the element’s symbol in each square. Notice that atomic weight of boron (symbol B) is 10.8, which is what we calculated in example 5, and the atomic weight of neon (symbol Ne) is 20.20, which is very close to what we calculated in example 6. Take time to notice that not all periodic tables have the atomic number above the element’s symbol and the atomic weight below it. If you are ever confused, remember that the atomic number should always be the smaller of the two and will be a whole number, while the atomic weight should always be the larger of the two and will be a decimal number.

Lesson Summary

• • Electrons are a type of subatomic particle with a negative charge.

  • Protons are a type of subatomic particle with a positive charge.

  • Neutrons are a type of subatomic particle with no charge (they’re neutral).

  • Protons and neutrons have approximately the same mass, but they are both much more massive than electrons (approximately 2,000 times as massive as an electron).

  • The positive charge on a proton is equal in magnitude to the negative charge on an electron. As a result, a neutral atom must have an equal number of protons and electrons.

  • Each element has a unique number of protons. An element’s atomic number is equal to the number of protons in the nucleus of any of its atoms.

  • The mass number of an atom is the sum of the protons and neutrons in the atom

  • Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons in their atomic nuclei.

  • An element’s atomic weight is the average mass of one atom of that element. An element’s atomic weight can be calculated if the relative abundances of the element’s naturally occurring isotopes and the masses of those isotopes are known.

  • The periodic table is a convenient way to summarize information about the different elements. In addition to the element’s symbol, most periodic tables will also contain the element’s atomic number and atomic weight.

Vocabulary

Neutron

A subatomic particle with no charge.

Atomic mass unit (amu)

A unit of mass equal to one-twelfth the mass of a carbon-12 atom.

Atomic number

The number of protons in the nucleus of an atom.

Mass number

The total number of protons and neutrons in the nucleus of an atom.

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons.

Atomic weight

The weighted average of the masses of the isotopes of an element.