Lecture Notes - Protons, neutrons, and electrons

Chem 1010 Lecture Notes

Protons, Neutrons, and Electrons: What Atoms are Made of

Today, we'll talk more about how atoms themselves are put together.

First, we'll look more closely at the three elementary particles that an atom is made up of.

Then we will show how the number of protons, neutrons, and electrons in an atom determines its atomic number, its charge, and its atomic mass.

We will see how to write symbols for the different ions and isotopes of an element.

Finally, we'll look at how the mass of a single atom of an element is different from the mass of a natural sample of atoms of that element.

The Elementary Particles

As we have learned, there are three types of particles which make up an atom – protons, neutrons, and electrons. These are called the elementary particles or sometimes subatomic particles. We are now ready to learn more about them and how they are put together to make atoms.

In this lecture, we will talk about three important properties of an atom: atomic number, charge, and atomic mass. These properties come from the number of elementary particles that make up each atom (the protons, neutrons, and electrons). First, we need some more information about these particles.

Location – As we previously discussed, protons and neutrons are located in the nucleus of the atom, while the electrons are outside the nucleus, surrounding it. It is somewhat like a solar system, where the nucleus is like the sun in the middle and the electrons are like planets rotating around it.

Symbol – Rather than having to write out the words “proton,” “neutron,” and “electron,” we will often use symbols. However, they must be different from the symbols for the elements so that we don't get the two confused. The symbol that is used by scientists for a proton is p⁺ (lower case p so that you don't mistake it for phosphorus, whose symbol is a capital P). The symbol that is used for a neutron is n^o (lower case to avoid confusion with nitrogen, N). The symbol for an electron is e^-.

Charge – Two of the elementary particles are charged. A proton has a +1 charge, and an electron has a -1 charge. A neutron has no charge; it is neutral (which is how it got its name). These charges are used in the symbols for each particle because charge is such an important property.

Mass – Atoms and the particles that make them up are so small that in order to talk about their masses, we need a new unit – if we tried to use grams or pounds, we'd be writing zeros all day. The unit that scientists use is the “atomic mass unit” abbreviated “amu” (sadly, this is about the least creative unit ever – I'd have voted for the “dalton” or some such thing). To give you an idea of how tiny an amu is:

1 amu = 0.0000000000000000000000000036 pounds (the same as 3.6 x 10^-27)

Using these units, a proton has a mass of 1 amu, and a neutron also has a mass of 1 amu. (When we talk about nuclear reactions we will find one instance where we need to be more exact, but this will do for the present.) An electron is much smaller - it weighs only 0.0005 amu. If an atom were enlarged so that a proton was the size of a basketball, an electron would only be the size of a grain of rice.

Here is a summary of these characteristics of the elementary particles:

	location	symbol	charge	mass
proton	in the nucleus	p⁺	+1	1 amu
neutron	in the nucleus	n^o	0	1 amu
electron	outside the nucleus	e^-	-1	0.0005 amu

Atomic Number, Charge, and Atomic Mass

The number of protons, neutrons, and electrons that an atom has can be used to determine the atomic number, charge, and atomic mass of the atom.

The atomic number is determined by the number of protons.

The charge is determined by the number of protons and electrons, since these are the two particles which have a charge. The number of neutrons doesn't affect the charge because the neutrons have a charge of 0.

The atomic mass is determined by the number of protons and neutrons. The electrons have such a small mass that by comparison, they have no effect of the mass of the atom (basketballs vs. rice...).

In order to discuss these three concepts, we will use four atoms as examples: atom A, atom B, atom C, and atom D. The number of elementary particles and a representation using beans is given below.

red bean = proton
white bean = neutron
black bean = electron

Atomic number

As we have previously discussed, the atomic number is the number of protons in the nucleus. This number determines the identify of the atom – what element it is.

The atomic number for an element can be found by looking at the Periodic Table of Elements. So if you know the symbol for an element, a quick glance at the Periodic Table will tell you how many protons it has. Likewise, if you know how many protons an atom has, you have only to look at the Periodic Table to see what element it is.

atomic number = number of protons

Charge

The two elementary particles that determine the charge on an atom are the protons and the electrons. If an atom has the same number of protons and electrons, it is neutral, and has a charge of 0. However, if the number protons and electrons are not equal, the atom will have a charge.

It is much more likely for an atom to lose or gain electrons than protons. This is because the protons are tucked away inside the nucleus, and only a force of enormous energy (enough to cause a nuclear reaction) can cause the number of protons to change. The electrons, on the other hand, are on the outside, and can often be gained or lost.

If a neutral atom loses an electron, its charge becomes positive, because now there are more protons than electrons. If a neutral atom gains an electron, its charge becomes negative, because now there are more electrons than protons. It is easy to get this mixed up if you forget that electrons are negatively charged (we don't usually think of “gain” and “negative” together!).

charge = protons - electrons

When writing the symbol for an atom, the charge is indicated by writing it in the upper right corner. Please note that if the charge is +1 or -1, often only a + or - is used to write the symbol for the atom (Na⁺). However, if the charge is more than one, the number must be given too, but it can be before or after the + or - (Ca⁺² or Ca²⁺). If you are asked for the charge, you must not only tell whether it is + or -, but also give the numerical amount (-1, +2, -3, etc).

Charged atoms are called ions. Positively charged atoms are also called cations, while negatively charged atoms are also called anions.

Atomic mass

The atomic mass is the mass of one atom of the element. Since the electrons have very little mass, most of the mass of the atom comes from the nucleus of the atom. The atomic mass of an atom in amu is the sum of the number of protons and neutrons.

atomic mass = protons + neutrons

The mass of an atom can be shown by writing it in the upper left corner of the symbol for the atom (on the opposite side as the charge). When writing the symbol for a specific atom, including the mass is necessary so that you can tell how many neutrons it has.

Atoms of the same element which have different masses are called isotopes. Here is a simple representation of three isotopes of carbon:

Carbon-12 is the most common isotope; about 99% of all naturally occurring carbon is carbon-12. Most of the rest of the carbon is carbon-13 (it is useful in nuclear resonance spectroscopy). Carbon-14 is extremely rare, but it is radioactive, so we can detect small amounts of it, making it useful for dating of archeological artifacts.

Each of these atoms has the number of protons, and thus same atomic number, and the same element. They also have the same number of electrons. However, they do not have the same number of neutrons, which means that they also do not have the same mass. They may sometimes be written by using the name for the element, a hyphen, and the mass, as shown in the pictures above.

Isotopes of an element have the same chemical behavior – they will undergo the same reactions, and form the same compounds. However, when they undergo nuclear reactions, their behavior may be quite different.

It is not very difficult for an atom to gain or lose electrons, so it is not hard to convert one ion of an element into different ion. However, it is very difficult to change either the number of protons or neutrons in an atom, so it is very difficult to change one isotope of an element to another.

Atomic Mass and the Periodic Table

So far we have been talking about the mass of single atoms. These always have masses that are whole numbers, without any decimals. Atomic masses on the Periodic Table of Elements, however, often have masses with decimals. What's going on here?

In the real world, we don't encounter single atoms very often. Because they are so small, we are usually dealing with huge numbers of atoms. These large groups of atoms always contain the same ratio of each of their isotopes. The carbon atoms found in an apple, for example, have a ratio of 99% carbon-12, 1% carbon-13, and a tiny fraction of carbon-14. This same ratio is found anywhere in nature that you find carbon atoms.

In order to measure the mass of these atoms, we need to take an average of the masses of the three isotopes, and that average needs to take into account the ratio of them that is found. This is called a weighted average. The atomic mass of carbon that is given in the Periodic Table is 12.01. This reflects the percentages of the three carbon isotopes that are found in nature.

Nearly all elements have one isotope which is much more common than the others. By looking at the atomic mass given on the Periodic Table, you can often predict what the most common isotope is. For example, the atomic mass of vanadium is 50.94. This number is closest to 51 - and in fact is 99.7% of the vanadium found in nature is vanadium-51. Only 0.3% is vanadium-50.

Elements with atomic masses in parenthesis on the Periodic Table are radioactive elements that don't have enough stable isotopes to get a measurement of the average. Instead, the most stable isotope is listed. However, other isotopes are possible. For example, the most stable isotope of californium has an atomic mass of 251. But californium-249 has also been created.

Putting it all together

Let's sum up what we've talked about today:

atomic number = number of protons

charge = protons – electrons

atomic mass of an atom = protons + neutrons

complete symbol for an atom = ^massSymbol^charge

atomic mass for an element on the Periodic Table = weighted average of all naturally occurring isotopes

How many protons, neutrons, and electrons would you need to make the following atoms?

	protons	neutrons	electrons
¹⁰B	5	5	5
¹⁰B⁺	5	5	4
¹⁰B⁺²	5	5	3
¹⁰B^-	5	5	6
¹¹B⁺	5	6	4
¹⁰B⁺²	5	6	3

What element are the following atoms:

29 protons, 32 neutrons, 30 electrons – copper

91 neutrons, 83 protons, 85 electrons – bismuth

How many protons does a dysprosium atom have? 66

What is the atomic number of a krypton atom? 36

What charge do the following atoms have?

29 protons, 32 neutrons, 30 electrons -1

91 neutrons, 83 protons, 81 electrons +2

If an atom has a charge of +1, how many electrons has it gained or lost?

lost one electron

If an atom has a charge of -3, how many electrons has it gained or lost?

gained 3 electrons

If an atom has 29 protons, 32 neutrons, and 30 electrons, what is its mass?

mass = 29 protons + 32 neutrons = 61 amu

If an atom has 91 neutrons, 83 protons, and 85 electrons, what is its mass?

mass = 91 neutrons + 83 protons = 174 amu

How many protons and neutrons does a 75As atom have?

As = arsenic, 33 protons; 75 – 33 protons = 42 neutrons

What is the average mass of an atom in a naturally occurring sample of indium?

114.82 amu

What is the most stable isotope of polonium?

polonium-209

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