Activities Handout: Build An Atom

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Build an Atom part 1: Prelab Objectives:

• Use information about the number of protons, neutrons, and electrons to identify an element

• Determine whether an atom is neutral or an ion

• Explain element symbol, charge, atomic number, mass number, and isotope.

Web link:

PhET Build an Atom https://phet.colorado.edu/en/simulations/build-an-atom

Introduction:

Rutherford Model of the Atom

In 1911, Ernest Rutherford discovered that 99.97% of the mass of an atom was concentrated in a tiny

core, or nucleus. From this discovery, Rutherford proposed the “nuclear model” of the atom. In this model

there is a dense center of positive charge (the nucleus) around which the negatively charged electrons

orbited. Rutherford’s model envisioned the electrons as circulating in some way around a positively

charged core. The Rutherford model of the atom is shown in figure 1. (It wasn’t until 1932, that another

physicist, James Chadwick announced that the nucleus also contained an uncharged particle called the

neutron.)

Figure 1: Rutherford “Nuclear Model” of the atom

Subatomic Particles

Protons and neutrons are in the nucleus of the atom and are sometimes called nucleons. Because protons,

neutrons, and electrons are the building blocks of atoms, we often refer to these particles as sub-atomic

particles. Some of the properties of these sub-atomic particles are shown in table 1. Notice that the

protons and electrons have the same magnitude (or size) charge. Also note that protons and neutrons have

almost the same size mass, while electrons have almost negligible mass in comparison.

Table 1: Comparative Properties of Protons, Electrons, and Neutrons

Particle Symbol Charge Weight Weight Ratio (e-)

proton p+ +1 1.007 amu 1838 heavy

electron e- -1 0.0005 amu 1 light

neutron n0 0 1.008 amu 1840 heavy

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Examples

1. Which two subatomic particles have the same magnitude of charge?

Proton and electron

2. Which two subatomic particles have about the same mass?

Proton and neutron

3. Which two subatomic particles are located in the nucleus of the atom?

Proton and neutron ------------------------------------------------------------------------------------------------------------------------------

Bohr Model

In 1913, Niels Bohr proposed a new model of the atom. In the Bohr model of the atom, electrons can

only exist at specific radii, and they travel in orbits around the nucleus. The electron orbits are sometimes

called electron shells. The innermost electron shell can hold two electrons, and the second electron shell

can hold eight electrons. There are more electron shells, but for this lab we will focus on elements that

have 10 or less electrons (up to 2 for the first shell and 8 for the second shell).

Figure 2: Bohr Models of Neutral Hydrogen, Helium, and Lithium Atoms

Electron Configurations

Figure 3: Electron Configurations for the First 10 Elements and List of Names and Chemical Symbols

In figure 3, the electron arrangements for the first ten elements are shown as well as a list of the element

names and their chemical symbols. Elements are defined by their number of protons; so the first ten

elements means the elements that have 1 to 10 protons. Each square in figure 3 contains the chemical

symbol for the element, the number of protons in the nucleus (each proton has a 1+ charge), and the

number of electrons in each shell for that element. The number above each column 1A-8A represents the

group number from the periodic table and the number in front of each row 1-2 represents the period

number. Note that for each element shown, there are the same number of protons as electrons. Protons and

electrons have the same magnitude (size) of charge. Since neutral atoms contain the same number of

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protons and electrons, the charges of the protons and electrons cancel each other. When an atom loses or

gain electrons, it is no longer neutral. The name for a charged atom is an ion.

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Examples

4. What is the chemical symbol for nitrogen and how many protons does it have?

N and 7 protons

5. How many electrons are in the first shell of a neutral carbon atom?

2 - Carbon has a full first shell which can hold 2 electrons

6. How many electrons are in the second shell of a neutral carbon atom?

4 - Carbon has 6 protons and a total of 6 electrons. Two of the electrons are in the first shell and

the remaining 4 electrons are in the second shell which can hold up to 8 electrons

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Atomic Number and Isotopes

All atoms of an element have the same number of protons. For example, all hydrogen atoms have 1

proton and only one proton. All helium atoms have 2 protons. All lithium atoms have 3 protons, and so

on. The number of protons for an element is called the atomic number and is abbreviated with the letter

Z. However, not all atoms of an element have the same number of neutrons. For example, most hydrogen

atoms have zero neutrons, but a few have 1 or 2 neutrons. Most carbon atoms have 6 neutrons, but some

have 7 neutrons and others have 8 neutrons. When the number of neutrons vary in the nucleus of a given

element, it is called an isotope.

Isotope Notation

There is a common notation used when writing about different isotopes. This notation is shown in figure

4.

Figure 4: Generic Isotope Notation

The chemical symbol for the element is written. Then on the left a subscript is written with the atomic

number (number of protons). Also, on the left but in the superscript position is the mass number. The

mass number (symbol A) is the number of protons + neutrons, or the total number of nucleons. A

neutral atom has the same number of protons and electrons, so the atomic number determines the number

of both particles for the neutral atom. If the isotope being described is an ion, then the atom has gained or

lost electrons, and therefore has a charge. An atom with one extra electron would have one more electron

than protons and would have a 1- charge. An atom with two extra electrons would have a 2- charge and so

on. If the atom has lost one electron, then there would be one more proton than electrons. That atom

would have a 1+ charge. An atom missing two electrons would have a 2+ charge. On the isotope

notation, if the isotope has a charge, the value for that charge is written on the right side of the chemical

symbol in the superscript position. If the atom is neutral, nothing is written in the charge position.

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Examples

7. A lithium ion has a 1+ charge. How many total electrons does that lithium ion have?

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2- A lithium ion has 3 protons which means a neutral lithium atom would have 3 electrons. A

lithium ion with 1+ charge has one more proton than electrons, so this ion would have only 2

electrons.

8. If the lithium 1+ ion has 4 neutrons? What is the isotope notation for that ion?

𝐿𝑖1+3 7 The chemical symbol for lithium is Li. It has 3 protons and an atomic number of 3. This isotope

has 4 neutrons which means its mass number (protons + neutrons) is 3+4=7. The ion has a 1+

charge.

9. If an oxygen ion has a 2- charge and 9 neutrons? What is the isotope notation for that ion?

𝑂2− 8 17 The chemical symbol for oxygen is O. It has 8 protons and an atomic number of 8. This isotope

has 9 neutrons which means its mass number (protons + neutrons) is 8+9=17. The ion has a 2-

charge. ------------------------------------------------------------------------------------------------------------------------------

Naming Isotopes

In addition to the isotope notation, we can represent atoms by name and mass number. For example, the

hydrogen isotope, that has 1 neutron in addition to the 1 proton, can be written as hydrogen-2. The

isotope of hydrogen with two neutrons is hydrogen-3. nitrogen-15 has 7 protons and 15-7 or 8 neutrons.

Another example could be for very large elements like uranium which has 92 protons. The most common

isotope of uranium is uranium-238. It has 92 protons, because it is uranium and 238-92 or 146 neutrons.

Figure 5: Screenshot of PhET Build an Atom Simulation Symbol Tab

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Examples

10. An oxygen isotope is shown in figure 5. What would be the name of this isotope using the element

name and mass number?

Oxygen-16 It is an oxygen atom and the mass number is shown under the symbol location of the

screenshot in figure 5.

11. How many neutrons does the isotope shown in figure 5 have?

8- The mass number of the isotope is 16 and the atomic number is 8. 16-8 =8. ------------------------------------------------------------------------------------------------------------------------------