The Modern Atomic Model

The Modern Atomic Model
In the modern atomic model, electrons form an electron cloud. An electron cloud is an area around an atomic nucleus where an electron is most likely to be located. Imagine taking a time-lapse photograph of bees around a hive. You might see a blurry cloud. The cloud might be denser near the hive than farther away because the bees spend more time near the hive.
In a similar way, electrons constantly move around the nucleus. It is impossible to know both the speed and exact location of an electron at a given moment in time. Instead, scientists only can predict the likelihood that an electron is in a particular location. The electron cloud shown in Figure 10 is mostly empty space but represents the likelihood of finding an electron in a given area. The darker areas represent areas where electrons are more likely to be.
Atoms are made up of electrons, protons and neutrons.
Air fills most of an atom.

The nucleus of the atom is made of protons and neutrons at the center of an electron cloud.

The Parts of the Atom

If you could see inside any atom, you probably would see the same thingempty space surrounding a very tiny nucleus.
A look inside the nucleus would reveal positively charged protons and neutral neutrons. Negatively charged electrons would be whizzing by in the empty space around the nucleus.
Table 1 compares the properties of protons, neutrons, and electrons. Protons and neutrons have about the same mass. The mass of electrons is much smaller than the mass of protons or neutrons. That means most of the mass of an atom is found in the nucleus. In this lesson, you will learn that, while all atoms contain protons, neutrons, and electrons, the numbers of these particles are different for different types of atoms.
Table 1 Properties of Protons, Neutrons, and Electrons
Different ElementsDifferent Numbers of Protons
Look at the periodic table in the Reference Handbook. Notice that there are more than 115 different elements. Recall that an element is a substance made from atoms that all have the same number of protons. For example, the element carbon is made from atoms that all have six protons. Likewise, all atoms that have six protons are carbon atoms. The number of protons in an atom of an element is the elements atomic number. The atomic number is the whole number listed with each element on the periodic table.
What makes an atom of one element different from an atom of another element? Atoms of different elements contain different numbers of protons. For example, oxygen atoms contain eight protons; nitrogen atoms contain seven protons. Different elements have different atomic numbers. Figure 1 shows some common elements and their atomic numbers.
Neutral atoms of different elements also have different numbers of electrons. In a neutral atom, the number of electrons equals the number of protons. Therefore, the number of positive charges equals the number of negative charges.
1. Reading CheckWhat two numbers can be used to identify an element?
Figure 1 Atoms of different elements contain different numbers of protons.
Visual CheckExplain the difference between an oxygen atom and a carbon atom.
Neutrons and Isotopes
You have read that atoms of the same element have the same numbers of protons. However, atoms of the same element can have different numbers of neutrons.
For example, carbon atoms all have six protons, but some carbon atoms have six neutrons, some have seven neutrons, and some have eight neutrons.
These three different types of carbon atoms, shown in Table 2, are called isotopes.
Isotopes are atoms of the same element that have different numbers of neutrons. Most elements have several isotopes.
Table 2 Naturally Occurring Isotopes of Carbon
Protons, Neutrons, and Mass Number
The mass number of an atom is the sum of the number of protons and neutrons in an atom. This is shown in the following equation.
Mass number = number of protons + number of neutrons
Any one of these three quantities can be determined if you know the value of the other two quantities. For example, to determine the mass number of an atom, you must know the number of neutrons and the number of protons in the atom.
The mass numbers of the isotopes of carbon are shown in Table 2. An isotope often is written with the element name followed by the mass number. Using this method, the isotopes of carbon are written carbon-12, carbon-13, and carbon-14.
1. Reading CheckHow do two different isotopes of the same element differ?
Average Atomic Mass
You might have noticed that the periodic table does not list mass numbers or the numbers of neutrons. This is because a given element can have several isotopes. However, you might notice that there is a decimal number listed with most elements, as shown in Figure 2. This decimal number is the average atomic mass of the element. The average atomic mass of an element is the average mass of the elements isotopes, weighted according to the abundance of each isotope.
Table 2 shows the three isotopes of carbon. The average atomic mass of carbon is 12.01. Why isnt the average atomic mass 13? After all, the average of the mass numbers 12, 13, and 14 is 13.
The average atomic mass is weighted based on each isotopes abundancehow much of each isotope is present on Earth. Almost 99 percent of Earths carbon is carbon-12. That is why the average atomic mass is close to 12.
2. Reading CheckWhat does the term weighted average mean?
Figure 2 The element carbon has several isotopes. The decimal number 12.01 is the average atomic mass of these isotopes.
Math Skills: Use Percentages
You can calculate the average atomic mass of an element if you know the percentage of each isotope in the element. Lithium (Li) contains 7.5% Li-6 and 92.5% Li-7. What is the average atomic mass of Li?
1. Divide each percentage by 100 to change to decimal form.
2. Multiply the mass of each isotope by its decimal percentage.
6 × 0.075 = 0.45
7 × 0.925 = 6.475
3. Add the values together to get the average atomic mass.
0.45 + 6.475 = 6.93
Nitrogen (N) contains 99.63% N-14 and 0.37% N-15.
1. What is the average atomic mass of nitrogen?

IonsGaining or Losing Electrons
What happens to a neutral atom if it gains or loses electrons? Recall that a neutral atom has no overall charge. This is because it contains equal numbers of positively charged protons and negatively charged electrons. When electrons are added to or removed from an atom, that atom becomes an ion. An ion is an atom that is no longer neutral because it has gained or lost electrons. An ion can be positively or negatively charged depending on whether it has lost or gained electrons.
Positive Ions
When a neutral atom loses one or more electrons, it has more protons than electrons. As a result, it has a positive charge. An atom with a positive charge is called a positive ion. A positive ion is represented by the elements symbol followed by a superscript plus sign (+). For example, Figure 6 shows how sodium (Na) becomes a positive sodium ion (Na+).
Figure 6 An ion is formed when a neutral atom gains or loses an electron.
Negative Ions
When a neutral atom gains one or more electrons, it now has more electrons than protons. As a result, the atom has a negative charge. An atom with a negative charge is called a negative ion. A negative ion is represented by the elements symbol followed by a superscript negative sign (). Figure 6 shows how fluorine (F) becomes a fluoride ion (F).
1. Key Concept CheckHow does a neutral atom change when its number of protons, electrons, or neutrons changes?
Lesson Review
Visual Summary
Different elements contain different numbers of protons.
Two isotopes of a given element contain different numbers of neutrons.
When a neutral atom gains or loses an electron, it becomes an ion.
1. All atoms of the same element have the same number of protons.
2. Atoms of one element cannot be changed into atoms of another element.
3. Ions form when atoms lose or gain electrons.
Use Vocabulary
1. The number of protons in an atom of an element is its __________.
2. Nuclear decay occurs when an unstable atomic nucleus changes into another nucleus by emitting
3. Describe how two isotopes of nitrogen differ from two nitrogen ions.

Key Concepts
4. An elements average atomic mass is calculated using the masses of its
      A. electrons.
      B. isotopes.
      C. neutrons.
      D. protons.
5. Compare and contrast oxygen-16 and oxygen-17.
6. Show what happens to the electrons of a neutral calcium atom (Ca) when it is changed into      
ion (Ca2+).
7. If an ion contains 10 electrons, 12 protons, and 13 neutrons, what is the ions charge?
      A. 2
      B. 1
      C. 2+
      D. 3+
8. How many neutrons does iron-59 have?
      A. 30
      B. 33
      C. 56
      D. 59
9. Which determines the identity of an element?
      A. its mass number
      B. the charge of the atom
      C. the number of its neutrons
      D. the number of its protons
10. The figure below shows which of the following?

two atoms.  One on left hast 11 protons and 11 electrons.  One on the right has 9 protons and 9 electrons.
      A. two different elements
      B. two different ions
      C. two different isotopes
      D. two different protons
Interpret Graphics
11. Contrast Copy and fill in this graphic organizer to contrast how different elements, isotopes, and
are produced.

Graphic organizer titled How is each produced?
Critical Thinking
12. Consider Find two neighboring elements on the periodic table whose positions would be reversed
they were arranged by atomic mass instead of atomic number.
13. Infer Can an isotope also be an ion?
14. Summarize how radioactive decay can produce new elements.
15. Hypothesize What might happen if a negatively charged ion comes into contact with a positively
16. Infer Why isnt mass number listed with each element on the periodic table?
17. Explain How is the average atomic mass calculated?
18. Infer Oxygen has three stable isotopes.

Table to show oxygen has three stable isotopes

What can you determine about the average atomic mass of oxygen without calculating it?
19. Describe the current model of the atomExplain the size of atomsAlso explain the charge, the
, and the size and mass of protons, neutrons, and electrons.
Math Skills
Use Percentages
20. A sample of copper (Cu) contains 69.17% Cu-63. The remaining copper atoms are Cu-65. What I
the average atomic mass of copper?
Use the information in the table to answer questions 21 and 22.

Table for magnesium isotopes and the percent found in nature
21. What is the percentage of Mg-26 found in nature?
22. What is the average atomic mass of magnesium?