Atoms With The Same Number Of Protons But Different Numbers Of Neutrons

• Atoms With The Same Number Of Protons But Different Numbers Of Neutrons Are Called ____
• How To Find Protons Neutrons And Electrons

Learning Objectives

Isotopes - An isotope of a chemical element is an atom that has a different number of neutrons (that is, a greater or lesser atomic mass) than the standard for that element. The atomic number is the number of protons in an atom's nucleus. Ions - An ion is an atom or molecule that has a net electrical charge. If two atoms have different numbers of protons, they are different elements. However, if two atoms have the same number of protons, but different numbers of neutrons we refer to them as isotopes. Two terms we use to identify nuclides (isotopes) are atomic numberand mass number.

1. Define and differentiate between the atomic number and the mass number of an element.
2. Explain how isotopes differ from one another.

Now that we know how atoms are generally constructed, what do atoms of any particular element look like? How many protons, neutrons, and electrons are in a specific kind of atom?

First, if an atom is electrically neutral overall, then the number of protons equals the number of electrons. Because these particles have the same but opposite charges, equal numbers cancel out, producing a neutral atom.

Atomic Number

In the 1910s, experiments with X rays led to this useful conclusion: the magnitude of the positive charge in the nucleus of every atom of a particular element is the same. In other words, all atoms of the same element have the same number of protons. Furthermore, different elements have a different number of protons in their nuclei, so the number of protons in the nucleus of an atom is characteristic of a particular element. This discovery was so important to our understanding of atoms that the number of protons in the nucleus of an atom is called the atomic number.

For example, hydrogen has the atomic number 1; all hydrogen atoms have 1 proton in their nuclei. Helium has the atomic number 2; all helium atoms have 2 protons in their nuclei. There is no such thing as a hydrogen atom with 2 protons in its nucleus; a nucleus with 2 protons would be a helium atom. The atomic number defines an element. Chapter 21 'Appendix: Periodic Table of the Elements' lists the elements and their atomic numbers. From this table, you can determine the number of protons in the nucleus of any element. The largest atoms have over 100 protons in their nuclei.

Example 3

What is the number of protons in the nucleus of each element? (Use the table in Chapter 21 'Appendix: Periodic Table of the Elements'.)

1. aluminum
2. iron
3. carbon

Solution

1. According to the table, aluminum has an atomic number of 13. Therefore, every aluminum atom has 13 protons in its nucleus.
2. Iron has an atomic number of 26. Therefore, every iron atom has 26 protons in its nucleus.
3. Carbon has an atomic number of 6. Therefore, every carbon atom has 6 protons in its nucleus.

Skill-Building Exercise

What is the number of protons in the nucleus of each element? (Use the table in Chapter 21 'Appendix: Periodic Table of the Elements'.)

1. oxygen

How many electrons are in an atom? Previously we said that for an electrically neutral atom, the number of electrons equals the number of protons, so the total opposite charges cancel. Thus, the atomic number of an element also gives the number of electrons in an atom of that element. (Later we will find that some elements may gain or lose electrons from their atoms, so those atoms will no longer be electrically neutral. Thus we will need a way to differentiate the number of electrons for those elements.)

Example 4

How many electrons are present in the atoms of each element?

1. sulfur
2. tungsten
3. argon

Solution

1. The atomic number of sulfur is 16. Therefore, in a neutral atom of sulfur, there are 16 electrons.
2. The atomic number of tungsten is 74. Therefore, in a neutral atom of tungsten, there are 74 electrons.
3. The atomic number of argon is 18. Therefore, in a neutral atom of argon, there are 18 electrons.

Skill-Building Exercise

How many electrons are present in the atoms of each element?

1. potassium

Isotopes

How many neutrons are in atoms of a particular element? At first it was thought that the number of neutrons in a nucleus was also characteristic of an element. However, it was found that atoms of the same element can have different numbers of neutrons. Atoms of the same element that have different numbers of neutrons are called isotopes. For example, 99% of the carbon atoms on Earth have 6 neutrons and 6 protons in their nuclei; about 1% of the carbon atoms have 7 neutrons in their nuclei. Naturally occurring carbon on Earth, therefore, is actually a mixture of isotopes, albeit a mixture that is 99% carbon with 6 neutrons in each nucleus.

An important series of isotopes is found with hydrogen atoms. Most hydrogen atoms have a nucleus with only a single proton. About 1 in 10,000 hydrogen nuclei, however, also has a neutron; this particular isotope is called deuterium. An extremely rare hydrogen isotope, tritium, has 1 proton and 2 neutrons in its nucleus. Figure 2.6 'Isotopes of Hydrogen' compares the three isotopes of hydrogen. Disk drill pro for windows.

Figure 2.6 Isotopes of Hydrogen. Most hydrogen atoms have only a proton in the nucleus (a). A small amount of hydrogen exists as the isotope deuterium, which has one proton and one neutron in its nucleus (b). A tiny amount of the hydrogen isotope tritium, with one proton and two neutrons in its nucleus, also exists on Earth (c). The nuclei and electrons are proportionately much smaller than depicted here.

The discovery of isotopes required a minor change in Dalton's atomic theory. Dalton thought that all atoms of the same element were exactly the same.

Most elements exist as mixtures of isotopes. In fact, there are currently over 3,500 isotopes known for all the elements. When scientists discuss individual isotopes, they need an efficient way to specify the number of neutrons in any particular nucleus. The mass number of an atom is the sum of the numbers of protons and neutrons in the nucleus. Given the mass number for a nucleus (and knowing the atomic number of that particular atom), you can determine the number of neutrons by subtracting the atomic number from the mass number.

A simple way of indicating the mass number of a particular isotope is to list it as a superscript on the left side of an element's symbol. Atomic numbers are often listed as a subscript on the left side of an element's symbol. Thus, we might see

[latex]_{atomic numberrightarrow 26}^{mass numberrightarrow 55}[/latex]Fe

which indicates a particular isotope of iron. The 26 is the atomic number (which is the same for all iron atoms), while the 56 is the mass number of the isotope. To determine the number of neutrons in this isotope, we subtract 26 from 56: 56 − 26 = 30, so there are 30 neutrons in this atom.

Example 5

How many protons and neutrons are in each atom?

1. [latex]_{17}^{35}[/latex]Cl
2. [latex]_{53}^{127}[/latex]I

Solution

1. In [latex]_{17}^{35}[/latex]Cl, there are 17 protons, and 35 − 17 = 18 neutrons in each nucleus.
2. In [latex]_{53}^{127}[/latex]I, there are 53 protons, and 127 − 53 = 74 neutrons in each nucleus.

Skill-Building Exercise

How many protons and neutrons are in each atom?

1. [latex]_{11}^{23}[/latex]Na

It is not absolutely necessary to indicate the atomic number as a subscript because each element has its own unique atomic number. Many isotopes are indicated with a superscript only, such as ¹³C or ²³⁵U. You may also see isotopes represented in print as, for example, carbon-13 or uranium-235.

Concept Review Exercises

1. Why is the atomic number so important to the identity of an atom?

2. What is the relationship between the number of protons and the number of electrons in an atom?

3. How do isotopes of an element differ from each other?

Answers

1. The atomic number defines the identity of an element.

2. In an electrically neutral atom, the number of protons equals the number of electrons.

3. Isotopes have different numbers of neutrons in their nuclei.

4. The mass number is the sum of the numbers of protons and neutrons in the nucleus of an atom.

• Elements can be identified by their atomic number and mass number.
• Isotopes are atoms of the same element that have different masses.

Exercises

1. How many protons are in the nucleus of each element?

   1. radon
   2. tungsten
   3. chromium
   4. beryllium

2. How many protons are in the nucleus of each element?

   1. sulfur
   2. uranium
   3. calcium
   4. lithium

3. What are the atomic numbers of the elements in Exercise 1?

4. What are the atomic numbers of the elements in Exercise 2?

5. How many electrons are in neutral atoms of the elements in Exercise 1?

6. How many electrons are in neutral atoms of the elements in Exercise 2?

7. Complete the following table.

   Number of Protons	Number of Neutrons	Element Name	Isotope Symbol
   80	120
   [latex]_{26}^{55}[/latex]Fe
   2	hydrogen

8. Complete the following table.

   Number of Protons	Number of Neutrons	Element Name	Isotope Symbol
   [latex]_2^3[/latex]H
   95	153
   21	potassium

9. State the number of protons, neutrons, and electrons in neutral atoms of each isotope.

   1. ¹³¹I
   2. ⁴⁰K
   3. ²⁰¹Hg
   4. ¹⁹F

10. State the number of protons, neutrons, and electrons in neutral atoms of each isotope.

    1. ³H
    2. ¹³³Cs
    3. ⁵⁶Fe
    4. ²⁰⁷Pb

11. What is the mass number of a gallium atom that has 38 neutrons in it?

12. What is the mass number of a uranium atom that has 143 neutrons in it?

13. Complete each sentence.

    1. ⁴⁸Ti has _____ neutrons.
    2. ⁴⁰Ar has _____ neutrons.
    3. ³H has _____ neutrons.

14. Complete each sentence.

    1. ¹⁸O has _____ neutrons.
    2. ⁶⁰Ni has _____ neutrons.
    3. ¹²⁷I has _____ neutrons.

Answers

1.

1. 1. 86
   2. 74
   3. 24
   4. 4

3. 86, 74, 24, and 4

5. 86, 74, 24, and 4

7.

9.

1. 1. protons: 53; neutrons: 78; electrons: 53
   2. protons: 19; neutrons: 21; electrons: 19
   3. protons: 80; neutrons: 121; electrons: 80
   4. protons: 9; neutrons: 10; electrons: 9

11. 69

13.

1. 1. 26
   2. 22
   3. 2

Atoms With The Same Number Of Protons But Different Numbers Of Neutrons Are Called ____

Learning Objectives

1. State the modern atomic theory.
2. Learn how atoms are constructed.

The smallest piece of an element that maintains the identity of that element is called an atom. Individual atoms are extremely small. It would take about fifty million atoms in a row to make a line that is 1 cm long. The period at the end of a printed sentence has several million atoms in it. Atoms are so small that it is difficult to believe that all matter is made from atoms—but it is.

The concept that atoms play a fundamental role in chemistry is formalized by the modern atomic theory, first stated by John Dalton, an English scientist, in 1808. It consists of three parts:

1. All matter is composed of atoms.
2. Atoms of the same element are the same; atoms of different elements are different.
3. Atoms combine in whole-number ratios to form compounds.

These concepts form the basis of chemistry.

Although the word atom comes from a Greek word that means 'indivisible,' we understand now that atoms themselves are composed of smaller parts called subatomic particles. The first part to be discovered was the electron, a tiny subatomic particle with a negative charge. It is often represented as e⁻, with the right superscript showing the negative charge. Later, two larger particles were discovered. The proton is a more massive (but still tiny) subatomic particle with a positive charge, represented as p⁺. The neutron is a subatomic particle with about the same mass as a proton but no charge. It is represented as either n or n⁰. We now know that all atoms of all elements are composed of electrons, protons, and (with one exception) neutrons. Table 3.1 'Properties of the Three Subatomic Particles' summarizes the properties of these three subatomic particles.

Table 3.1 Properties of the Three Subatomic Particles

How To Find Protons Neutrons And Electrons

How are these particles arranged in atoms? They are not arranged at random. Experiments by Ernest Rutherford in England in the 1910s pointed to a nuclear model of the atom. The relatively massive protons and neutrons are collected in the center of an atom, in a region called the nucleus of the atom (plural nuclei). The electrons are outside the nucleus and spend their time orbiting in space about the nucleus. (See Figure 3.1 'The Structure of the Atom'.)

Figure 3.1 The Structure of the Atom

Atoms have protons and neutrons in the center, making the nucleus, while the electrons orbit the nucleus.

The modern atomic theory states that atoms of one element are the same, while atoms of different elements are different. What makes atoms of different elements different? The fundamental characteristic that all atoms of the same element share is the number of protons. All atoms of hydrogen have one and only one proton in the nucleus; all atoms of iron have 26 protons in the nucleus. This number of protons is so important to the identity of an atom that it is called the atomic number of the element. Thus, hydrogen has an atomic number of 1, while iron has an atomic number of 26. Each element has its own characteristic atomic number.

Atoms of the same element can have different numbers of neutrons, however. Atoms of the same element (i.e., atoms with the same number of protons) with different numbers of neutrons are called isotopes. Most naturally occurring elements exist as isotopes. For example, most hydrogen atoms have a single proton in their nucleus. However, a small number (about one in a million) of hydrogen atoms have a proton and a neutron in their nuclei. This particular isotope of hydrogen is called deuterium. A very rare form of hydrogen has one proton and two neutrons in the nucleus; this isotope of hydrogen is called tritium. The sum of the number of protons and neutrons in the nucleus is called the mass number of the isotope.

Neutral atoms have the same number of electrons as they have protons, so their overall charge is zero. However, as we shall see later, this will not always be the case.

Example 1

1. The most common carbon atoms have six protons and six neutrons in their nuclei. What are the atomic number and the mass number of these carbon atoms?
2. An isotope of uranium has an atomic number of 92 and a mass number of 235. What are the number of protons and neutrons in the nucleus of this atom?

Solution

1. If a carbon atom has six protons in its nucleus, its atomic number is 6. If it also has six neutrons in the nucleus, then the mass number is 6 + 6, or 12.
2. If the atomic number of uranium is 92, then that is the number of protons in the nucleus. Because the mass number is 235, then the number of neutrons in the nucleus is 235 − 92, or 143.

Test Yourself

The number of protons in the nucleus of a tin atom is 50, while the number of neutrons in the nucleus is 68. What are the atomic number and the mass number of this isotope?

Answer

Atomic number = 50, mass number = 118

When referring to an atom, we simply use the element's name: the term sodium refers to the element as well as an atom of sodium. But it can be unwieldy to use the name of elements all the time. Instead, chemistry defines a symbol for each element. The atomic symbol is a one- or two-letter abbreviation of the name of the element. By convention, the first letter of an element's symbol is always capitalized, while the second letter (if present) is lowercase. Thus, the symbol for hydrogen is H, the symbol for sodium is Na, and the symbol for nickel is Ni. Most symbols come from the English name of the element, although some symbols come from an element's Latin name. (The symbol for sodium, Na, comes from its Latin name, natrium.) Table 3.2 'Names and Symbols of Common Elements' lists some common elements and their symbols. You should memorize the symbols in Table 3.2 'Names and Symbols of Common Elements', as this is how we will be representing elements throughout chemistry.

Table 3.2 Names and Symbols of Common Elements

Element NameSymbol

The elements are grouped together in a special chart called the periodic table. A simple periodic table is shown in Figure 3.2 'A Simple Periodic Table', while a more extensive one is presented in Chapter 17 'Appendix: Periodic Table of the Elements'. The elements on the periodic table are listed in order of ascending atomic number. The periodic table has a special shape that will become important to us when we consider the organization of electrons in atoms (see Chapter 8 'Electronic Structure'). One immediate use of the periodic table helps us identify metals and nonmetals. Nonmetals are in the upper right corner of the periodic table, on one side of the heavy line splitting the right-hand part of the chart. All other elements are metals.

Figure 3.2 A Simple Periodic Table

There is an easy way to represent isotopes using the atomic symbols. We use the construction

where X is the symbol of the element, A is the mass number, and Z is the atomic number. Thus, for the isotope of carbon that has 6 protons and 6 neutrons, the symbol is

where C is the symbol for the element, 6 represents the atomic number, and 12 represents the mass number.

Example 2

1. What is the symbol for an isotope of uranium that has an atomic number of 92 and a mass number of 235?
2. How many protons and neutrons are in ₂₆Fe?

Solution

1. The symbol for this isotope is
2. This iron atom has 26 protons and 56 − 26 = 30 neutrons.

Test Yourself

How many protons are in ₁₁N?

Answer

11 protons

It is also common to state the mass number after the name of an element to indicate a particular isotope. Carbon-12 represents an isotope of carbon with 6 protons and 6 neutrons, while uranium-238 is an isotope of uranium that has 146 neutrons.

Key Takeaways

• Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms.
• Atoms themselves are composed of protons, neutrons, and electrons.
• Each element has its own atomic number, which is equal to the number of protons in its nucleus.
• Isotopes of an element contain different numbers of neutrons.
• Elements are represented by an atomic symbol.
• The periodic table is a chart that organizes all the elements.

Exercises

List the three statements that make up the modern atomic theory.

Which is larger, a proton or an electron?

What are the charges for each of the three subatomic particles?

Sketch a diagram of a boron atom, which has five protons and six neutrons in its nucleus.

Sketch a diagram of a helium atom, which has two protons and two neutrons in its nucleus.

Define atomic number. What is the atomic number for a boron atom?

Define isotope and give an example.

What is the difference between deuterium and tritium?

a)

_{b) 26}F and ₂₅M

_{c) 14}S and ₁₅P

14. Which pair represents isotopes?

a) ₂₀C and ₁₉K

_{b) 26}F and ₂₆F

c) ₉₂U and ₉₂U

15. Give complete symbols of each atom, including the atomic number and the mass number.

a) an oxygen atom with 8 protons and 8 neutrons

b) a potassium atom with 19 protons and 20 neutrons

c) a lithium atom with 3 protons and 4 neutrons

16. Give complete symbols of each atom, including the atomic number and the mass number.

a) a magnesium atom with 12 protons and 12 neutrons

b) a magnesium atom with 12 protons and 13 neutrons

c) a xenon atom with 54 protons and 77 neutrons

17. Americium-241 is an isotope used in smoke detectors. What is the complete symbol for this isotope?

18. Carbon-14 is an isotope used to perform radioactive dating tests on previously living material. What is the complete symbol for this isotope?

19. Give atomic symbols for each element.

a) sodium

b) argon

c) nitrogen

d) radon

20. Give atomic symbols for each element.

a) silver

b) gold

c) mercury

d) iodine

21. Give the name of the element.

Vim cheat sheet for programmers 2018. a) Si

b) Mn

c) Fe

d) Cr

22. Give the name of the element.

a) F

b) Cl

c) Br

d) I

Answers

1.

All matter is composed of atoms; atoms of the same element are the same, and atoms of different elements are different; atoms combine in whole-number ratios to form compounds.

3.

A proton is larger than an electron.

5.

proton: 1+; electron: 1−; neutron: 0

7.

9.

The atomic number is the number of protons in a nucleus. Boron has an atomic number of five.

11.

Isotopes are atoms of the same element but with different numbers of neutrons. Poweramp deezer app. are examples.

13.

a) isotopes

b) not isotopes

c) not isotopes

15.

a)

b) ₁₉K

c)

17.

₉₅A

19.

a) Na

b) Ar

c) N

d) Rn

21.

a) silicon

b) manganese

c) iron

d) chromium