All Atoms Of The Same Element Must Have The Same Number Of

The reason JJ Thompson reasons that electrons must be part of the atoms of all elements was because the charge to mass ratio of the electrons was the same, regardless of the gas used or material used as the anode or cathode. Correct answers: 2 question: All atoms of the same element must have the same number of A. I love you, thxxx ⭐️. All atoms of the same element must contain the same number of. Electrons plus protons d. Protons plus neutrons.

Learning Objectives

  • Explain what isotopes are and how an isotope affects an element's atomic mass.
  • Determine the number of protons, electrons, and neutrons of an element with a given mass number.

All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same physical and chemical properties because they have the same numbers of protons and electrons.

An Example: Hydrogen Isotopes

Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in Figure (PageIndex{1}). Most hydrogen atoms have just one proton, one electron, and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium.

For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13.

Example (PageIndex{1}): Lithium Isotopes

  1. What is the atomic number and the mass number of an isotope of lithium containing 3 neutrons?
  2. What is the atomic number and the mass number of an isotope of lithium containing 4 neutrons?

Solution

A lithium atom contains 3 protons in its nucleus irrespective of the number of neutrons or electrons.

a.

[ begin{align}text{atomic number} = left( text{number of protons} right) &= 3 nonumber left( text{number of neutrons} right) &= 3 nonumberend{align} nonumber ]

[ begin{align} text{mass number} & = left( text{number of protons} right) + left( text{number of neutrons} right) nonumber text{mass number} & = 3 + 3 nonumber &= 6 nonumber end{align}nonumber]

b.

[ begin{align}text{atomic number} = left( text{number of protons} right) &= 3 nonumber left( text{number of neutrons} right) & = 4nonumberend{align}nonumber]

[ begin{align}text{mass number} & = left( text{number of protons} right) + left( text{number of neutrons} right)nonumber text{mass number} & = 3 + 4nonumber &= 7 nonumber end{align}nonumber]

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Notice that because the lithium atom always has 3 protons, the atomic number for lithium is always 3. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in the isotope with 4 neutrons. In nature, only certain isotopes exist. For instance, lithium exists as an isotope with 3 neutrons, and as an isotope with 4 neutrons, but it doesn't exist as an isotope with 2 neutrons or as an isotope with 5 neutrons.

Stability of Isotopes

Atoms need a certain ratio of neutrons to protons to have a stable nucleus. Having too many or too few neutrons relative to protons results in an unstable, or radioactive, nucleus that will sooner or later break down to a more stable form. This process is called radioactive decay. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. When they decay, they release particles that may be harmful. This is why radioactive isotopes are dangerous and why working with them requires special suits for protection. The isotope of carbon known as carbon-14 is an example of a radioisotope. In contrast, the carbon isotopes called carbon-12 and carbon-13 are stable.

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 as well! How did Dalton miss this? It turns out that elements found in nature exist as constant uniform mixtures 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 in the same relative amounts (or 'relative abundance'). In a chunk of lithium, (93%) will always be lithium with 4 neutrons, while the remaining (7%) will always be lithium with 3 neutrons.

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. For most of our purposes in chemistry, we will do the same thing and deal with the average mass of the atoms. Luckily, aside from having different masses, most other properties of different isotopes are similar.

There are two main ways in which scientists frequently show the mass number of an atom they are interested in. It is important to note that the mass number is not given on the periodic table. These two ways include writing a nuclear symbol or by giving the name of the element with the mass number written.

To write a nuclear symbol, the mass number is placed at the upper left (superscript) of the chemical symbol and the atomic number is placed at the lower left (subscript) of the symbol. The complete nuclear symbol for helium-4 is drawn below:

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

[ce{^{59}_{28}Ni}]

[ ce{ ^{238}_{92}U}]

In the nickel nucleus represented above, the atomic number 28 indicates that the nucleus contains 28 protons, and therefore, it must contain 31 neutrons in order to have a mass number of 59. The uranium nucleus has 92 protons, as all uranium nuclei do; and this particular uranium nucleus has 146 neutrons.

Another way of representing isotopes is by adding a hyphen and the mass number to the chemical name or symbol. Thus the two nuclei would be Nickel-59 or Ni-59 and Uranium-238 or U-238, where 59 and 238 are the mass numbers of the two atoms, respectively. Note that the mass numbers (not the number of neutrons) are given to the side of the name.

Example (PageIndex{2}): Potassium-40

How many protons, electrons, and neutrons are in an atom of (^{40}_{19}ce{K})?

Solution

[text{atomic number} = left( text{number of protons} right) = 19]

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

[text{number of electrons} = 19]

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.

[text{number of neutrons} = 40 - 19 = 21.]

Example (PageIndex{3}): Zinc-65

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

Solution

[text{number of protons} = 30]

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

[text{number of electrons} = 30]

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.

[text{number of neutrons} = 65 - 30 = 35]

Exercise (PageIndex{3})

How many protons, electrons, and neutrons are in each atom?

  1. (^{60}_{27}ce{Co})
  2. Na-24
  3. (^{45}_{20}ce{Ca})
  4. Sr-90
Answer a:
27 protons, 27 electrons, 33 neutrons
Answer b:
11 protons, 11 electrons, 13 neutrons
Answer c:
20 protons, 20 electrons, 25 neutrons
Answer d:
38 protons, 38 electrons, 52 neutrons

Summary

  • The number of protons is always the same in atoms of the same element.
  • The number of neutrons can be different, even in atoms of the same element.
  • Atoms of the same element that contain the same number of protons, but different numbers of neutrons, are known as isotopes.
  • Isotopes of any given element all contain the same number of protons, so they have the same atomic number (for example, the atomic number of helium is always 2).
  • Isotopes of a given element contain different numbers of neutrons, therefore, different isotopes have different mass numbers.

Contributions & Attributions

This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:

  • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

  • Marisa Alviar-Agnew (Sacramento City College)

  • Henry Agnew (UC Davis)

2.4 Nuclei of Atoms

Learning Objectives

  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 numberThe number of protons in the nucleus of an atom..

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

All Atoms Of The Same Element Must Have The Same Number Of Units

    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 isotopesAtoms of the same element that have different numbers of neutrons.. 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.

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.

Note

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 numberThe sum of the numbers of protons and neutrons in a nucleus of an atom. 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

2656atomic numbermass numberFe

All Atoms Of The Same Element Must Have The Same Number Of Numbers

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. C1735l
  2. I53127

Solution

All Atoms Of The Same Element Must Have The Same Number Of Steps

Number
  1. In C1735l, there are 17 protons, and 35 − 17 = 18 neutrons in each nucleus.
  2. In I53127, 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. N1123a

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?

All Atoms Of The Same Element Must Have The Same Number Of Elements

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.

Key Takeaways

  • 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 ProtonsNumber of NeutronsElement NameIsotope Symbol
    80120
    F2655e
    2hydrogen
  8. Complete the following table.

    Number of ProtonsNumber of NeutronsElement NameIsotope Symbol
    H23e
    95153
    21potassium
  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. Number of ProtonsNumber of NeutronsElement NameIsotope Symbol
    80120mercuryH80200g
    2629ironF2655e
    12hydrogenH13
    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
  2. 69

    1. 26
    2. 22
    3. 2
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