Learning Objectives

Define and also differentiate in between the atomic number and the mass variety of an element. Describe how isotope differ indigenous one another.

Now that us know just how atoms are typically constructed, what perform atoms the any certain element look at like? How numerous protons, neutrons, and electrons space in a details kind that atom?

First, if one atom is electrically neutral overall, climate the number of protons amounts to the number of electrons. Because these particles have the same yet opposite charges, same numbers release out, producing a neutral atom.

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Atomic Number

In the 1910s, experiments v X rays brought about this advantageous conclusion: the size of the positive charge in the cell core of every atom that a specific element is the same. In other words, every atoms that the same facet have the same variety of protons. Furthermore, different aspects have a different number of protons in their nuclei, so the number of protons in the cell nucleus of an atom is characteristic of a particular element. This exploration was so important to our knowledge of atoms that the variety of protons in the cell core of an atom is referred to as the atomic numberThe number of protons in the cell core of one atom..

For example, hydrogen has actually the atom number 1; all hydrogen atoms have actually 1 proton in your nuclei. Helium has the atom number 2; all helium atoms have 2 protons in their nuclei. There is no such point as a hydrogen atom through 2 proton in its nucleus; a nucleus through 2 protons would be a helium atom. The atom number defines an element. Thing 21 \"Appendix: periodic Table the the Elements\" list the elements and their atomic numbers. Indigenous this table, you deserve to determine the variety of protons in the cell nucleus of any type of element. The largest atoms have actually over 100 proton in their nuclei.

Example 3

What is the number of protons in the cell core of every element? (Use the table in chapter 21 \"Appendix: routine Table the the Elements\".)

aluminum iron carbon


follow to the table, aluminum has actually an atomic number of 13. Therefore, every aluminum atom has 13 proton in that nucleus. Iron has actually an atomic variety of 26. Therefore, every stole atom has 26 protons in the nucleus. Carbon has actually an atomic number of 6. Therefore, every carbon atom has actually 6 protons in the nucleus.

Skill-Building Exercise

What is the variety of protons in the cell nucleus of each element? (Use the table in thing 21 \"Appendix: periodic Table of the Elements\".)




How many electrons are in one atom? formerly we claimed that because that an electrically neutral atom, the variety of electrons equals the number of protons, therefore the total opposite dues cancel. Thus, the atomic number of an element also gives the variety of electrons in an atom of that element. (Later us will discover that some facets may gain or shed electrons from your atoms, so those atoms will certainly no much longer be electrically neutral. For this reason we will need a way to differentiate the number of electrons for those elements.)

Example 4

How plenty of electrons are present in the atom of every element?

sulfur tungsten argon


The atomic number of sulfur is 16. Therefore, in a neutral atom that sulfur, there space 16 electrons. The atomic variety of tungsten is 74. Therefore, in a neutral atom that tungsten, there space 74 electrons. The atomic number of argon is 18. Therefore, in a neutral atom of argon, there room 18 electrons.


How many neutrons room in atom of a details element? At very first it was assumed that the number of neutrons in a nucleus was also characteristic of an element. However, that was found that atoms of the same facet can have actually different numbers of neutrons. Atoms of the same element that have different numbers of neutron are referred to as isotopesAtoms that the same aspect that have various numbers that neutrons.. Because that example, 99% that the carbon atom on earth have 6 neutrons and 6 proton in their nuclei; about 1% that the carbon atoms have 7 neutron in their nuclei. Naturally occurring carbon on Earth, therefore, is actually a mixture the isotopes, albeit a mixture that is 99% carbon through 6 neutrons in each nucleus.

An important collection of isotopes is uncovered with hydrogen atoms. Most hydrogen atoms have a cell nucleus with only a single proton. About 1 in 10,000 hydrogen nuclei, however, also has a neutron; this specific isotope is dubbed deuterium. Very rare hydrogen isotope, tritium, has actually 1 proton and also 2 neutron in the nucleus. Number 2.6 \"Isotopes the Hydrogen\" compares the 3 isotopes of hydrogen.

Figure 2.6 isotope of Hydrogen


Most hydrogen atoms have only a proton in the cell nucleus (a). A tiny amount of hydrogen exists together the isotope deuterium, which has one proton and one spirit in its cell nucleus (b). A tiny amount that the hydrogen isotope tritium, with one proton and two neutrons in that nucleus, additionally exists on planet (c). The nuclei and electrons room proportionately much smaller than shown here.


The discovery of isotopes required a minor readjust in Dalton’s atomic theory. Dalton thought that all atoms the the same facet were specifically the same.

Most elements exist together mixtures the isotopes. In fact, there are right now over 3,500 isotopes known for all the elements. When scientists discuss individual isotopes, they need an efficient means to specify the number of neutrons in any details nucleus. The mass numberThe sum of the numbers of protons and neutrons in a cell core of one atom. Of an atom is the amount of the numbers of protons and also neutrons in the nucleus. Provided the mass number for a cell core (and learning the atomic number of that specific atom), you have the right to determine the variety of neutrons by individually the atomic number from the mass number.

A simple means of denote the mass variety of a specific isotope is to list it as a superscript ~ above the left next of one element’s symbol. Atom numbers space often listed as a subscript ~ above the left side of one element’s symbol. Thus, we can see

2656atomic number→mass number→Fe

which indicates a specific isotope of iron. The 26 is the atomic number (which is the very same for all iron atoms), while the 56 is the mass number of the isotope. To recognize the number of neutrons in this isotope, we subtract 26 indigenous 56: 56 − 26 = 30, so there space 30 neutron in this atom.

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Example 5

How plenty of protons and also neutrons room in every atom?

C1735l I53127


In C1735l, there room 17 protons, and 35 − 17 = 18 neutron in every nucleus. In I53127, there are 53 protons, and 127 − 53 = 74 neutrons in each nucleus.

Skill-Building Exercise

How many protons and neutrons are in every atom?



It is no absolutely crucial to indicate the atom number as a subscript because each facet has that own distinct atomic number. Plenty of isotopes are shown with a superscript only, such together 13C or 235U. You may additionally see isotopes stood for in print as, for example, carbon-13 or uranium-235.