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

You are watching: How are isotopes of a particular element alike

Now that us know how atoms are typically constructed, what carry out atoms the any details element look at like? How numerous protons, neutrons, and also electrons room in a specific kind that atom? First, if an atom is electrically neutral overall, then the number of protons amounts to the variety of electrons. Because these particles have actually the same however opposite charges, same numbers cancel out, developing a neutral atom.

Atomic Number

In the 1910s, experiments v x-rays resulted in this valuable conclusion: the magnitude of the positive charge in the cell core of every atom the a particular element is the same. In various other words, every atoms of the same aspect have the same variety of protons. Furthermore, different aspects have a different number of protons in your nuclei, for this reason the variety of protons in the cell nucleus of one atom is characteristic of a particular element. This exploration was so essential to our knowledge of atoms the the variety of protons in the nucleus of one atom is dubbed the atom number (Z).

For example, hydrogen has actually the atomic number 1; every hydrogen atoms have actually 1 proton in their nuclei. Helium has the atomic number 2; all helium atoms have 2 protons in your nuclei. There is no such thing as a hydrogen atom through 2 proton in its nucleus; a nucleus through 2 protons would be a helium atom. The atomic number defines one element. Table \(\PageIndex1\) perform some typical elements and also their atom numbers. Based on its atomic number, you can determine the number of protons in the cell nucleus of one atom. The largest atoms have actually over 100 protons in your nuclei.

Table \(\PageIndex1\): Some typical Elements and Their atom Numbers aspect Atomic Number facet Atomic Nmbers
aluminum (Al) 13 magnesium (Mg) 12
americium (Am) 95 manganese (Mn) 25
argon (Ar) 18 mercury (Hg) 80
barium (Ba) 56 neon (Ne) 10
beryllium (Be) 4 nickel (Ni) 28
bromine (Br) 35 nitrogen (N) 7
calcium (Ca) 20 oxygen (O) 8
carbon (C) 6 phosphorus (P) 15
chlorine (Cl) 17 platinum (Pt) 78
chromium (Cr) 24 potassium (K) 19
cesium (Cs) 55 radon (Rn) 86
fluorine (F) 9 silver (Ag) 47
gallium (Ga) 31 sodium (Na) 11
gold (Au) 79 strontium (Sr) 38
helium (He) 2 sulfur (S) 16
hydrogen (H) 1 titanium (Ti) 22
iron (Fe) 26 tungsten (W) 74
iodine (I) 53 uranium (U) 92
lead (Pb) 82 zinc (Zn) 30
lithium (Li) 3 zirconium (Zr) 40

Example \(\PageIndex1\)

What is the number of protons in the nucleus of each element?

aluminum steel carbon Answer a

According come Table 2.4.1, aluminum has actually an atomic number of 13. Therefore, every aluminum atom has 13 protons in that is nucleus.

Answer b

Iron has an atomic number of 26. Therefore, every iron atom has 26 protons in the nucleus.

Answer c

Carbon has an atomic variety of 6. Therefore, every carbon atom has 6 proton in the nucleus.

Exercise \(\PageIndex1\)

What is the number of protons in the cell core of every element? usage Table 2.4.1.

salt oxygen chlorine Answer a

Sodium has actually 11 protons in the nucleus.

Answer b

Oxygen has 8 proton in that nucleus.

Answer c

Chlorine has actually 17 proton in its nucleus

How countless electrons space in one atom? previously we stated that for an electrically neutral atom, the number of electrons amounts to the variety of protons, so the total opposite fees cancel. Thus, the atomic number of one element also gives the number the electrons in an atom of the element. (Later we will uncover that some elements may gain or lose electrons from your atoms, for this reason those atoms will certainly no longer be electrically neutral. Therefore we will need a means to distinguish the variety of electrons for those elements.)

Example \(\PageIndex2\)

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

sulfur tungsten argon Answer a

The atomic variety of sulfur is 16. Therefore, in a neutral atom of sulfur, there space 16 electrons.

Answer b

The atomic number of tungsten is 74. Therefore, in a neutral atom of tungsten, there are 74 electrons.

Answer c

The atomic variety of argon is 18. Therefore, in a neutral atom that argon, there space 18 electrons.

Exercise \(\PageIndex2\)

How countless electrons are present in the atom of each element?

magnesium potassium iodine Answer a

Mg has actually 12 electrons.

Answer b

K has actually 19 electrons.

Answer c

I has actually 53 electrons.


How countless neutrons space in atoms of a certain element? At very first it was thought that the number of neutrons in a cell core was also characteristic of an element. However, it was discovered that atoms of the same aspect can have actually different number of neutrons. Atoms of the same facet (i.e., same atomic number, Z) that have various numbers of neutrons are called isotopes. Because that example, 99% that the carbon atom on planet have 6 neutrons and also 6 proton in your nuclei; around 1% that the carbon atoms have actually 7 neutron in your nuclei. Naturally occurring carbon ~ above Earth, therefore, is in reality a mixture of isotopes, albeit a mixture that is 99% carbon with 6 neutrons in every nucleus.

An important series of isotope is discovered with hydrogen atoms. Many hydrogen atoms have actually a nucleus with only a single proton. Around 1 in 10,000 hydrogen nuclei, however, also has a neutron; this specific isotope is referred to as deuterium. An extremely rare hydrogen isotope, tritium, has actually 1 proton and 2 neutron in that is nucleus. Number \(\PageIndex1\) compare the three isotopes of hydrogen.

Figure \(\PageIndex1\): isotopes of Hydrogen. Most hydrogen atoms have actually only a proton in the cell core (a). A tiny amount of hydrogen exists together the isotope deuterium, which has one proton and one spirit in its nucleus (b). A tiny amount the the hydrogen isotope tritium, through one proton and two neutron in the nucleus, likewise exists on planet (c). The nuclei and electrons room proportionately much smaller than portrayed here.

Most elements exist together mixtures the isotopes. In fact, over there are currently over 3,500 isotopes recognized for every the elements. As soon as scientists discuss individual isotopes, they need an efficient method to clues the variety of neutrons in any certain nucleus. The mass number (A) of an atom is the sum of the numbers of protons and also neutrons in the nucleus. Given the fixed number because that a nucleus (and understanding the atomic number of that details atom), you deserve to determine the variety of neutrons by individually the atom number indigenous the massive number.

A simple way of indicating the mass number of a particular isotope is to list it together a superscript top top the left side of an element’s symbol. Atom numbers are often listed as a subscript top top the left next of an element’s symbol. Thus, we can see

\<\mathrm^mass\: number\xrightarrow\hspace45px 56_atomic\: number \xrightarrow\hspace35px 26Fe \labelEq1\>

which shows a particular isotope that iron. The 26 is the atom number (which is the same for every iron atoms), if 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 neutron in this atom.

Exercise \(\PageIndex3\)

How plenty of protons and neutrons room in each atom?

\(\mathrm^197_79Au\) \(\mathrm^23_11Na\) Answer a

In \(\mathrm^197_79Au\) there room 79 protons, and 197 − 79 = 118 neutrons in each nucleus.

Answer b

In \(\mathrm^23_11Na\) there space 11 protons, and also 23 − 11 = 12 neutron in each nucleus.


The atom is composed of discrete particles the govern that is tasiilaq.netical and physical behavior. Each atom the an aspect contains the same variety of protons, i beg your pardon is the atomic number (Z). Neutral atoms have actually the same number of electrons and protons. Atom of an aspect that contain various numbers of neutrons are dubbed isotopes. Each isotope of a given aspect has the same atomic number but a various mass number (A), i beg your pardon is the sum that the number of protons and neutrons.

See more: How Many Single Bonds Can Carbon Form ? Carbon And Carbon Bonding

Almost every one of the mass of an atom is native the complete protons and neutrons contained within a small (and therefore really dense) nucleus. The majority of the volume of an atom is the surrounding room in i beg your pardon the electrons reside. A depiction of a carbon-12 atom is shown below in number \(\PageIndex2\).

Figure \(\PageIndex2\): Formalism offered for identifying particular nuclide (any certain kind the nucleus)