Each isotope of an element has a whole number of protons and neutrons, but the atomic weight is the average of all the isotopes found, and it is affected by the observed ratios of the different isotopes. If the number was a whole number, then no known isotopes have been found rare other than some man-made elements. Short answer: The reason the atomic mass is not a whole number is because it's the weighted average mass of the isotopes of that specific element. The atomic mass of an element is the average mass of all its isotopes, weighted by their abundance. For example, the mass of Carbon is given as The atomic number refers to the number of protons in the atom.

Since the proton cannot be an in-between number, the atomic number will have to be a whole number. On the other hand, the atomic mass does not have to be a whole number because it is the mass of an atom and is roughly equivalent to the number of protons plus the average number of neutrons in that particular element. The mass number of an element is equal to it's atomic weight rounded to the nearest whole number.

The atomic number of an element is a whole number. It's the mass number that's not whole. This is due mainly to the fact that each element has several isotopes with different masses. The mass number for a given element is the average of those isotopes. Mass number is atomic mass rounded to the nearest whole number. In this case the element with a mass number of 32 would be Sulfur. It is its Atomic Mass and why atomic mass is frequently not a whole number.

The mass number of an element is the atomic mass rounded to the nearest whole number. For Example, if the atomic mass is You can use the mass number to calculate the amount of neutrons in an isotope. Atomic mass is the mass of 1 mole of the element. Atomic number is the number of protons the element has. To get the number of neutrons in an element you simply subtract the atomic mass from the atomic number.

The atomic mass of an element is the number of protons and neutrons added together. The atomic number is the number of protons the element has. Mass number is the average of all the naturally occurring isotopes of that element.

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When calculated, this average is not a whole number.Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other.

Carbon is taken as the standard atom and has a relative atomic mass A r of The table shows some A r values:. These values tell you that a magnesium atom has twice the mass of a carbon atom, and 24 times more mass than a hydrogen atom. They also tell you that hydrogen atoms have 12 times less mass than a carbon atom.

The A r values also allow you to work out that three oxygen atoms have the same mass as two magnesium atoms. Chlorine's A r of The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.

### Why does copper have a weird atomic mass?

For example, chlorine has two isotopes: 35 Cl and 37 Cl. But the relative atomic mass of chlorine is not In any sample of chlorine, 75 per cent of the atoms are 35 Cl and the remaining 25 per cent are 37 Cl. The relative atomic mass is worked out using the following formula, illustrated for two isotopes, where the abundances are given in percentage values.

For example, using chlorine:. Relative atomic mass Different atoms have different masses. Atoms with an A r of less than this have a smaller mass than a carbon atom. Atoms with an A r that is more than this have a larger mass than a carbon atom. Calculating relative atomic mass from isotopic abundance The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.

Hydrogen H. Carbon C. Oxygen O. Magnesium Mg. Chlorine Cl.They are not whole numbers because there are different isotopes of the elements.

The atomic weight is the average based on the proportions that they occur in naturally. The isotopes with the neutrons occur in very small amounts so the average weight is slightly greater than 1. If the atomic mass is not a whole number, then that means it is an average atomic mass of all the element's isotopes' atomic masses.

Basically for convenience, to avoid using powers of 10 as part of numbers more often than necessary; there is no commonly accepted prefix for the basic mass unit "gram" to make the mass of a single atom greater than a small fraction of such a unit. Yes, because some elements have more than one stable isotope. The isotopes individually have integral numbers of atomic mass units, but the value for the element is the average of the mass of all the naturally occurring isotopes, weighted by their relative abundances in nature, and this average is often fractional.

Chlorine, for example, has isotopes of mass numbers 35 and 37, and the elemental atomic mass of about They aren't. The atomic numbers of the elements give the numbers of electrons and protons for each atom. However, the number of neutrons in a nucleus often differs from the number of protons.

In fact, this is reflected in the atomic masses for the elements. You will notice that these are never integers because elements are always mixtures of a number of isotopes, that is, versions of atoms with varying numbers of neutrons in their nuclei. When dealing with reactions on the atomic or molecular level, it is often necessary to deal with masses that are extremely small.

Consider that the mass of an electron is only 9. The mass number is the combined number of protons and neutrons in the nucleus of the particular element you are looking at. NOTE: don't be confused by the periodic table Different elements have isotopes with varying mass numbers, so the mass number displayed on a periodic table is the ratio of those isotopic mass numbers in any given sample of the element your examining.

This ratio is often confused with with the mass number of the element when it is displayed on periodic tables, it is actually the relative atomic mass. You can tell if a number is the mass number or a relative atomic mass by whether or not it is a whole number if it is then it's a mass number if it has decimal places out beside it then you're looking at relative atomic mass.

The number of protons and neutrons added together gives the atomic mass number, nucleon number or often simply the mass number". The short answer is no. What makes one isotope different from another is its mass. Isotopes are atoms of the same element with different masses. All the atoms of an element have the same number of protons, which is what defines the element. They have different numbers of neutrons. Number of protons is atomic number, and protons plus neutrons is mass number. Thus you can say isotopes have the same number of protons but different numbers of neutrons, or alternatively they have the same atomic or proton number but different mass numbers.

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For instance, chlorine has two common isotopes. Both have 17 protons that's what makes them chlorine but one has 18 neutrons and the other has Their mass numbers are 35 and Having said that, I have been careful not to use the term atomic mass.

Atomic mass, or more properly relative atomic mass, is a term more usually applied to a natural sample of an element rather than to one isotope, and is defined as the average mass of 1 atom of the element on a scale where one atom of the isotope carbon- 12 has a mass of 12 units.

A natural sample of an element often consists of a mixture of isotopes in various proportions, and it behaves as if the mass of each atom is a weighted average of all the isotopes present.

Thus for chlorine, the relative atomic mass is The atomic mass ma is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is defined as the mass of an atom, which can only be one isotope at a time and is not an abundance-weighted average as in the case of atomic weight. For elements with more than one common isotope the difference even to the most common atomic mass can be half a mass unit or more e.

The atomic mass of an uncommon isotope can differ from the relative atomic mass or standard atomic weight by several mass units. Isotopes are species of atoms having same atomic no. So an isotope has either lesser or more neutrons than the usual atom of the element often called the most abundant isotope.Follow-up on this answer.

Related Questions. Still Curious? Good observation! This is a really important question with three contributions. But the masses listed at least in the periodic table on my bookshelf are not for any particular atom, but for an average over the naturally-occurring isotopes, taking their natural abundances into account.

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An isotope of a chemical element is defined to be an atom with the same number of protons and therefore electrons, if it's not ionizedand a different number of neutrons -- each possible number of neutrons in an atom corresponds to a different isotope.

Because most atoms with several stable isotopes have natural abundances that are shared in some fractional way between the possible isotopes, you will get fractional masses. Another very interesting effect is that the mass of an atom corresponds to the total energy of everything inside. A big piece of this energy is in fact negative -- it's the potential energy of the forces which bind the protons and neutrons together. Pull a nucleus apart into its component protons and neutrons, add up all the masses you get, and you will have more than if all the protons and neutrons were stuck together.

The binding energy contributes to the fractional masses of atoms. Furthermore, the electrons and their kinetic and binding energies also contribute to the total mass of an atom. Another effect is that a proton and a neutron have slightly different masses. The mass of Carbon and Boron, for example, will be different because Boron has one more neutron, and one fewer proton and one fewer electron.

A free neutron will in fact decay into a proton and an electron and an electron antineutrino, so it has more mass, and enough to give the electron and neutrino some kinetic energy in the decay. A neutron bound in a stable nucleus cannot decay in this manner because the decay of a neutron into a proton changes the binding energy of the nucleus by more than what you get by turning the neutron into a less massive proton minus the minimum energy needed to create the electron and neutrino.Because of the existence of isotopes.

Let's look at the simplest atom, hydrogen. As Z increases, the nucleus can support different numbers of neutrons, and thus for heavier elements there is an isotopic distribution. The mass quoted on the Periodic Table is the weighted average of the individual isotopes.

Why Aren't All Atomic Masses Whole Numbers? - Properties of Matter - Chemistry - FuseSchool

Why is the atomic mass of an element not a whole number? Chemistry Matter Atomic Mass. Sep 26, Explanation: Let's look at the simplest atom, hydrogen. Related questions How do you calculate the atomic mass of carbon?

## Why is the Relative Atomic Mass not a whole number?

How are atomic mass and mass number different? How do you calculate atomic mass from isotopic composition? How do atomic mass and atomic weight differ? How do atomic masses reflect isotope abundances?

How do atomic masses vary throughout the periodic table? How much atomic mass is in hydrogen? What atomic mass does ekasilicon have?

Why are atomic masses of most of the elements fractional? What is atomic mass? See all questions in Atomic Mass. Impact of this question views around the world.One of my students was scanning through a table of atomic weights and noticed that copper was odd because it has an atomic mass which isn't a whole number. Copper has a relative atomic mass of I thought I'd share our discussion because it draws in a lot of interesting Physics and Chemistry.

Copper is actually a mixture of different isotopes. Remember the chemistry of an atom is determined by the number of electrons it has. All solid elements have to be electrically neutral, otherwise their atoms would all have the same charge and the element would blow itself apart thanks to electric repulsion. Clearly, this doesn't happen. That means the number of positively charged protons in the nucleus has to be exactly equal to the number of negatively charged electrons in orbit around the nucleus.

Copper has 29 protons in its nucleus, and that means it must have 29 electrons whizzing about outside the nucleus. But the number of neutrons can vary without affecting the chemistry of copper because neutrons don't have any charge. That means its relative atomic mass depends on the proportions of these two isotopes. That means the overall atomic mass is a weighted average of the two:. In fact, if you look in detail copper is not that unusual in having an atomic mass which isn't even near a round number. To see this for yourself, take a look at the top-left hand corner for each element in the table below. So what is relative atomic mass and why is it useful? In Chemistry we're usually interested in reactions between atoms and molecules, so an important thing is how many atoms and molecules we have.

To understand why imagine we have to pair lots of gloves together. Gloves require a left hand and a right hand glove to make a pair. But what if we had one thousand left-hand gloves and only one right hand glove? Well, we could only make one pair. The lack of right-hand gloves would be rate-limiting. In Chemistry we usually deal with weights of chemicals. If we had the weight of left-hand and right-hand gloves we would need to work out the pairs we could form in two steps. In case you don't remember what gloves look like here's a nice French 18th century pair made of silk.

The left-hand glove is shown palm-upwards and the right hand one is palm downwards to show off the beautiful design to best effect.The relative atomic mass of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance.

The atomic mass of chlorine listed in the periodic table is a weighted average of the masses of all the different isotopes of chlorine. Add to that the fact that the masses of each isotope are not integers Chlorine 35 [ It's a weighted average. Basically, it means that on average, each chlorine atom weighs whatever the mass is.

However, about 75 percent of the chlorine atoms in the world weigh 35 daltons grams per moland another some percent weigh 37 daltons. So the average one weighs about The atomic masses of most elements are not whole numbers because they are average values of weight. Every single atom of oxygen does not weigh exactly 16amu, and every atom of carbon is not exactly It is an average of all of the potential isotopes of that element.

Update: thx everybody for helping me. Update 2: If u can, answer my another question. Answer Save. Favourite answer. Chlorine has two isotopes available in Nature. One has At. Another has At.

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You can see that its not a whole number. Similar case is for the element Copper, Iron etc. Chlorine Atomic Mass. How could it be? What do you think of the answers? You can sign in to give your opinion on the answer. Brian L Lv 7. Seiko K. Mary Lv 4. Still have questions? Get answers by asking now.