All atoms of an element have the same number of electrons. Question 12 options: True False. All atoms of an element have the same number of electrons.

Mosley discovered that all atoms of an element have the same number of neutrons in there nuclei False-atomic mass The average mass of an elements atoms is called the atomic number. The atomic number is the number of Protons in an atom and, every element has a different atomic number, from 1 to 118 elements in the Periodic Table. The atomic mass is the number of Protons PLUS. All atoms of a given element have the same. Number of nucleons. Number of neutrons. Number of protons.

Learning Objectives

• Explain what isotopes are and how an isotope affect 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. That's 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 and 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 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]

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 abundances'). 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 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 do all uranium nuclei 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}): Krypton-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, containing 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.

Contributors and Attributions

• 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)

Many people might think atoms and elements are the same. However, atoms and elements do have a few differences when you start breaking it down. The main difference is elements are made of atoms. Learn other differences between atoms and elements by dissecting these two terms. Explore examples of elements and atoms.

What Is an Atom?

You might have heard the term atom thrown around in chemistry. And, there's a good reason for that; it’s extremely important. Every compound, molecule, or element you come across will be made of atoms. For example, humans are made of atoms. Air is made of atoms. Your computer… made of atoms. Everything is made of atoms.

Which Of The Following Is An Acid

This is why you can’t begin to understand the difference between atoms and elements without first understanding what an atom is and what it’s made of. Simply put, atoms are the building blocks of elements. They are some of the smallest bits of what you would call ordinary matter.

Structure of an Atom

Like everything else, atoms have a few different things floating around inside of them. These subatomic particles include:

• neutrons - with no charge
• protons - positively charged
• electrons - negatively charged

The protons and neutrons are in the atomic nucleus of the atom, while the electrons orbit the atomic nucleus. Think of this as similar to the way the planets all orbit the sun. That’s an atom in a nutshell. Well, if a nutshell was orbited by lots of little electrons, that is.

What Is an Element?

With atoms out of the way, it’s time to look at elements. If you’ve ever seen a Periodic Table of Elements, then you probably have some idea of what elements are. But to break it down into a simple definition, elements are all the different types of atoms we know exist on Earth. They are arranged by their atomic number on the Periodic Table of Elements.

All Atoms Of Element Have The Same Number Of

For example, gold is an element. If you were to hold a chunk of pure gold in your hand, you would be holding an element. Other elements include:

• Hydrogen
• Boron
• Carbon
• Neon
• Magnesium
• Silicon
• Aluminum
• Chloride
• Oxygen
• Calcium

Elements and Atomic Number

What makes something an element is the fact that all the atoms have the same number of protons in the nucleus. While you can find them all on the periodic table, let’s look at the common elements’ mercury and copper.

• Mercury is an element with 80 protons in its nucleus. It has an atomic number of 80.
• Copper is made of atoms with 29 protons in the nucleus. Therefore, it has an atomic number of 29.

What Is the Difference Between an Element and a Molecule?

With atoms and elements all cleared up, it’s important to understand the difference between a molecule and an element. Because like all things in the world, elements and molecules are both made of atoms. You know elements are all the different types of atoms on the periodic table. Molecules are what you get when those atoms are combined. Unlike elements, molecules can be made from the same or different elements. The key to a molecule is that two or more atoms are bonded together.

For example, water is a molecule made of hydrogen and oxygen. It’s actually made of two hydrogen atoms and one oxygen atom. You can also have molecules of a single atom bonded together like two oxygen atoms. This makes up the oxygen humans breathe.

All Atoms Of An Element Have The Same Number Of

Difference Between Atoms and Elements

It can be easy to see why elements and atoms get confused because elements are atoms. They are just a group of all the same kind of atoms. All the known elements on Earth can be found in the periodic table of elements.

Science is fun right? Keep the chemistry fun going by exploring the difference between atoms and molecules. You can also have more chemistry fun by reading up on molecules and compounds.

All Atoms Of An Element Have The Same Number Of Electrons

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