Atomic Number

In the modern periodic table, the elements are actually arranged in order of increasing atomic number–that’s the number of protons in one atom of a particular element. An undisturbed atom is electrically neutral, so the number of electrons in it is the same as its atomic number.

Atomic weight almost always increases with atomic number, so Mendeleev’s sequence of elements was almost exactly the same as the one used today, though there are a couple of weird exceptions. In general, it’s correct to think of atoms getting heavier as you go down a column or to the right across a row.

The atomic number is equal to the number of protons in an atom’s nucleus. The atomic number determines which element an atom is. For example, any atom that contains exactly 47 protons in its nucleus is an atom of silver.

 

Protons are positively charged particles found within atomic nuclei. Protons were discovered by Ernest Rutherford in experiments conducted between the years 1911 and 1919.

 

Discovered by Ernest Rutherford in 1911, the nucleus is the central part of an atom. Composed of protons and neutrons, the nucleus contains most of an atom’s mass.

 

The neutron is a particle found in almost every atomic nucleus (the tiny spec of matter at the heart of an atom). Only the hydrogen nucleus has no neutron. All other atoms have one or more.

It has no electric charge and its mass is nearly 1,840 times that of the electron.

Neutrons and protons constitute almost all of an atom’s mass. They stick together because of the strong nuclear force to form all the different kinds of atoms.

The neutron was discovered in 1932 by the English physicist James Chadwick.

How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon, silver, gold, etc…?

To find the number of protons, electrons and neutrons in an atom, just follow these easy steps:

Step 1 – Gather Information

The first thing you will need to do is find some information about your element. Go to the Periodic Table of Elements. If it makes things easier, you can select your element from an alphabetical listing.

Use the Table of Elements to find your element’s atomic number and atomic weight. The atomic number is the number located in the upper left corner and the atomic weight is the number located on the bottom, as in this example for krypton:

Step 2 – The Number of Protons is…

The atomic number is the number of protons in an atom of an element. In our example, krypton’s atomic number is 36. This tells us that an atom of krypton has 36 protons in its nucleus.

The interesting thing here is that every atom of krypton contains 36 protons. If an atom doesn’t have 36 protons, it can’t be an atom of krypton. Adding or removing protons from the nucleus of an atom creates a different element. For example, removing one proton from an atom of krypton creates an atom of bromine.

Step 3 – The Number of Electrons is…

By definition, atoms have no overall electrical charge. That means that there must be a balance between the positively charged protons and the negatively charged electrons. Atoms must have equal numbers of protons and electrons. In our example, an atom of krypton must contain 36 electrons since it contains 36 protons.

Electrons are arranged around atoms in a special way. If you need to know how the electrons are arranged around an atom, take a look at the ‘How do I read an electron configuration table?‘ page.

An atom can gain or lose electrons, becoming what is known as an ion. An ion is nothing more than an electrically charged atom. Adding or removing electrons from an atom does not change which element it is, just its net charge.

For example, removing an electron from an atom of krypton forms a krypton ion, which is usually written as Kr⁺. The plus sign means that this is a positively charged ion. It is positively charged because a negatively charged electron was removed from the atom. The 35 remaining electrons were outnumbered by the 36 positively charged protons, resulting in a charge of +1.

Step 4 – The Number of Neutrons is…

The atomic weight is basically a measurement of the total number of particles in an atom’s nucleus. In reality, it isn’t that clean cut. The atomic weight is actually a weighted average of all of the naturally occurring isotopes of an element relative to the mass of carbon-12. Didn’t understand that? Doesn’t matter. All you really need to find is something called the mass number. Unfortunately, the mass number isn’t listed on the Table of Elements. Happily, to find the mass number, all you need to do is round the atomic weight to the nearest whole number. In our example, krypton’s mass number is 84 since its atomic weight, 83.80, rounds up to 84.

The mass number is a count of the number of particles in an atom’s nucleus. Remember that the nucleus is made up of protons and neutrons. So, if we want, we can write:

Mass Number = (Number of Protons) + (Number of Neutrons)

For krypton, this equation becomes:

84 = (Number of Protons) + (Number of Neutrons)

If we only knew how many protons krypton has, we could figure out how many neutrons it has. Wait a minute… We do know how many protons krypton has! We did that back in Step 2! The atomic number (36) is the number of protons in krypton. Putting this into the equation, we get:

84 = 36 + (Number of Neutrons)

What number added to 36 makes 84? Hopefully, you said 48. That is the number of neutrons in an atom of krypton.

The interesting thing here is that adding or removing neutrons from an atom does not create a different element. Rather, it creates a heavier or lighter version of that element. These different versions are called isotopes and most elements are actually a mixture of different isotopes.

If you could grab atoms of krypton and count the number of neutrons each one had, you would find that most would have 48, others would have 47, some would have 50, some others would have 46, a few would have 44 and a very few would have 42. You would count different numbers of neutrons because krypton is a mixture of six isotopes.

In Summary…

For any element:

Number of Protons = Atomic Number

Number of Electrons = Number of Protons = Atomic Number

Number of Neutrons = Mass Number – Atomic Number

For krypton:

Number of Protons = Atomic Number = 36

Number of Electrons = Number of Protons = Atomic Number = 36

Number of Neutrons = Mass Number – Atomic Number = 84 – 36 = 48

Lewis Structures of atoms

The Lewis structure for an individual atom is drawn by placing a dot around the atom for each valence electron available. There are four positions available for dots to be placed; most chemists draw them on the top, left, bottom, and right of the atom.

Each of the four positions must receive one electron (one dot) before any electrons can be paired with one another. (Helium is an exception; the two electrons are shown as a pair.)

• The chemical symbol for the atom is surrounded by a number of dots corresponding to the number of valence electrons.

Number of Valence Electrons	1		2		3	4	5	6	7	8
Example	Hydrogen	Group I (Alkali metals)	Helium	Group II (alkali earth metals)	Group III	Group IV	Group V	Group VI	Group VII (Halogens)	Group VIII except Helium (Noble Gases)
Lewis Structure (electron dot diagram)

Lewis Structures for Ions of Elements

• The chemical symbol for the element is surrounded by the number of valence electrons present in the ion.
  The whole structure is then placed within square brackets, with a superscript to indicate the charge on the ion.
• Atoms will gain or lose electrons in order to achieve a stable, Noble Gas (Group VIII), electronic configuration.
• Negative ions (anions) are formed when an atom gains electrons.
• Positive ions (cations) are formed when an atom loses electrons.

Charge on Ion	1+		2+	3+	4+	4-	3-	2-	1-
No. electrons gained or lost	1e lost		2e lost	3e lost	4e lost	4e gained	3e gained	2e gained	1e gained
Example	H+	Group I + (Alkali metals)	Group II 2+ (alkali earth metals)	Group III 3+	Group IV 4+	Group IV 4-	Group V 3-	Group VI 2-	Group VII - (Halogens)	H- (hydride)
Lewis Structure (electron dot diagram)	OR H+	OR Li+	OR Be2+	OR B3+	OR C4+