# Atomic Number Is Equal To

Learning Objectives

• List the properties of the three main subatomic particles.
• Define atomic mass unit (amu).
• Define atomic number and mass number.
• Define isotopes.
• Determine the number of protons, neutrons, and electrons in an atom with a given mass number.

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. Click to see full answer.

The atomic number of an element is equal to the number of in the nucleus. 1 Educator answer. ENotes.com will help you with any book or any question. Our summaries and analyses are written. The atomic number equals the charge on the nucleus. It therefore also equals the number of protons in the nucleus and also equals numerically the number of electrons in the neutral atom. The atomic number has the symbol Z. The atomic number is equal to the number of protons in thenucleus and in a neutral atom will also equal the number ofelectrons. Home Science Math History Literature Technology Health Law Business.

The historical development of the different models of the atom’s structure is summarized in Figure (PageIndex{1}). J.J. Thomson and Robert Millikan conducted experiments to study the properties of electrons. Rutherford established that the nucleus of the hydrogen atom was a positively charged particle, for which he coined the name proton in 1920. He also suggested that the nuclei of elements other than hydrogen must contain electrically neutral particles with approximately the same mass as the proton. The neutron, however, was not discovered until 1932, when James Chadwick (1891–1974, a student of Rutherford; Nobel Prize in Physics, 1935) discovered it. As a result of Rutherford’s work, it became clear that an α particle contains two protons and neutrons, and is therefore the nucleus of a helium atom.

Figure (PageIndex{1}) A summary of the historical development of models of the components and structure of the atom. The dates in parentheses are the years in which the key experiments were performed. (CC BY-SA-NC). Rutherford’s model of the atom is essentially the same as the modern model, except that it is now known that electrons are not uniformly distributed throughout an atom’s volume. Instead, they are distributed according to a set of principles described by Quantum Mechanics.

Figure (PageIndex{2}) shows how the model of the atom has evolved over time from the indivisible unit of Dalton to the modern view taught today.

Figure (PageIndex{2})The Evolution of Atomic Theory, as Illustrated by Models of the Oxygen Atom. Bohr’s model and the current model are described in Chapter 6, 'The Structure of Atoms.' Image used with Permission (CC BY-SA-NC). The nucleus (plural, nuclei) is a positively charged region at the center of the atom. It consists of two types of subatomic particles packed tightly together. The particles are protons, which have a positive electric charge, and neutrons, which are neutral in electric charge. Outside of the nucleus, an atom is mostly empty space, with orbiting negative particles called electrons whizzing through it. Figure (PageIndex{3}) below shows these parts of the atom.

Figure (PageIndex{3}) The nuclear atom

The nucleus of the atom is extremely small. Its radius is only about 1/100,000 of the total radius of the atom. If an atom were the size of a football stadium, the nucleus would be about the size of a pea! Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense. If an object the size of a penny had the same density as the nucleus of an atom, its mass would be greater than 30 million tons!

Holding It All Together

Particles with opposite electric charges attract each other. This explains why negative electrons orbit the positive nucleus. Particles with the same electric charge repel each other. This means that the positive protons in the nucleus push apart from one another. So why doesn't the nucleus fly apart? An even stronger force - called the strong nuclear force - holds protons and neutrons together in the nucleus.

Table (PageIndex{1}) gives the properties and locations of electrons, protons, and neutrons. The third column shows the masses of the three subatomic particles in 'atomic mass units.' An atomic mass unit ((text{amu})) is defined as one-twelfth the mass of a carbon-12 atom. Atomic mass units ((text{amu})) are useful, because, as you can see, the mass of a proton and the mass of a neutron are almost exactly (1) in this unit system.

Table (PageIndex{1}): Properties of Subatomic Particles
ParticleSymbolMass (amu)Relative Mass (proton = 1)Relative ChargeLocation
protonp+11+1inside the nucleus
electrone5.45× 10−40.00055−1outside nucleus
neutronn0110inside the nucleus

amu in gram and kilogram

1 amu = 1.6605 × 10−24 g = 1.6605 × 10−27 kg

## Atomic Number

Negative and positive charges of equal magnitude cancel each other out. This means that the negative charge on an electron perfectly balances the positive charge on the proton. In other words, a neutral atom must have exactly one electron for every proton. If a neutral atom has 1 proton, it must have 1 electron. If a neutral atom has 2 protons, it must have 2 electrons. If a neutral atom has 10 protons, it must have 10 electrons. You get the idea. In order to be neutral, an atom must have the same number of electrons and protons.

Scientists distinguish between different elements by counting the number of protons in the nucleus (Table (PageIndex{2})). If an atom has only one proton, we know it's a hydrogen atom. An atom with two protons is always a helium atom. If scientists count four protons in an atom, they know it's a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . the list goes on.

Since an atom of one element can be distinguished from an atom of another element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs between different elements. The number of protons in an atom is called its atomic number ((Z)). This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A)
Table (PageIndex{2}): Atoms of the First Six Elements
Hydrogen10111
Helium22224
Lithium34337
Beryllium45449
Boron565511
Carbon666612

Of course, since neutral atoms have to have one electron for every proton, an element's atomic number also tells you how many electrons are in a neutral atom of that element. For example, hydrogen has an atomic number of 1. This means that an atom of hydrogen has one proton, and, if it's neutral, one electron as well. Gold, on the other hand, has an atomic number of 79, which means that an atom of gold has 79 protons, and, if it's neutral, 79 electrons as well.

Neutral Atoms

Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons (Table (PageIndex{2})). Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atom's chemical properties.

## Mass Number

The mass number ((A)) of an atom is the total number of protons and neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit (left( text{amu} right)). One atomic mass unit is the mass of a proton, or about (1.67 times 10^{-27}) kilograms, which is an extremely small mass. A neutron has just a tiny bit more mass than a proton, but its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units (Table (PageIndex{2})).

Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units ((2 : text{amu}) for the protons + (2 : text{amu}) for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two (these are called isotopes and are discussed in detail later on)

Why do you think that the 'mass number' includes protons and neutrons, but not electrons? You know that most of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the number of protons and neutrons. You have already learned that the mass of an electron is very, very small compared to the mass of either a proton or a neutron (like the mass of a penny compared to the mass of a bowling ball). Counting the number of protons and neutrons tells scientists about the total mass of an atom. An atom's mass number is very easy to calculate provided you know the number of protons and neutrons in an atom. The mass number of a carbon atom with 6 protons and 7 neutrons is calculated and shown as follows:

[text{mass number} : A = left( text{number of protons} right) + left( text{number of neutrons} right)]

[text{mass number} = text{6} + text{6} = text{12}]

Example 4.5.1

What is the mass number of an atom of helium that contains 2 neutrons?

Solution

(left( text{number of protons} right) = 2) (Remember that an atom of helium always has 2 protons.)

(left( text{number of neutrons} right) = 2)

(text{mass number} = left( text{number of protons} right) + left( text{number of neutrons} right))

(text{mass number} = 2 + 2 = 4)

## Isotopes

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{4}). 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.

Figure (PageIndex{4}):The three most stable isotopes of hydrogen: protium (A = 1), deuterium (A = 2), and tritium (A = 3). (CC SA-BY 3.0; Balajijagadesh).

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.

## Symbols for Isotopes

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}): POTASSIUM-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{1})

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
27 protons, 27 electrons, 33 neutrons
11 protons, 11 electrons, 13 neutrons
20 protons, 20 electrons, 25 neutrons
38 protons, 38 electrons, 52 neutrons

The properties of the isotopes of hydrogen, helium, lithium, berrylium, boron, and carbon are in Table (PageIndex{3}).

Table (PageIndex{3}): Properties of Isotopes of the First Six Elements.
ElementSymbolAtomic NumberNumber of ProtonsNumber of NeutronsMass (amu)% Natural Abundance
hydrogen(ce{^1_1H})
(protium)
1101.007899.989
(ce{^2_1H})
(deuterium)
1112.01410.0115
(ce{^3_1H})
(tritium)
1123.01605— (trace)
helium(ce{^3_2He})2213.016030.00013
(ce{^4_2He})2224.0026100
lithium(ce{^6_3Li})3336.01517.59
(ce{^7_3Li})3347.016092.41
beryllium(ce{^9_4Be})4459.0122100
boron(ce{^{10}_5B})55510.012919.9
(ce{^{11}_5B})55611.009380.1
carbon(ce{^{12}_6C})66612.000098.89
(ce{^{13}_6C})66713.00341.11
(ce{^{14}_6C})66814.0032— (trace)

## Summary

• The atom consists of discrete particles that govern its chemical and physical behavior.
• Each atom of an element contains the same number of protons, which is the atomic number (Z).
• Neutral atoms have the same number of electrons and protons.
• Atoms of an element that contain different numbers of neutrons are called isotopes.
• Each isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons.
• The relative masses of atoms are reported using the atomic mass unit (amu) which is defined as one-twelfth of the mass of one atom of carbon-12, with 6 protons, 6 neutrons, and 6 electrons. The nuclear model of the atom consists of a small and dense positively charged interior surrounded by a cloud of electrons.

### Atomic Number Definition

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

• TextMap: Chemistry-The Central Science (Brown et al.)
• Marisa Alviar-Agnew (Sacramento City College)

• Henry Agnew (UC Davis)

An explanation of the superscripts and subscripts seen in atomic number notation. Atomic number is the number of protons, and therefore also the total positive charge, in the atomic nucleus.
The Rutherford–Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one orbital to another be proportional to the mathematical square of atomic charge (Z2). Experimental measurement by Henry Moseley of this radiation for many elements (from Z = 13 to 92) showed the results as predicted by Bohr. Both the concept of atomic number and the Bohr model were thereby given scientific credence.

The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.

The sum of the atomic number Z and the number of neutronsN gives the mass numberA of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the 'relative isotopic mass'), is within 1% of the whole number A.

Atoms with the same atomic number but different neutron numbers, and hence different mass numbers, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.

The conventional symbol Z comes from the German word Zahl meaning number, which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.

## History

### The periodic table and a natural number for each element

Russian chemist Dmitri Mendeleev, creator of the periodic table.

Loosely speaking, the existence or construction of a periodic table of elements creates an ordering of the elements, and so they can be numbered in order.

Dmitri Mendeleev claimed that he arranged his first periodic tables (first published on March 6, 1869) in order of atomic weight ('Atomgewicht'). However, in consideration of the elements' observed chemical properties, he changed the order slightly and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9). This placement is consistent with the modern practice of ordering the elements by proton number, Z, but that number was not known or suspected at the time.

### Atomic Number Is Equal To The Number Of Neutrons In An Atom's Nucleus

Niels Bohr, creator of the Bohr model.

A simple numbering based on periodic table position was never entirely satisfactory, however. Besides the case of iodine and tellurium, later several other pairs of elements (such as argon and potassium, cobalt and nickel) were known to have nearly identical or reversed atomic weights, thus requiring their placement in the periodic table to be determined by their chemical properties. However the gradual identification of more and more chemically similar lanthanide elements, whose atomic number was not obvious, led to inconsistency and uncertainty in the periodic numbering of elements at least from lutetium (element 71) onward (hafnium was not known at this time).

### The Rutherford-Bohr model and van den Broek

In 1911, Ernest Rutherford gave a model of the atom in which a central nucleus held most of the atom's mass and a positive charge which, in units of the electron's charge, was to be approximately equal to half of the atom's atomic weight, expressed in numbers of hydrogen atoms. This central charge would thus be approximately half the atomic weight (though it was almost 25% different from the atomic number of gold (Z = 79, A = 197), the single element from which Rutherford made his guess). Nevertheless, in spite of Rutherford's estimation that gold had a central charge of about 100 (but was element Z = 79 on the periodic table), a month after Rutherford's paper appeared, Antonius van den Broek first formally suggested that the central charge and number of electrons in an atom was exactly equal to its place in the periodic table (also known as element number, atomic number, and symbolized Z). This proved eventually to be the case.

### Moseley's 1913 experiment

Henry Moseley in his lab.

The experimental position improved dramatically after research by Henry Moseley in 1913. Moseley, after discussions with Bohr who was at the same lab (and who had used Van den Broek's hypothesis in his Bohr model of the atom), decided to test Van den Broek's and Bohr's hypothesis directly, by seeing if spectral lines emitted from excited atoms fitted the Bohr theory's postulation that the frequency of the spectral lines be proportional to the square of Z.

To do this, Moseley measured the wavelengths of the innermost photon transitions (K and L lines) produced by the elements from aluminum (Z = 13) to gold (Z = 79) used as a series of movable anodic targets inside an x-ray tube. The square root of the frequency of these photons (x-rays) increased from one target to the next in an arithmetic progression. This led to the conclusion (Moseley's law) that the atomic number does closely correspond (with an offset of one unit for K-lines, in Moseley's work) to the calculated electric charge of the nucleus, i.e. the element number Z. Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.

### Missing elements

After Moseley's death in 1915, the atomic numbers of all known elements from hydrogen to uranium (Z = 92) were examined by his method. There were seven elements (with Z < 92) which were not found and therefore identified as still undiscovered, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, all seven of these missing elements were discovered. By this time, the first four transuranium elements had also been discovered, so that the periodic table was complete with no gaps as far as curium (Z = 96).

### The proton and the idea of nuclear electrons

In 1915, the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood. An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or 'protyles') of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one. However, as early as 1907, Rutherford and Thomas Royds had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times. If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms.

In 1917, Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas, and believed he had proven Prout's law. He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles). It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei. A helium nucleus was presumed to be composed of four protons plus two 'nuclear electrons' (electrons bound inside the nucleus) to cancel two of the charges. At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of +79, consistent with its atomic number.

### The discovery of the neutron makes Z the proton number

All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932. An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive charge now was realized to come entirely from a content of 79 protons. After 1932, therefore, an element's atomic number Z was also realized to be identical to the proton number of its nuclei.

## The symbol of Z

The conventional symbol Z possibly comes from the German word Atomzahl (atomic number). However, prior to 1915, the word Zahl (simply number) was used for an element's assigned number in the periodic table.

## Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is Z (the atomic number). The configuration of these electrons follows from the principles of quantum mechanics. The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. Hence, it is the atomic number alone that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of any mixture of atoms with a given atomic number.

## New elements

### Atomic Number Is Equal To Protons And Electrons The quest for new elements is usually described using atomic numbers. As of 2021, all elements with atomic numbers 1 to 118 have been observed. Synthesis of new elements is accomplished by bombarding target atoms of heavy elements with ions, such that the sum of the atomic numbers of the target and ion elements equals the atomic number of the element being created. In general, the half-life of a nuclide becomes shorter as atomic number increases, though undiscovered nuclides with certain 'magic' numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability.

 Look up atomic number in Wiktionary, the free dictionary.

## References

1. ^ abThe Periodic Table of Elements, American Institute of Physics
2. ^The Development of the Periodic Table, Royal Society of Chemistry
3. ^Ordering the Elements in the Periodic Table, Royal Chemical Society
4. ^Moseley, H.G.J. (1913). 'XCIII.The high-frequency spectra of the elements'. Philosophical Magazine. Series 6. 26 (156): 1024. doi:10.1080/14786441308635052. Archived from the original on 22 January 2010.
5. ^Eric Scerri, A tale of seven elements, (Oxford University Press 2013) ISBN978-0-19-539131-2, p.47
6. ^Scerri chaps. 3–9 (one chapter per element)
7. ^Ernest Rutherford NZHistory.net.nz, New Zealand history online. Nzhistory.net.nz (19 October 1937). Retrieved on 2011-01-26.
8. ^Origin of symbol Z. frostburg.edu