5 Simple Ways to Identify an Element on the Periodic Table

Every atom in the universe is a particular element. But how do we tell which of the 100+ elements it is? A larger pile of stuff might give us helpful clues: we can tell that iron is heavy, and grey, and magnetic. As you study chemistry, you'll learn that all of those qualities come from small differences in the structure of atoms. This understanding of atomic structure is the foundation for the tools actual scientists use to identify elements.

1

By proton number

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An element is defined by the number of protons in one atom. For example, every single atom of hydrogen has exactly one proton. We say that hydrogen has a proton number or atomic number of 1.^{[1]XResearch source} The periodic table is arranged in order of proton number, which is why hydrogen is in the very first box with a 1 next to it.
- Atomic number is abbreviated "Z". If your homework says an element has Z=13, you can look for atomic number 13 on the periodic table and identify it as aluminum (Al).
- An atom can gain or lose neutrons and still be the same element. For instance, $_{11}^{22}Na$ is a sodium atom with 11 protons and 22 neutrons. If it gains a neutron, it is still sodium and becomes $_{11}^{23}Na$ (with 23 neutrons). But if you add a proton, it transforms from sodium to magnesium, $_{12}Mg$ .

2

By electron count

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The total electron count equals the atomic number. In a neutral atom, the number of electrons is exactly equal to the number of protons. This number is the atomic number of the element, which you can look up on the periodic table. If you are a little further in your chemistry studies, you might be given an electron configuration to read. All of the superscript numbers (^{like this}) are electron counts, so add all these together to find the total number of electrons.^{[2]XResearch source}
- For example, if you are asked which element has 8 electrons, look for the element with atomic number 8: oxygen.
- For a more advanced example, the configuration $1s^{2}2s^{2}2p^{2}$ has $^{2}$ electrons in the 1s shell, $^{2}$ in the 2s shell, and $^{2}$ in the 2p shell, for a total of 2+2+2=6. This is carbon, with atomic number 6.
- Note that this only holds true when the atoms are in electrically neutral states, not ionized. But unless specified otherwise, this is the state we talk about when we discuss element characteristics.^{[3]XResearch source}

3

By electron configuration with the periodic table

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Memorize the periodic table structure to read electron configurations quickly. The structure of the periodic table is closely related to how electron orbitals are filled. With a little practice you can jump directly to the right region of the periodic table.^{[4]XResearch source} Note that the electron configuration must be in its ground state for this to work.
- The first row (hydrogen and helium) fills up the 1s orbital from left to right. Think of these, plus all elements in the first two columns, as the "s-block". Each row of the "s-block" fills up one s orbital.
- The right-hand side of the table is the "p-block", starting with boron through neon. Each row of the "p-block" fills up one p orbital (starting with 2p).
- The transition metals in the center form the "d-block". Each row fills up one d orbital, starting with scandium through zinc filling 3d.
- The lanthanides and actinides at the bottom of the table fill the 4f and 5f orbitals. (Some elements here break the pattern, so double-check these.^{[5]XResearch source})
- For example, look at $[Kr]5s^{2}4d^{10}5p^{2}$ and focus on the last orbital: $5p^{2}$ . Go to the "p-block" on the right, and count rows down from 2p (boron) until you reach 5p (indium). Since this element has two electrons in 5p, count two elements into this row of the p-block to get the answer: tin.

4

By spectroscopy

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Compare the spectra to the known spectra of elements. In spectroscopy, scientists examine how light interacts with an unknown material. Each element releases a unique pattern of light, which you can see on the spectroscopy results, called "spectra".^{[6]XResearch source}
- For example, a lithium spectrum has a very bright, thick green line, and several other fainter ones in different colors. If your spectrum has all those same lines on it, the light came from the element lithium.^{[7]XResearch source} (Some types of spectra will show dark gaps instead of bright lines, but you can compare these the same way.)
- Want to know why this works? Electrons only absorb and emit light at very specific wavelengths (meaning specific colors). Different elements have different arrangements of electrons, which leads to different colors of bands.^{[8]XResearch source}
- A more advanced spectroscope shows a detailed graph instead of a few lines. You can match the x-axis value at each peak to a table of known values to identify molecules. As you learn about different types of molecules, you'll learn to focus on just a few useful spots on the graph to save time.^{[9]XResearch source}

5

By mass spectrum

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Look for elements whose atomic masses match the graph. A mass spectrometer sorts the components of a sample by mass. To read the bar graph showing the results, check the "m/z" axis for the values of the taller bars. Some values will match the atomic mass of an element that was part of the sample. Others (usually the larger ones) represent compounds, so that mass will equal the sum of masses of multiple atoms.^{[10]XResearch source}
- Let's say the tallest bar is at m/z 18, with short bars at 1, 16, and 17. Only two of these match the atomic mass of an element: hydrogen (atomic mass 1) and oxygen (atomic mass 16). Adding these atoms together gives you the compounds HO (mass 1 + 16 = 17) and H₂O (mass 1 + 1 + 16 = 18). This sample was water!^{[11]XResearch source}
- Technically, a mass spectrometer ionizes the sample and sorts by the ratio of mass to charge (or m/z). But most ions will have a charge of 1, and so you can ignore the division problem and just look at mass. The smallest bars often represent small amounts of more charged particles that you can ignore for identification purposes.^{[12]XResearch source}

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What kind of test can I perform to identify an element?
Anne Schmidt
Chemistry Instructor
Anne Schmidt is a Chemistry Instructor in Wisconsin. Anne has been teaching high school chemistry for over 20 years and is passionate about providing accessible and educational chemistry content. She has over 9,000 subscribers to her educational chemistry YouTube channel. She has presented at the American Association of Chemistry Teachers (AATC) and was an Adjunct General Chemistry Instructor at Northeast Wisconsin Technical College. Anne was published in the Journal of Chemical Education as a Co-Author, has an article in ChemEdX, and has presented twice and was published with the AACT. Anne has a BS in Chemistry from the University of Wisconsin, Oshkosh, and an MA in Secondary Education and Teaching from Viterbo University.
Anne Schmidt
Chemistry Instructor
Expert Answer
A fun way to do this is by performing a flame test. Although some elements may not have a reaction when ignited. But usually, it's metal elements that have lost electrons that tend to burn a specific color in a flame, or at least a fairly unique color. And so fireworks are based on this property; the age of stars is based on this property; and an instrument called an atomic absorption emission spectrum called an AE is based on this property. Each element has a unique color based on those electron transitions.
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Tips

Mass spectrometer readings get complicated when you sample large molecules that can break into many parts.^{[13]XResearch source} If you can narrow down the sample to a few possibilities, you can look up the mass spectra of each one and compare that to your actual results.
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Electron configurations can also be written in noble gas notation, which uses a noble gas element symbol to stand in for that element's electron shell.^{[14]XResearch source} For instance, $[Ne]3s^{2}3p^{4}$ can be expanded with neon's electron configuration into $1s^{2}2s^{2}2p^{6}3s^{2}3p^{4}$ . To identify the element by electron count, add the electrons in the expanded configuration.^{[15]XResearch source} In this case there are 2+2+6+2+4 = 16 electrons, so this must be the sixteenth element, sulfur.
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Most periodic tables show two numbers next to each element. The smaller one (that always goes up by 1 as you go left to right) is the atomic number or proton number. The larger number is the atomic mass.
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Warnings

If you are given an electron configuration in an excited state, you can't use the highest energy electron to identify the element. Identify the ground state for that number of electrons first.
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