The periodic table, electron shells, and orbitals (article) | Khan Academy (2023)


At some point in your chemistry education, you may have been introduced to the song “The Elements,” in which Tom Lehrer does a rapid-fire musical rendition of all the elements' names. Like me, you may even have been offered the opportunity to memorize this song for extra credit. If so, it’s possible that you still remember the names of all the elements, which is an impressive feat—not to mention a fun trick to pull out at parties.

If you’ve memorized the names of the elements, does that mean you’ll never need a periodic table again? Well ... probably not. That’s because the periodic table isn’t just a big bucket that holds all of the elements. Instead, it’s more like a filing system. The position of each element in the table gives important information about its structure, properties, and behavior in chemical reactions. Specifically, an element’s position in the periodic table helps you figure out its electron configuration, how the electrons are organized around the nucleus. Atoms use their electrons to participate in chemical reactions, so knowing an element’s electron configuration allows you to predict its reactivity—whether, and how, it will interact with atoms of other elements.

In this article, we’ll look in more detail at the periodic table, how atoms organize their electrons, and how this allows us to predict the reactivity of elements.

The periodic table

By convention, elements are organized in the periodic table, a structure that captures important patterns in their behavior. Devised by Russian chemist Dmitri Mendeleev (1834–1907) in 1869, the table places elements into columns—groups—and rows—periods—that share certain properties. These properties determine an element’s physical state at room temperature—gas, solid, or liquid—as well as its chemical reactivity, the ability to form chemical bonds with other atoms.

In addition to listing the atomic number for each element, the periodic table also displays the element’s relative atomic mass, the weighted average for its naturally occurring isotopes on earth. Looking at hydrogen, for example, its symbol, H,\text{H,}H,start text, H, comma, end text and name appear, as well as its atomic number of one—in the upper left-hand corner—and its relative atomic mass of 1.01.

The periodic table of the elements

(Video) Shells, subshells, and orbitals | Atomic structure and properties | AP Chemistry | Khan Academy

Image credit: modified from OpenStax Biology. An accessible version of the periodic table is available here.

Differences in chemical reactivity between elements are based on the number and spatial distribution of their electrons. If two atoms have complementary electron patterns, they can react and form a chemical bond, creating a molecule or compound. As we will see below, the periodic table organizes elements in a way that reflects their number and pattern of electrons, which makes it useful for predicting the reactivity of an element: how likely it is to form bonds, and with which other elements.

Electron shells and the Bohr model

An early model of the atom was developed in 1913 by the Danish scientist Niels Bohr (1885–1962). The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular electron shells at specific distances from the nucleus, similar to planets orbiting around the sun. Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus. By convention, each shell is assigned a number and the symbol n—for example, the electron shell closest to the nucleus is called 1n. In order to move between shells, an electron must absorb or release an amount of energy corresponding exactly to the difference in energy between the shells. For instance, if an electron absorbs energy from a photon, it may become excited and move to a higher-energy shell; conversely, when an excited electron drops back down to a lower-energy shell, it will release energy, often in the form of heat.

Bohr model of an atom, showing energy levels as concentric circles surrounding the nucleus. Energy must be added to move an electron outward to a higher energy level, and energy is released when an electron falls down from a higher energy level to a closer-in one.

Image credit: modified from OpenStax Biology

(Video) What are Shells, Subshells, and Orbitals? | Chemistry

Atoms, like other things governed by the laws of physics, tend to take on the lowest-energy, most stable configuration they can. Thus, the electron shells of an atom are populated from the inside out, with electrons filling up the low-energy shells closer to the nucleus before they move into the higher-energy shells further out. The shell closest to the nucleus, 1n, can hold two electrons, while the next shell, 2n, can hold eight, and the third shell, 3n, can hold up to eighteen.

The number of electrons in the outermost shell of a particular atom determines its reactivity, or tendency to form chemical bonds with other atoms. This outermost shell is known as the valence shell, and the electrons found in it are called valence electrons. In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule. Some atoms can be stable with an octet even though their valence shell is the 3n shell, which can hold up to 18 electrons. We will explore the reason for this when we discuss electron orbitals below.

Examples of some neutral atoms and their electron configurations are shown below. In this table, you can see that helium has a full valence shell, with two electrons in its first and only, 1n, shell. Similarly, neon has a complete outer 2n shell containing eight electrons. These electron configurations make helium and neon very stable. Although argon does not technically have a full outer shell, since the 3n shell can hold up to eighteen electrons, it is stable like neon and helium because it has eight electrons in the 3n shell and thus satisfies the octet rule. In contrast, chlorine has only seven electrons in its outermost shell, while sodium has just one. These patterns do not fill the outermost shell or satisfy the octet rule, making chlorine and sodium reactive, eager to gain or lose electrons to reach a more stable configuration.

Bohr diagrams of various elements

Image credit: OpenStax Biology

(Video) Shells, Subshells, and Orbitals - BIOLOGY/CHEMISTRY EP5

Electron configurations and the periodic table

Elements are placed in order on the periodic table based on their atomic number, how many protons they have. In a neutral atom, the number of electrons will equal the number of protons, so we can easily determine electron number from atomic number. In addition, the position of an element in the periodic table—its column, or group, and row, or period—provides useful information about how those electrons are arranged.

If we consider just the first three rows of the table, which include the major elements important to life, each row corresponds to the filling of a different electron shell: helium and hydrogen place their electrons in the 1n shell, while second-row elements like Li start filling the 2n shell, and third-row elements like Na continue with the 3n shell. Similarly, an element’s column number gives information about its number of valence electrons and reactivity. In general, the number of valence electrons is the same within a column and increases from left to right within a row. Group 1 elements have just one valence electron and group 18 elements have eight, except for helium, which has only two electrons total. Thus, group number is a good predictor of how reactive each element will be:

  • Helium (He\text{He}Hestart text, H, e, end text), neon (Ne\text{Ne}Nestart text, N, e, end text), and argon (Ar\text{Ar}Arstart text, A, r, end text), as group 18 elements, have outer electron shells that are full or satisfy the octet rule. This makes them highly stable as single atoms. Because of their non-reactivity, they are called the inert gases or noble gases.

  • Hydrogen (H\text{H}Hstart text, H, end text), lithium (Li\text{Li}Listart text, L, i, end text), and sodium (Na\text{Na}Nastart text, N, a, end text), as group 1 elements, have just one electron in their outermost shells. They are unstable as single atoms, but can become stable by losing or sharing their one valence electron. If these elements fully lose an electron—as Li\text{Li}Listart text, L, i, end text and Na\text{Na}Nastart text, N, a, end text typically do—they become positively charged ions: Li+\text{Li}^+Li+start text, L, i, end text, start superscript, plus, end superscript and Na+\text{Na}^+Na+start text, N, a, end text, start superscript, plus, end superscript.

  • Fluorine (F\text{F}Fstart text, F, end text) and chlorine (Cl\text{Cl}Clstart text, C, l, end text), as group 17 elements, have seven electrons in their outermost shells. They tend to achieve a stable octet by taking an electron from other atoms, becoming negatively charged ions: F\text{F}^-Fstart text, F, end text, start superscript, minus, end superscript and Cl\text{Cl}^-Clstart text, C, l, end text, start superscript, minus, end superscript.

  • Carbon (C\text{C}Cstart text, C, end text), as a group 14 element, has four electrons in its outer shell. Carbon typically shares electrons to achieve a complete valence shell, forming bonds with multiple other atoms.

Thus, the columns of the periodic table reflect the number of electrons found in each element’s valence shell, which in turn determines how the element will react.

Subshells and orbitals

The Bohr model is useful to explain the reactivity and chemical bonding of many elements, but it actually doesn’t give a very accurate description of how electrons are distributed in space around the nucleus. Specifically, electrons don’t really circle the nucleus, but rather spend most of their time in sometimes-complex-shaped regions of space around the nucleus, known as electron orbitals. We can’t actually know where an electron is at any given moment in time, but we can mathematically determine the volume of space in which it is most likely to be found—say, the volume of space in which it will spend 90% of its time. This high-probability region makes up an orbital, and each orbital can hold up to two electrons.

So, how do these mathematically defined orbitals fit in with the electron shells we saw in the Bohr model? We can break each electron shell down into one or more subshells, which are simply sets of one or more orbitals. Subshells are designated by the letters ssss, pppp, dddd, and ffff, and each letter indicates a different shape. For instance, ssss subshells have a single, spherical orbital, while pppp subshells contain three dumbbell-shaped orbitals at right angles to each other. Most of organic chemistry—the chemistry of carbon-containing compounds, which are central to biology—involves interactions between electrons in ssss and pppp subshells, so these are the most important subshell types to be familiar with. However, atoms with many electrons may place some of their electrons in dddd and ffff subshells. Subshells dddd and ffff have more complex shapes and contain five and seven orbitals, respectively.

3D diagram of circular 1s and 2s orbitals and dumbbell-shaped 2p orbitals. There are three 2p orbitals, and they are at right angles to each other.

(Video) Electron Configuration - Basic introduction

Image credit: modified from OpenStax Biology

The first electron shell, 1n, corresponds to a single 1s1s1s1, s orbital. The 1s1s1s1, s orbital is the closest orbital to the nucleus, and it fills with electrons first, before any other orbital. Hydrogen has just one electron, so it has a single spot in the 1s1s1s1, s orbital occupied. This can be written out in a shorthand form called an electron configuration as 1s11s^ 11s11, s, start superscript, 1, end superscript, where the superscripted 1 refers to the one electron in the 1s1s1s1, s orbital. Helium has two electrons, so it can completely fill the 1s1s1s1, s orbital with its two electrons. This is written out as 1s21s^ 21s21, s, squared, referring to the two electrons of helium in the 1s1s1s1, s orbital. On the periodic table, hydrogen and helium are the only two elements in the first row, or period, which reflects that they only have electrons in their first shell. Hydrogen and helium are the only two elements that have electrons exclusively in the 1s1s1s1, s orbital in their neutral, non-charged, state.

The second electron shell, 2n, contains another spherical ssss orbital plus three dumbbell-shaped pppp orbitals, each of which can hold two electrons. After the 1s1s1s1, s orbital is filled, the second electron shell begins to fill, with electrons going first into the 2s2s2s2, s orbital and then into the three pppp orbitals. Elements in the second row of the periodic table place their electrons in the 2n shell as well as the 1n shell. For instance, lithium (Li\text{Li}Listart text, L, i, end text) has three electrons: two fill the 1s1s1s1, s orbital, and the third is placed in the 2s2s2s2, s orbital, giving an electron configuration of 1s21s^ 21s21, s, squared 2s12s^ 12s12, s, start superscript, 1, end superscript. Neon (Ne\text{Ne}Nestart text, N, e, end text), on the other hand, has a total of ten electrons: two are in its innermost 1s1s1s1, s orbital and eight fill the second shell—two each in the 2s2s2s2, s and three pppp orbitals, 1s21s^ 21s21, s, squared 2s22s^ 22s22, s, squared 2p62p^62p62, p, start superscript, 6, end superscript. Because its 2n shell is filled, it is energetically stable as a single atom and will rarely form chemical bonds with other atoms.

The third electron shell, 3n, also contains an ssss orbital and three pppp orbitals, and the third-row elements of the periodic table place their electrons in these orbitals, much as second-row elements do for the 2n shell. The 3n shell also contains a dddd orbital, but this orbital is considerably higher in energy than the 3s3s3s3, s and 3p3p3p3, p orbitals and does not begin to fill until the fourth row of the periodic table. This is why third-row elements, such as argon, can be stable with just eight valence electrons: their ssss and pppp subshells are filled, even though the entire 3n shell is not.

While electron shells and orbitals are closely related, orbitals provide a more accurate picture of the electron configuration of an atom. That’s because orbitals actually specify the shape and position of the regions of space that electrons occupy.

[Attribution and references]

(Video) Introduction to electron configurations | AP Chemistry | Khan Academy


What is the 2 8 8 18 rule in chemistry? ›

It is an arrangement of electrons in various shells, sub-shells and orbitals in an atom. It is written as 2, 8, 8, 18, 18, 32. It is written as nlx ( where n indicates the principal quantum number), l indicates the azimuthal quantum number or sub-shell, and x is the number of electrons.

What are electron shells and orbitals? ›

Subshell (electron): A grouping of electrons in a shell according to the shape of the region of space they occupy. Within each subshell, electrons are grouped into orbitals, regions of space within an atom where the specific electrons are most likely to be found.

How can you determine how many orbitals or shells an element has from the periodic table? ›

Period: A period is the horizontal row of the periodic table. All elements in a row have the same number of electron shells. So, if we know the period of an element then we can predict the number of electron shells of the neutral atom.

What is the 2 8 8 rule periodic table? ›

We should start with the atoms that have atomic numbers between 1 and 18. There is a 2-8-8 rule for these elements. The first shell is filled with 2 electrons, the second is filled with 8 electrons, and the third is filled with 8. You can see that sodium (Na) and magnesium (Mg) have a couple of extra electrons.

Why is it 2 8 8 1 and not 2 8 9? ›

According to octet rule, the outermost shell of an atom can accommodate maximum 8 electrons (except K shell which can accommodate maximum 2 electrons). Hence, the electronic configuration of potassium is 2,8,8,1 and not 2,8,9.

What are the 4 types of atomic orbitals? ›

An orbital is a region of space where there is a high probability of finding an electron. There are four basic types of orbitals: s, p, d, and f.

How many shells are in orbitals? ›

The number of orbitals in a shell is the square of the principal quantum number: 12 = 1, 22 = 4, 32 = 9. There is one orbital in an s subshell (l = 0), three orbitals in a p subshell (l = 1), and five orbitals in a d subshell (l = 2).

Are there 7 electron shells? ›

The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from innermost shell outwards. Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells.

What is the difference between an orbital and an orbit? ›

Differences between Orbit and Orbitals

An orbit is the simple planar representation of an electron. An orbital refers to the dimensional motion of an electron around the nucleus in a three-dimensional motion. An orbital can be defined as the space or region where the electron is most likely to be found.

Why there are only 8 electrons in the third shell? ›

The third period contains only eight elements even though the electron capacity of the third shell is 18 because when the other shells get filled and the resultant number of electrons becomes eighteen, it gets added up and settles in the third electron shell and three shells are acquired by the fourth period.

What are the 4 quantum numbers? ›

There are four quantum numbers, namely, principal, azimuthal, magnetic and spin quantum numbers.

What group is the element having the structure 1s2 2s2 2p6 3s2 3p2 in? ›

The electron configuration 1s22s22p63s23p2 is the element Silicon.

Which of the two outermost orbitals must be filled to satisfy the octet rule? ›

Atoms follow the octet rule because they always seek the most stable electron configuration. Following the octet rule results in completely filled s- and p- orbitals in an atom's outermost energy level.

Why do electrons enter the 4s orbital before entering the 3d orbital? ›

We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first.

Why do the electron shells fill to 2 8 8 8 instead of filling their shells completely? ›

So the two have to go to the next energy level so that the electronic configuration becomes 2,8,8,2. Now, there are only two electrons in the valence shell and hence when you give energy, 2 electrons in the valence shell move out of the atom, thereby making the atom stable.

Is the octet rule always 8? ›

While most atoms obey the duet and octet rules, there are some exceptions. For example, elements such as boron or beryllium often form compounds in which the central atom is surrounded by fewer than eight electrons (e.g., BF₃ or BeH₂).

What is the 8 electron rule called? ›

The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds.

What is the electron configuration of 2 8 8 8 1? ›

Potassium with atomic number 19 has the electronic configuration (2,8,8,1).

What is the electronic configuration of 2 8 8 1? ›

Therefore, the element K or Potassium has the electronic configuration 2, 8, 8, 1.

What is the electron configuration of 2 8 9 2? ›

Electron Shell Configuration of the elements
Calcium{2, 8, 8, 2}
Scandium{2, 8, 9, 2}
Titanium{2, 8, 10, 2}
Vanadium{2, 8, 11, 2}
54 more rows

How do you identify an orbital? ›

Each orbital is denoted by a number and a letter. The number denotes the energy level of the electron in the orbital. Thus 1 refers to the energy level closest to the nucleus; 2 refers to the next energy level further out, and so on. The letter refers to the shape of the orbital.

How do orbitals work? ›

Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible. This filling of orbitals singly where possible is known as Hund's rule.

Which orbital has highest energy? ›

The f orbital has the highest energy among these, check the (n+l) rule for this.

Which shell has 7 orbitals? ›

Subshells d and f have more complex shapes and contain five and seven orbitals, respectively.

What is the order of the orbitals? ›

The order of the electron orbital energy levels, starting from least to greatest, is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Since electrons all have the same charge, they stay as far away as possible because of repulsion.

Which shell has 9 orbitals? ›

On adding all the orbitals of 3s, 3p and 3d we get a total of 9 orbitals. Therefore, there are 9 orbitals in the third shell.

What is the first shell of an atom called? ›

The innermost shell of electrons that surrounds an atomic nucleus and constitutes the lowest available energy level of the electrons is called as K-shell. K denotes the first shell or the energy level.

What are electron shells called? ›

Also called an orbital or atomic shell, it is used to describe the discreet region occupied by electrons around the nucleus of an atom. Electron shells are sometimes called energy levels because electrons contain energy. Electrons near the nucleus have less energy than those farther away.

Why do shells start with K? ›

He later renamed these two to K and L since he realized that the highest energy X-rays produced in his experiment might not be the highest energy X-rays possible. It later turned out that K has the highest energy possible. Thus, the innermost shell was called the K- shell.

What is the 2 8 8 2 rule in chemistry? ›

The 2–8–8 rule is the electron filling rule in the shells of an atom. It is used for predicting the no. Of electron in each shell. The innermost shell will have maximum of 2 electrons, second will have 8 and so on. It follows a rule of 2n^2, where n is equal to the position of shell .

What is the magic number 2 8 8 18? ›

The numbers 2, 8, 8, 18, 18, 32 are known as magic numbers. It is because of the fact that the elements which occur after these intervals have similar properties. For example, atomic number of Li is 3.

What is the 8 rule electrons? ›

The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds.

Why does the second shell hold 8 electrons? ›

The second shell has two subshells (labeled 2s and 2p). The 2s subshell holds a maximum of 2 electrons, and the 2p subshell holds a maximum of 6 electrons. This means that the second shell can hold a maximum of eight electrons (2+6=8). Notice that there are eight elements in the second row of the periodic table.

Why is the octet rule 8? ›

It is based on the observation that the atoms of the main group elements have a tendency to participate in chemical bonding in such a way that each atom of the resulting molecule has eight electrons in the valence shell. The octet rule is only applicable to the main group elements.

Why is the third shell 8 or 18? ›

The third period contains only eight elements even though the electron capacity of the third shell is 18 because when the other shells get filled and the resultant number of electrons becomes eighteen, it gets added up and settles in the third electron shell and three shells are acquired by the fourth period.

Why is 33 a magic number? ›

Number 33 is a Master Number (Master Teacher) and resonates with the energies of compassion, blessings, inspiration, honesty, discipline, bravery and courage. Number 33 tells us that 'all things are possible'. 33 is also the number that symbolizes 'guidance'.

Why is 4 the magic number? ›

Four is a very magical number, all camp counselors know this to be true. All numbers, no matter how big or small, equal four. Here are a few examples for you: 1: One is three, three is five, five is four, and four is four, so 4 is the magical number!

Why is 15 a magic number? ›

Fifteen is a number of deep esoteric significance to astrologers, numerologists and magicians across the world. Magicians consider it as alchemic vibration through which all magic is manifested. Fifteen is considered “an extremely lucky number,” carrying the essence of enchantment.

Can each shell only hold 8 electrons? ›

Each shell can contain only a fixed number of electrons: the first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on.

Which rule says that the last shell Cannot have more than 8 electrons? ›

The octet rule states that atoms with 8 electrons in their outer shell are stable. Thus, even if the shell has the capacity to hold more electrons, it does not have more than 8 electrons. If the outermost shell holds up more than 8 electros, the atom becomes unstable.

What is the magic number in chemistry 2 8 8 18 18 32? ›

Magic Numbers: It has been discovered that when elements are placed in order of increasing atomic number, elements with comparable properties tend to recur at intervals of 2, 8, 18, or 32 elements. 2,8,8,18,18,32 sets include only magic number.


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6. Electron shells Elements 1-18
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