Lesson Explainer: The Periodic Table Chemistry

In this explainer, we will learn how to define groups, periods, and blocks and link the properties of elements to their positions in the periodic table.

All matter is composed of chemical substances. There are millions of known chemicals and they are all made from different arrangements of about 118 elements, with only about half of these elements making up most of the things we see around us.

The elements have been studied over several centuries, and scientists have arranged them into the periodic table of elements.

The periodic table is very useful to chemists. The elements are listed in it in a very specific order, in rows and columns. Where each element is located on the periodic table tells us a lot of information about that particular element.

There are 18 vertical columns called groups; you may see groups 1 and 2 and groups 13–18 labeled with Roman numerals in some versions of the periodic table. The 7 horizontal rows are called periods. The bottom two rows, shown in green and purple, fit into the green and purple spaces on the main table in periods 6 and 7 respectively.

Definition: Groups

Groups are the vertical columns in the periodic table.

Definition: Periods

Periods are the horizontal rows in the periodic table.

At room temperature, all the elements exist as solids, except for the elements shown in orange and blue. The elements shown in orange are liquids at room temperature (298 K or 25C). They are

  • bromine (Br),
  • mercury (Hg).

The elements shown in blue are gases at room temperature. The gases are

  • hydrogen (H),
  • nitrogen (N),
  • oxygen (O),
  • fluorine (F),
  • chlorine (Cl),
  • all the elements of group 18.

The elements outlined in red are artificially synthesized in a laboratory, for example, Tc (technetium) in group 7. All the others are naturally occurring, although some only occur in very small quantities in nature.

The periodic table can be divided into blocks.

The blocks are the s-, d-, p-, and f-blocks. The block where an element is located is related to the arrangement of its atoms’ outermost electrons.

Most of the d-block elements are also called transition elements, except for the last column of the d-block that includes the elements zinc (Zn), cadmium (Cd), mercury (Hg), and copernicium (Cn).

The f-block elements are also called inner transition elements.

The majority of the elements are metals, some are nonmetals, and a few are what we call metalloids. The table below reminds us of some of the general properties of metals and nonmetals and shows us the properties of metalloids. Metalloids are elements that display properties of both metals and nonmetals. However, we must remember that these are general properties and there are some exceptions; for example, graphite is a nonmetal but is an excellent conductor of electricity.

Some Properties of Metals, Nonmetals, and Metalloids
MetalsNonmetalsMetalloids (Semimetals)
Conduct heat and electricityAre poor conductors of heat and electricityRelatively good conductors of heat and electricity (semiconductors)
Malleable: can be hammered into flat sheetsBrittle: shatter when hammered and cannot be hammered into flat sheetsBrittle: most shatter when hammered and cannot be hammered into flat sheets
Ductile: can be pulled into thin wiresCannot be made into thin wiresCannot be made into thin wires
Lustrous (shiny) when surfaces are cleanDull appearanceSome are lustrous

In the following version of the periodic table, we can see information about which elements display metallic, nonmetallic, or metalloid properties.

The metalloids are shown in blue and form distinctive “steps” going diagonally down from left to right, starting at group 13.

The metalloids are

  • boron (B),
  • silicon (Si),
  • germanium (Ge),
  • arsenic (As),
  • antimony (Sb),
  • tellurium (Te),
  • polonium (Po),
  • astatine (At).

Not much is known about the elements circled in red, because they have only been artificially synthesized in very small quantities. We might expect them to act as metals or nonmetals, as shown by the colors; however, some scientists disagree about how these elements will behave. We must also be aware that some periodic tables have minor differences in terms of which elements are labeled as metalloids.

Some of the groups and periods have special names. These names are shown in the following diagram.

These names are given because of the manner in which a set of elements react. For example, the group 1 metals all react in the same way with water: they produce alkaline hydroxide solutions. For this reason, they are called the alkali metals. The group 2 metals are called the alkaline earth metals. In the middle of the periodic table are 8 elements, shown in red, collectively called the noble metals. Another example are the group 18 elements that all exist as individual atoms, unbonded to other atoms, and are generally stable and unreactive. They are collectively known as the noble gases. All the elements outlined in pink belong to the transition metals. The transition metals include the bottom two rows, in green and purple, that are specifically known as the lanthanides and actinides respectively.

Example 1: Identifying Alkaline Earth Metals

Which of the following is an alkaline earth metal?

  1. Lithium
  2. Rubidium
  3. Strontium
  4. Yttrium
  5. Scandium

Answer

The collective name for the metals in group 2 of the periodic table is alkaline earth metals. The elements in group 2 are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The only element from this list that is one of the possible answer options is strontium. So, the correct answer is option C, strontium.

Lithium (Li) is found in group 1, the alkali metals. Rubidium (Rb) is also an alkali metal. Yttrium (Y) and scandium (Sc) are in group 3.

The diagram below is a more detailed version of the periodic table containing specific information about the atoms of each element. This periodic table will commonly be found in a book or on the Internet.

An atom is the smallest particle of an element. An atom usually contains one or more positively charged protons and neutral neutrons, in the nucleus, and one or more negatively charged electrons, found in orbitals outside of the central nucleus.

Definition: The Atom

An atom is the smallest particle of an element, containing positively charged protons and neutral neutrons, in the nucleus, and negatively charged electrons.

Atoms are electrically neutral particles, with the same number of positively charged protons and negatively charged electrons.

Let us look closely at the element lithium, for example, to see what information the periodic table provides.

The chemical symbol of the element lithium is uppercase L and lowercase i. Every element has a different chemical symbol.

The elements are listed in the periodic table, from left to right, in order of their atomic number. Atomic number means the number of protons in the nucleus, or center of an atom. The atomic number of lithium is 3 as there are 3 protons in the nucleus.

Definition: Atomic Number

An atomic number is the number of protons in the nucleus of an atom.

Example 2: Recalling the Organizing Principle of the Periodic Table

According to which property are the elements in the modern periodic table organized left to right?

Answer

Elements are organized in the periodic table in order of increasing atomic number, from left to right. Atomic number is the number of protons in the nucleus of an atom of an element.

The number of protons in the nucleus of an atom is unique for each element. It does not matter whether we are discussing an atom of sodium or an ion of sodium; there will be 11 protons in the nucleus. Only an atom of the element sodium has 11 protons in its nucleus. If there were 10 protons, it would not be an atom of sodium; it would be an atom of neon. And if there were 12 protons, it would not be an atom of sodium; it would be an atom of magnesium.

When an atom of an element reacts with an atom of another element, it either loses one or more electrons, or gains one or more electrons, or shares electrons with the other atom. We can get information about the number of electrons lost or gained from the periodic table.

The group that an element is in indicates the number of electrons that an atom of that element will either gain or lose when it reacts with another atom.

In general, the following is true.

  • Metals tend to lose electrons when they react and form positively charged ions that have more protons than electrons: metalion:numberofprotonsnumberofelectrons>.
  • Nonmetals tend to gain electrons when they react and form negatively charged ions that have more electrons than protons: nonmetalion:numberofprotonsnumberofelectrons<.

Definition: Ions

An ion is an atom (or molecule) that has either gained or lost one or more electrons and therefore has a net negative or positive charge respectively.

The specific number of electrons lost by a metal, as well as the specific number of electrons gained by a nonmetal, is often linked to the specific group that each element is in. In other words, the group number gives us information about the number of electrons lost or gained by an atom of an element when it reacts.

Valence is the number of electrons lost or gained by an atom when it reacts, or the combining power, or the maximum number of other particles an atom can bond with.

Valence values are shown in the periodic table below.

  • The metals in group 1 tend to lose 1 electron when they react. Hydrogen is also in this group, and though it is not a metal, it reacts in the same way. The group 1 elements have similar chemical properties because of this shared tendency.
  • Elements in group 2 lose 2 electrons to form ions with a 2+ charge.
  • Elements in group 13 lose 3 electrons to form ions with a 3+ charge.
  • Group 14 elements can either lose or gain 4 electrons, and elements in groups 15 and 16 tend to gain 3 and 2 electrons respectively. However, sometimes the elements in groups 14, 15, and 16 neither lose nor gain electrons, but they share electrons with other atoms when they form covalent bonds.
  • The group 17 elements gain 1 electron when they react and form ions with a 1 charge.
  • Elements in group 18 generally do not react, and they neither gain nor lose electrons.

Example 3: Connecting the Group Number with the Charge of an Ion

Sulfur is in group 16. What is the charge of the sulfide anion?

Answer

The elements in group 16 tend to gain 2 electrons when they react with other elements and form negatively charged ions with a charge of 2. So, the sulfur atom gains 2 electrons to form the sulfide anion whose symbol is S2. (Sometimes the elements in this group form covalent bonds with certain elements, where electrons are shared.)

Knowing valence values helps us to understand and predict chemical formulae that involve a metal bonded to a nonmetal.

For example, when a metal in group 1, such as lithium, reacts with chlorine, a nonmetal in group 17, we can easily determine the chemical formula of the product. Lithium will lose 1 electron during the reaction and form Li+. Chlorine will gain 1 electron (from lithium) and form Cl. The charges are equal and opposite and so the product is lithium chloride with a formula of LiCl11, which we can simply write as LiCl: Li+ClLiClLiCl+11

Another example is when magnesium in group 2 reacts with bromine in group 17. Magnesium will lose 2 electrons to become Mg2+ and two bromine atoms will each gain 1 electron (from magnesium) to form two Br1 ions. The product is magnesium bromide with the formula MgBr12. We can write this simply as MgBr2: Mg+Br+BrMgBrMgBr2+11122

Example 4: Connecting the Group Number with the Subscript in an Ionic Formula

Consider the equation below: 2X+3ClXCl226

Which one of the following groups could X be in?

  1. Group 17
  2. Group 2
  3. Group 13
  4. Group 15

Answer

The compound XCl26 contains chlorine. Chlorine is in group 17 and so has a valency of 1 and forms a negative Cl1 ion when it reacts. There are six chlorine ions in this compound and thus six negative charges.

This compound is electrically neutral overall and so “X2” must carry six positive charges to cancel with the six negative charges from “Cl6.

We can divide the 6 positive charges by 2, since there are two X ions. This will give us the charge on one X ion.

Each X ion is X3+. Thus, X must be from group 13, as elements in group 13 tend to lose electrons when they react and form positive ions, and the valency of group 13 elements is 3, so they lose 3 electrons specifically.

The correct answer is option C, group 13.

Key Points

  • The periodic table is arranged into 18 vertical columns called groups and 7 horizontal rows called periods.
  • Most elements are solids at room temperature, two are liquids, and several are gases.
  • Most elements are naturally occurring, and several are artificially synthesized.
  • Most elements are metals, several are nonmetals, and several are metalloids.
  • Metals are separated from the nonmetals by a “stepped” arrangement of metalloids.
  • Some groups have special names based on how their elements react.
  • Each element has a unique symbol.
  • Elements are listed from left to right in the periodic table in order of increasing atomic number.
  • The periodic table can be divided into the s-, p-, d-, and f-blocks.
  • Atomic number is the number of protons in the nucleus, or center of an atom.
  • An atom is the smallest particle of an element, containing positively charged protons and neutral neutrons, in the nucleus, and negatively charged electrons. Atoms contain an equal number of protons and electrons and are electrically neutral.
  • Ions are particles that contain an unequal number of protons and electrons and are electrically charged.
  • Metals tend to lose electrons when they react and form positively charged cations.
  • Nonmetals tend to gain electrons when they react and form negatively charged anions.
  • Valence is the number of electrons lost or gained by an atom when it reacts. Elements in the same group have the same valence.
  • Elements in groups 14, 15, and 16 sometimes form covalent bonds where electrons are shared between two atoms.

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