Why elements react to form compounds
In this topic you will:
- describe the structure of an ion and compare it with that of an atom
- explain how ionic and covalent bonds are formed
- explain what a molecule is
- write the formulae of some ionic and covalent compounds
Key words
- chemical bonds
- covalent bond
- dot and cross diagram
- ion
- highest energy level
- ionic bond
- ionic compounds
- molecule
- stable
- outermost electron shell
Thinking about atomic structure
Atoms have electrons arranged in different electron shells or energy levels around the nucleus. The shell with the highest energy level is the one on the outside of the atom. It is called the outermost electron shell.
In the different shells, there is room for two electrons in the first electron shell, eight electrons in the second electron shell, and eight electrons in the third electron shell.
The number of electrons in an atom is the same as the number of protons in that atom.
The atomic number tells us how many protons there are in an atom.
The electrons have an electrical charge of −1 and protons a charge of +1.
Atoms have no overall charge because there are an equal number of protons and electrons.
Electrons are held in place by electrostatic forces.
Why do elements react together?
Atoms are more stable when the outermost electron shell (highest energy level) is completely full of electrons.
The elements in Group 8, the noble gases, have their outermost energy level of electrons full, so they do not react to form compounds.
All the other groups do not have full outer shells of electrons so they can react to form compounds. In doing so, they fill the outermost electron shells with electrons. The elements in compounds are held together by chemical bonds. These bonds can be formed in two ways:
- the atoms can lose or gain electrons
- atoms can share electrons.
Losing electrons
Sodium reacts with other elements by losing an electron. When this happens, the sodium atom forms an ion.
When a sodium atom loses the electron from the outer shell, which is the outermost electron shell, the next shell becomes the outermost electron shell. This electron shell is full. So, the sodium ion is more stable than the sodium atom.
We write the symbol for a sodium atom as Na. When a sodium ion is formed it has one less electron than the atom, so there is now one more positively charged proton than negatively charged electrons. So, we write the symbol for a sodium ion as Na⁺.
Gaining electrons
An atom can also become an ion by gaining electrons.
In some groups in the Periodic Table, the elements have their outermost electron shells almost full. In Group 7, elements such as chlorine have seven electrons in the outermost electron shell. To fill its outermost electron shell, a chlorine atom gains an electron and forms a chlorine ion. The outermost electron shell in the chlorine ion is now full, so the chlorine ion is more stable than the chlorine atom.
We write the symbol for a chlorine atom as Cl. When a chlorine ion is formed, it has one more negatively charged electron than the atom, so there is now one more electron than the positively charged protons. So, we write the symbol for a chlorine ion as Cl⁻.
Ionic bonding
A sodium ion and a chlorine ion can form an ionic chemical bond to produce the compound sodium chloride (NaCl). In an ionic bond, there is a strong attraction between the positively charged ion (cation), sodium, and the negatively charged ion (anion), chlorine.
When a sodium atom loses its one outermost electron, it becomes a positively charged sodium ion (Na⁺). It now has 10 electrons and 11 protons — a net charge of +1. The new electron arrangement is [2,8], a full outer shell.
When a chlorine atom gains one electron, it becomes a negatively charged chloride ion (Cl⁻). It now has 18 electrons and 17 protons — a net charge of −1. Its electron arrangement becomes [2,8,8], also a full outer shell.
These opposite charges attract, forming a strong electrostatic force between them. This is the basis of the ionic bond, resulting in the formation of sodium chloride (NaCl).
Sodium (Na) has the electron structure 2,8,1. It loses 1 electron to achieve a full outer shell: 2,8. Chlorine (Cl) has the electron structure 2,8,7. It gains 1 electron to become 2,8,8.
Na + Cl → Na⁺ + Cl⁻ → NaCl
Some students think that the atoms “share” electrons in ionic bonding. In fact, ionic bonding involves the full transfer of electrons from one atom to another — not sharing.
Other metals in Group 1 also react with Group 7 elements in a similar way. Here are some more examples of ionic compound formation:
- lithium + chlorine → lithium chloride
- lithium + fluorine → lithium fluoride
- potassium + chlorine → potassium chloride
- potassium + fluorine → potassium fluoride
Reactivity and Electron Structure
Look at the electronic structures for the elements lithium, sodium and potassium.
- The outermost electron in potassium is farther from the nucleus.
- This makes the electrostatic attraction weaker.
- It takes less energy to remove the electron.
As you move down Group 1, atoms become larger. This increases the distance between the outermost electron and the nucleus, making the electron easier to remove. So, reactivity increases down the group.
Now look at the electronic structures of fluorine and chlorine.
- Its outer shell is closer to the nucleus.
- There is a stronger attraction between the positive nucleus and the incoming electron.
As you go down Group 7, the outermost shell is further from the nucleus and the electrostatic pull is weaker. This makes it harder to gain an electron, so reactivity decreases down the group.
Some students believe that atoms get more reactive the bigger they are. This is only true for Group 1. For Group 7, it's the opposite: smaller atoms like fluorine are more reactive.
Questions
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A sodium atom has the electronic structure 2,8,1.
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A sodium ion (Na⁺) has the electronic structure 2,8.
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The symbol for a chlorine atom is Cl.
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The symbol for a chlorine ion is Cl⁻.
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A potassium ion (K⁺) has the electronic structure 2,8,8.
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A fluorine ion (F⁻) has the electronic structure 2,8.
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Potassium has more electron shells, so its outer electron is further from the nucleus and less strongly attracted, making it easier to lose.
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Fluorine has fewer electron shells, so the nucleus has a stronger attraction on incoming electrons, making it easier to gain one.
Other ionic compounds
Ionic compounds are those that are made from ions. They form when a metal reacts with a non-metal.
In some elements more than one electron is lost or gained. For example, when magnesium combines with oxygen to form magnesium oxide, MgO.
A magnesium atom has two electrons in the outermost electron shell, its electronic structure is 2,8,2. An oxygen atom has six electrons in its outermost electron shell, 2,6.
The magnesium atom loses the two electrons and forms a magnesium ion, Mg²⁺. The oxygen atom gains two electrons and becomes an oxygen ion O²⁻. The two ions are attracted to one another and form an ionic bond to form the ionic compound magnesium oxide, MgO.
Common Mistake
Students sometimes forget that a magnesium ion (Mg²⁺) has only 8 electrons in its outer shell after losing 2. Always check electron counts after ion formation.
Magnesium reacts with chlorine to form magnesium chloride
Extension material: When magnesium forms an ionic compound with chlorine, two ions of chlorine are formed.
The magnesium atom loses the two electrons from its outermost shell and forms a magnesium ion, Mg²⁺. Two chlorine atoms both gain one electron and become two chloride ions, 2 Cl⁻.
The two chlorine ions are attracted to the magnesium ion and form an ionic bond to produce the ionic compound magnesium chloride, MgCl₂.
Quick Fact
Each chlorine atom needs only one electron to become stable, so one magnesium atom can react with two chlorine atoms to form MgCl₂.
Questions
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A calcium atom has the electronic structure 2,8,8,2. A calcium ion (Ca²⁺) has lost two outer electrons, so its structure becomes 2,8,8. The ion has fewer electrons and a full outer shell.
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Two chloride ions (Cl⁻) are needed to balance the 2+ charge on one calcium ion (Ca²⁺), so the answer is two.
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The chemical formula for calcium chloride is CaCl₂.
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The chemical formula for calcium oxide is CaO.
Sharing electrons
When non-metals form compounds with other non-metals they do so by sharing electrons to fill their outermost electron shells.
Hydrogen and chlorine
An example of this is when hydrogen and chlorine react to form hydrogen chloride:
- a hydrogen atom has just one electron in its outermost electron shell; in this first shell there is only room for two electrons
- a chlorine atom has seven electrons in its outermost electron shell; in this electron shell there is room for eight electrons
- the atoms share a pair of electrons, so both hydrogen and chlorine have their outermost electron shells full of electrons
- a molecule of hydrogen chloride is formed; its formula is HCl.
The electron in the hydrogen atom is shown as a dot and the electrons in the chlorine atom are shown as crosses. This type of diagram is known as a dot and cross diagram.
This type of chemical bond where electrons are shared is called a covalent bond.
Important Concept
A covalent bond is a chemical bond formed when two non-metal atoms share a pair of electrons. It is the key bonding type in molecular compounds like HCl and H2O.
Hydrogen and hydrogen
Covalent molecules are formed when atoms of different non-metals join together to form compounds and when atoms of the same non-metal join together.
For example, two atoms of hydrogen join together to form a molecule of hydrogen:
- an atom of hydrogen has one electron in its outermost electron shell; this electron shell has room for only two electrons
- the hydrogen atoms share a pair of electrons, so both atoms have two electrons in the outermost electron shell so the shell is full and more stable
- a molecule of hydrogen is formed; the formula is written as H2
Hydrogen and nitrogen
Another example of a covalent compound is ammonia. This compound is formed when hydrogen and nitrogen share three pairs of electrons:
- an atom of hydrogen has only one electron in the outermost electron shell, but there is room for two electrons in this shell
- an atom of nitrogen has an atomic number of 7, so it has two electrons in the first electron shell and has five electrons in its outermost electron shell, but there is room for eight electrons in this shell
- three atoms of hydrogen combine with one atom of nitrogen and share three pairs of electrons, so all the atoms have their outermost electron shells full of electrons
- a stable molecule of ammonia is formed; its formula is NH3
The hydrogen chloride, hydrogen, and ammonia molecules can also be represented using diagrams like the ones shown below. There are some more examples of covalent compounds, too:
Questions
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The compounds are hydrogen chloride (HCl), ammonia (NH3), methane (CH4), water (H2O), and carbon dioxide (CO2) because they contain different elements.
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A chlorine molecule (Cl2) forms when two chlorine atoms each share one electron. The dot and cross diagram shows a single pair of shared electrons between the two atoms, giving both a full outer shell of eight electrons.
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Calcium chloride is an ionic compound because it is formed from a metal (calcium) and a non-metal (chlorine), and electrons are transferred from calcium to chlorine.
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Methane: CH4
Carbon dioxide: CO2
Nitrogen: N2