Structures and Properties of Materials

Difference between Atoms, Elements, Compounds, and Mixtures.

An element is a pure substance that contains only one type of atom. An atom is a substance cannot be split up into two or more simpler substances by chemical processes or electricity. In other words, atoms are indivisible. There are many different types of atoms, and we distinguish them according to their proton number. All the elements that we knew so far are presented in the Periodic Table.

Elements are made up of atoms. Take nitrogen for example. Nitrogen is an element because it only contains one type of atom, that is the nitrogen atom.

There are two types of elements: metal and non-metal elements. Some elements, which have both properties of metals and non-metals are called metalloids.

Metalloids are elements that have properties of metals and non-metals.

Using Periodic Table to Identify the Type of Elements

Metals are usually found in the left-hand side of the Periodic Table, while non-metal are usually found at the right side of the Periodic Table.

There is a ‘staircase’ line that separates metals and non-metals. Elements that are found near to the ‘staircase’ line are called metalloids.

Example to Differentiate Atoms, Compounds, and Elements

Examples of elements are oxygen and nitrogen. Elements can be found in the Periodic Table. Oxygen gas (state of oxygen in room temperature and pressure) is an element. This is because it only contains oxygen atoms. But, the oxygen atoms don’t exist alone, because oxygen atoms with itself are unstable.

Hence, oxygen gas is made up of oxygen molecules. In each oxygen molecule, it is made up of two oxygen atoms chemically combined. In between the two oxygen atoms, there is a strong bond, which requires a large amount of energy to overcome. A molecule is a substance when two or more atoms of an element chemically bond together.

What Are Molecules?

Molecules are also called compounds. A compound is a substance formed when two or more chemical elements.

Atoms that have full valence electrons are generally stable. Full valence electrons simply mean that the outermost electron shell is fully filled with electrons. If the atom has one electron shell, it needs two electrons to fully fill up its valence shell. On the other hand, if the atom has two or more electron shell, it needs eight electrons to fill up its valence shell.

When an atom has fully filled electron shell, the atom is said to achieve a noble gas configuration. Note that noble gases belong to Group 0 and have fully filled valence electron shells.

To achieve full valence electron (therefore a stable noble gas configuration), atoms can lose, gain or share electrons. Metals tend to lose electrons to achieve full valence electron.

What Happens When Atoms Do Not Have Full Valence Electrons?

Most atoms without full valence electron (noble gas configuration) are generally unstable. They tend to achieve a noble gas configuration by forming bonds. This is because the formation of bonds involved the losing, gaining and sharing of the valence electrons, which helps the atoms to attain a noble gas configuration.

Note: only the valence electrons are involved in bonding. The inner electrons, are not involved in bonding, simply because they are nearer to the positively charged nucleus. The negatively charged electrons in the inner shells are more attracted to the positively charged nucleus.

Types of Bonding

There are three types of bonding that we will cover in O Level Chemistry syllabus. They are

  1.  Ionic bond
  2. Covalent bond
  3. Metallic bond

(1) Ionic Bond

After reading through the introduction on bonding, you should have a better understanding of why atoms will like to form bond(s). They tend to form bonds so that they can achieve stability by obtaining a noble gas configuration.

An ionic bond is formed when metal and non-metal atoms react together. The metal atom will lose electron(s) to form a positively charged ion. On the other hand, the non-metal will gain the electron from the metal atom, to form a negatively charged ion.

Recall: An ion is a positively or negatively charged species that have an unequal number of protons and electrons. Due to the unbalanced number of protons and electrons, the net charge of the species will not be zero. Hence, ions are either negatively or positively charged.

Due to the difference in charge (positive and negative), both metal and non-metal attract to one another. We describe the attraction as an ionic bond.

Note: ionic bond is not a physical bond that you can touch or see. It is an imaginary bond due to the attraction of the oppositely charged ions. Also, there is an attraction between the two ions because the opposite charge attracts.

The above explanation contain many loose terms, to help you understand the concepts further. But, you cannot explain this way in O Level Chemistry examinations as it is too lengthy and it does not explicitly contain the necessary keywords examiners wants.

Hence, follow the complete definition for ionic bond presented below:

Ionic bond is the electrostatic forces of attraction between the positively charged and negatively charged ions.

The electrostatic forces of attraction is not limited to only two atoms. The ionic bond is formed continuously and extensively. This ionic molecule is then called a giant ionic molecule.

Note: Strictly speaking, ionic bond is formed not entirely with metals and non-metals. A more accurate description of ionic bond is the attraction between positively and negatively charged ions. Ammonium ion, for example, is a non-metal but it can form an ionic bond with negatively charged chloride ion.
Although ammonium ion is a non-metal polyatomic ion, it can form an ionic bond.

The strength of the ionic bond is dependent on the charge of the respective ions. The higher the charge of the individual ions, the stronger the ionic bond. This is because of the stronger electrostatic forces of attraction between the oppositely charged ions.

For example, you would expect the ionic bond in magnesium oxide to be stronger the sodium chloride. This is because magnesium oxide is made up of magnesium and oxygen ion, where both of them are doubly charged. On the other hand, you would expect the ionic bond in sodium chloride to be weaker, since sodium and chlorine ion are both singly charged. Therefore, you would expect the ionic bond strength of magnesium oxide to be greater.

Different atoms lose a different number of electrons, based on what Group the atoms belongs to. If you are not sure about this concept, please click here before continuing.

To show which molecule has a stronger ionic bond, we can look at the melting and boiling point as a reference. If an ionic molecule has a higher melting and boiling point, it has a stronger ionic bond. This is because higher melting and boiling point means more heat energy is required to overcome the ionic bond. This shows that the ionic bond is stronger for molecules with higher melting and boiling point.

In O Level examination, you are required to draw the ionic compound using dot and cross diagram. Dot and cross illustrate the electrons of individual atoms.

An ionic compound is made up of two or more atoms, where both metal and non-metal atoms are present. In that case, we need to use different symbols to indicate the electrons that belong to different atoms in the same compound.

We can use dot or cross to indicate the electrons.

(2) Covalent Bond

We have learned that metal and non-metal atoms form an ionic bond. Formation of an ionic bond is possible as metal atom loses electron(s) to form positively charged ion and non-metal atom gain electron to form a negatively charged ion. An ionic bond when the oppositely charged ions form electrostatic forces of attraction.

Now, we will learn about covalent bond. Covalent bond is formed between non-metal atoms only. They form covalent bonds by sharing electrons to attain a noble gas configuration. This is different from ionic bond, as atoms loses and gain electrons to attain noble gas configuration.

Remember: Atoms want to attain full valence electron shell (noble gas configuration) as it is more stable.

Covalent bond is the strong force of attraction between the nuclei of atoms of non-metal and the shared pair of valence electron.

As mentioned, covalent bonding involves only non-metal atoms, where they share electrons to attain noble gas configuration. Note: atoms only involve valence electron when forming a bond, the inner electrons are not involved in bonding. This is true for all types of bonding: ionic, covalent and metallic bonding.

Let us study oxygen. An oxygen molecule (O2) is made up of two non-metal oxygen atom. Oxygen atom belongs to Group VI of the periodic table. Hence there will be six valence electrons. Currently, it does not have full valence electron. It requires eight electrons to attain a noble gas configuration, as it is more stable than having six valence electrons.

Hence, two oxygen atoms will come together and each atom share two of its valence electron. For every one electron shared, it gains one other electron from another atom.

By sharing two of its valence electron, it gains two more valence electron. As a result, the oxygen in the molecule has eight valence electron. They are now stable as they have attained a noble gas configuration.

Also, there is two covalent bond between each oxygen atom. This is because there is two pair of shared electrons.

One pair of shared electrons-Single bond
Two pairs of shared electrons-Double bond
Three pairs of shared electrons-Triple bond

Covalent molecule (a molecule that contains covalent bond) is the only type of molecule where you can describe the number of bonds.

(3) Metallic Bond

We have now discussed ionic and covalent bond. But, we have not consider the bond formed between pure metal atoms. The bonds formed between metal atoms are called metallic bonding.

This type of bond is formed when many metal atoms lose electron to form positively charged metal ions. The electrons initially from the metal

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