Sec 2 Chem: Ionic and Covalent Bonding

Ionic Bonding

All atoms have a neutral net charge due to same number of protons and electrons present in the atom itself. However, the atoms’ charges can change due to electron transfer. When an atom gains one electron, is becomes negatively charged. Conversely, when it loses an electron, it becomes positively charged. Losing/ gaining one electron causes the atom to have a charge of 1+ or 1-. What about losing or gaining 2 or more? This is self-explanatory :P

* A positively charged ion is called a cation. A negatively charged on is called an anion.

Ionic bonds are formed between charged atoms where one loses electron and another gains. Hence, ionic compounds have a neutral charge. The ions are held together due to the electrostatic forces between the opposite charged particles.

For example, Sodium, with electronic configuration (EC) 2.8.1, loses 1 electron to obtain a noble gas EC (Stable octet structure, remember?); Chlorine, 2.7, receives 1 electron and its EC becomes one of a noble gas. As both are positively and negatively charges (1+ and 1-), there charges offset each other and they combine to form the compound sodium chloride, NaCl.

Na à Na⁺

Cl à Cl^-

Na⁺ + Cl^- à NaCl

Most commonly, metals lose electrons while non-metals gain electrons (Remember the periodic table and electronegativity?) But what determines how many electrons are transferred? For metals, it is the Group number: Group I metals lose 1 electron to obtain EC of noble gas, Group II loses 2 electrons, Group III loses 3. On the other hand, non-metals, the electrons they gain ic commonly 8 minus their group number. For instance, Chlorine gains 8-7=1 electrons while oxygen gains 8-6=2 electrons. So, for an interaction between Grp II and VI elements:

Ca à Ca²⁺

O à O^2-

Ca²⁺ + O^2- à CaO

Notice that for both compounds discussed, the ratio of cations and anion is 1:1. This is because, as mentioned, ionic compounds have neutral overall charges: Hence 2+ offsets 2-, 3+ offsets 3- (But this rarely happens as much energy is needed to transfer 3 elcetrons)

So, what about ions with different charges? Let’s take Na⁺ and O^2-.

Well, if you had paid attention during maths lesson, you would have came across the term Lowest Common Multiple. Ionic bonding utilizes this concept too! To offset the 2- charge on oxygen, 2 Na⁺ ions are required, hence yielding the compound Na₂O.

2 Na⁺ (This means 2 sodium ions) + O^2- à Na₂O

Here’s another problem: Aluminum Oxide.

From the name, we know the compound is made of 2 ions: Aluminum and oxygen. Aluminum, being in group 3, has an ion with a net charge of 3+. Oxygen has a net charge of 2-.

To offset the charges, we use the common LCM method: 3+ x 2 offset 2- x 3

Hence, the formula for aluminum oxide is Al₂O₃.

#A list of common cations and anions:

Common Cations
Name of ion                                           Formula              Other names
Aluminium	Al3+
Calcium	Ca2+
Copper(II)	Cu2+	cupric
Hydrogen	H+
Iron(II)	Fe2+	ferrous
Iron(III)	Fe3+	ferric
Magnesium	Mg2+
Mercury(II)	Hg2+	mercuric
Potassium	K+	kalic
Silver	Ag+
Sodium	Na+	natric

Simple Anions
Chloride	Cl−
Fluoride	F−
Oxide	O2−

*Polyatomic ions: These ions are made of more than one atom covalently bonded to one another. Yes, these ions are covalent compounds, but the ionic compound they form with opposite charge ions is ionic, and there is an overall net charge on the entire covalent compound. Take Copper (II) Sulphate:

Copper has a charge of 2+, hence a Sulphate ion would have a charge of 2-. Sulphate has a formula of SO₄^2-. This means one Sulphur atom is bonded to 4 oxygen atoms, with an OVERALL charge of 2-. To simplify this, lets break down the sulphate ion into 2 parts: SO₃ and O.

SO₃ is sulphur trioxide, a covalent compound with a neutral net charge. When bonded to another extra O^2- ion, it becomes SO₄ with a charge of 2-. Hence the formula SO₄^2-.

Let’s take a look at positive ions: The ammonium ion. The ammonium ion has the formula NH₄^-. When we break it down, it becomes NH₃ (Ammonia gas) and H⁺ ion. When ammonia gas reacts with a proton (H⁺ ion), it yields the polyatomic ions ammonium. It can from here form ionic compound with other anions.

#A list of polyatomic ions:

Anions
Acetate (ethanoate)	CH3COO− or C2H3O2-
Benzoate	C6H5COO− or C7H5O2-
Bicarbonate (hydrogen carbonate)	HCO3-
Carbonate	CO32-
Cyanide	CN−
Hydroxide	OH−
Nitrate	NO3-
Phosphate	PO43-
Sulfate	SO42−

Cations
Ammonium	NH4+
Hydronium or Oxonium	H3O+
Mercury(I)	Hg22+
Tropylium	C7H7+

# The more you look at these and get exposed to these terms, the easier it gets to remember the charges! :D

Covalent Bonding

Now here’s covalent bonding: In covalent bonding, electrons are shared instead of donated and received. Covalent bonds can involve a single bond (One pair of electrons shared), double triple…… But what determines how many electrons are shared? Again, it’s the group number the element is in. As covalent bonds usually only occurs among non-metals, the number of bonds an atom forms would be 8- its group number. For example, oxygen can form 8-6=2 bonds while carbon can form 8-4=4 bonds. Just like ionic bonds, covalent bonds allow atoms to achieve a noble gas electron configuration.

For example, in chlorine gas, Cl₂, each atom need to gain 1 more electron (8-7=1). Hence, a double bond is formed between them:

What about more complex compounds: Methane (CH₄) is made up of 1 carbon and 4 hydrogen atoms. The carbon atom can form 4 covalent bonds while each hydrogen atom can only form one, hence the structure of methane is:

Oxygen, O₂, consist of 2 oxygen atoms which form 2 covalent bonds. However, sine there are only this 2 atoms, they form a double bond instead of single bond, so each atom gains 2 electrons to obtain a noble gas configuration:

Another way of representing compounds, other than the dot-and-cross diagram and chemical formula, is the structural formula. For example, Chlorine would be Cl-Cl, where the - denotes a single bond between each chlorine atom. Oxygen is O=O where = is denoted as a single bond.