Chemistry Form 2: PERIODIC TABLE

Periodic Table

There are over 100 elements discovered so far. Scientists have tried to group them together in a periodic table to better understand their properties and relationships.

A periodic table is a horizontal and vertical arrangement of elements according to their atomic numbers.

This table was successfully arranged in 1913 by the British scientist Henry Moseley, building on the previous work of the Russian scientist Dmitri Mendeleev.

The horizontal arrangement forms a period. Atoms in the same period have the same number of energy levels in their electronic structure, i.e.

The number of energy levels in the electronic configuration of an element determines the period to which the element belongs in the periodic table.

For example:

Which period of the periodic table are the following isotopes/elements/atoms?

1. ¹²₆C
   Electron structure 2:4 => 2 energy levels used, thus Period 2
2. ²³₁₁Na
   Electron structure 2:8:1 => 3 energy levels used, thus Period 3
3. ³⁹₁₉K
   Electron structure 2:8:8:1 => 4 energy levels used, thus Period 4
4. ¹₁H
   Electron structure 1: => 1 energy level used, thus Period 1

The vertical arrangement of elements forms a group. Atoms in the same group have the same number of outer energy level electrons as per their electronic structure, i.e.

The number of electrons in the outer energy level of an element determines the group to which the element belongs in the periodic table.

1. ¹²₆C
   Electron structure 2:4 => 4 electrons in outer energy level, thus Group IV
2. ²³₁₁Na
   Electron structure 2:8:1 => 1 electron in outer energy level, thus Group I
3. ³⁹₁₉K
   Electron structure 2:8:8:1 => 1 electron in outer energy level, thus Group I
4. ¹₁H
   Electron structure 1: => 1 electron in outer energy level, thus Group I

By convention:

• Periods are named using English numerals: 1, 2, 3, 4…
• Groups are named using Roman numerals: I, II, III, IV…

There are eighteen groups in a standard periodic table.

There are seven periods in a standard periodic table.

When an atom has the maximum number of electrons in its outer energy level, it is said to be stable.

When an atom does not have the maximum number of electrons in its outer energy level, it is said to be unstable.

All stable atoms are in group 8/18 of the periodic table. All other elements are unstable.

All unstable atoms or isotopes try to become stable through chemical reactions. A chemical reaction involves gaining or losing outer electrons (electron transfer). When electron transfer takes place, an ion is formed.

An ion is formed when an unstable atom gains or donates electrons in its outer energy level in order to become stable. Whether an atom gains or donates electrons depends on the relative energy required to donate or gain extra electrons, i.e.

Examples

1. ¹⁹₉F has electronic structure/configuration 2:7.

It can donate the seven outer electrons to have stable electronic structure/configuration 2:.

It can gain one extra electron to have stable electronic structure/configuration 2:8. Gaining requires less energy, and thus Fluorine reacts by gaining one extra electron.

1. ²³₁₃Al has electronic structure/configuration 2:8:3.

It can donate the three outer electrons to have stable electronic structure/configuration 2:8.

It can gain five extra electrons to have stable electronic structure/configuration 2:8:8. Donating requires less energy, and thus Aluminium reacts by donating its three outer electrons.

Elements with less than four electrons in the outer energy level donate or lose the outer electrons to become stable and form a positively charged ion called a cation.

A cation therefore has more protons (positive charge) than electrons (negative charge).

Generally, metals usually form cations.

Elements with more than four electrons in the outer energy level gain or acquire extra electrons in the outer energy level to become stable and form a negatively charged ion called an anion.

An anion therefore has fewer protons (positive charge) than electrons (negative charge).

Generally, non-metals usually form anions, except Hydrogen.

The charge carried by an ion is equal to the number of electrons gained or acquired, or donated or lost.

Examples of ion formation

1. ¹₁H

H → H⁺ + e^–

(atom) (monovalent cation) (electron donated/lost)

Electronic configuration 1: (No electrons remain)

2. ²⁷₁₃Al

Al → Al³⁺ + 3e^–

(atom) (trivalent cation) (3 electrons donated/lost)

Electron structure 2:8:3 → 2:8

(unstable) (stable)

3. ²³₁₁Na

Na → Na⁺ + e^–

(atom) (cation) (1 electron donated/lost)

Electron structure 2:8:1 → 2:8

(unstable) (stable)

4. ²⁴₁₂Mg

Mg → Mg²⁺ + 2e^–

(atom) (cation) (2 electrons donated/lost)

Electron structure 2:8:2 → 2:8

(unstable) (stable)

5. ¹⁶₈O

O + 2e^– → O^2-

(atom) (2 electrons gained/acquired) (anion)

Electron structure 2:6 → 2:8

(unstable) (stable)

6. ¹⁴₇N

N + 3e^– → N^3-

(atom) (3 electrons gained/acquired) (anion)

Electron structure 2:5 → 2:8

(unstable) (stable)

7. ³¹₁₅P

P + 3e^– → P^3-

(atom) (3 electrons gained/acquired) (anion)

Electron structure 2:5 → 2:8

(unstable) (stable)

8. ¹⁹₉F

F + e^– → F^–

(atom) (1 electron gained/acquired) (anion)

Electron structure 2:7 → 2:8

(unstable) (stable)

9. ³⁵₁₇Cl

Cl + e^– → Cl^–

(atom) (1 electron gained/acquired) (anion)

Electron structure 2:8:7 → 2:8:8

(unstable) (stable)

3. ³⁹₁₉K

K → K⁺ + e^–

(atom) (cation) (1 electron donated/lost)

Electron structure 2:8:8:1 → 2:8:8

(unstable) (stable)

When an element donates or loses its outer electrons, the process is called oxidation. When an element acquires or gains extra electrons in its outer energy level, the process is called reduction. The charge carried by an atom, cation, or anion is its oxidation state.

Table showing the oxidation states of some isotopes

Note:

Some elements can exist in more than one oxidation state. They are said to have variable oxidation states.

Roman capital numerals are used to indicate the oxidation state of an element with a variable oxidation state in a compound.

Examples:

1. Copper (I) means Cu⁺ as in Copper(I) oxide
2. Copper (II) means Cu²⁺ as in Copper(II) oxide
3. Iron (II) means Fe²⁺ as in Iron(II) sulphide

(iv) Iron (III) means Fe³⁺ as in Iron(III) chloride

1. Sulphur (VI) means S⁶⁺ as in Iron(III) sulphate (VI)
2. Sulphur (VI) means S⁶⁺ as in sulphur (VI) oxide
3. Sulphur (IV) means S⁴⁺ as in sulphur (IV) oxide
4. Sulphur (IV) means S⁴⁺ as in sodium sulphate (IV)

(ix) Carbon (IV) means C⁴⁺ as in carbon (IV) oxide

(x) Carbon (IV) means C⁴⁺ as in Lead (II) carbonate (IV)

(xi) Carbon (II) means C²⁺ as in carbon (II) oxide

(xii) Manganese (IV) means Mn⁴⁺ as in Manganese (IV) oxide

A compound is a combination of two or more elements in fixed proportions. The ratio of the atoms making a compound is called the chemical formula. Elements combine together to form a compound depending on their combining power.

The combining power of atoms in an element is called valency. Valency of an element is equal to the number of:

• Hydrogen atoms that an atom of element can combine with or displace.
• Electrons gained or acquired in outer energy level by non-metals to be stable or attain duplet/octet.
• Electrons donated or lost by outer energy level of metals to be stable or attain octet/duplet.
• Charges carried by ions, cations, or anions.

Groups of atoms that react as a unit during chemical reactions are called radicals. Elements with variable oxidation states also have more than one valency.

Table showing the valency of common radicals

Table showing the valency of some common metals and non-metals

From the valency of elements, the chemical formula of a compound can be derived using the following procedure:

1. Identify the elements and radicals making the compound.
2. Write the symbol or formula of the elements making the compound, starting with the metallic element.
3. Assign the valency of each element or radical as superscript.
4. Interchange or exchange the valencies of each element as subscript.
5. Divide by the smallest or lowest valency to derive the smallest whole number ratios.

Ignore a valency of 1.

This is the chemical formula.

Practice examples

Write the chemical formula of:

(a) Aluminium oxide

Chemical formula of Aluminium oxide is thus: Al₂O₃

This means: 2 atoms of Aluminium combine with 3 atoms of Oxygen.

(b) Sodium oxide

Chemical formula of Sodium oxide is thus: Na₂O

This means: 2 atoms of Sodium combine with 1 atom of Oxygen.

(c) Calcium oxide

Chemical formula of Calcium oxide is thus: CaO

This means: 1 atom of Calcium combines with 1 atom of Oxygen.

(d) Lead (IV) oxide

Chemical formula of Lead (IV) oxide is thus: PbO₂

This means: 1 atom of Lead combines with 2 atoms of Oxygen.

(e) Lead (II) oxide

Chemical formula of Lead (II) oxide is thus: PbO

This means: 1 atom of Lead combines with 1 atom of Oxygen.

(f) Iron (III) oxide

Chemical formula of Iron (III) oxide is thus: Fe₂O₃

This means: 2 atoms of Iron combine with 3 atoms of Oxygen.

(g) Iron (II) sulphate (VI)

Chemical formula of Iron (II) sulphate (VI) is thus: FeSO₄

This means: 1 atom of Iron combines with 1 sulphate (VI) radical.

(h) Copper (II) sulphate (VI)

Chemical formula of Cu(II) sulphate (VI) is thus: CuSO₄

This means: 1 atom of Copper combines with 1 sulphate (VI) radical.

(i) Aluminium sulphate (VI)

Chemical formula of Aluminium sulphate (VI) is thus: Al₂(SO₄)₃

This means: 2 atoms of Aluminium combine with 3 sulphate (VI) radicals.

(j) Aluminium nitrate (V)

Chemical formula of Aluminium nitrate (V) is thus: Al(NO₃)₃

This means: 1 atom of Aluminium combines with 3 nitrate (V) radicals.

(k) Potassium manganate (VII)

Chemical formula of Potassium manganate (VII) is thus: KMnO₄

This means: 1 atom of Potassium combines with 4 manganate (VII) radicals.

(l) Sodium dichromate (VI)

Chemical formula of Sodium dichromate (VI) is thus: Na₂Cr₂O₇

This means: 2 atoms of Sodium combine with 1 dichromate (VI) radical.

(m) Calcium hydrogen carbonate

Chemical formula of Calcium hydrogen carbonate is thus: Ca(HCO₃)₂

This means: 1 atom of Calcium combines with 2 hydrogen carbonate radicals.

(n) Magnesium hydrogen sulphate (VI)

Chemical formula of Magnesium hydrogen sulphate (VI) is thus: Mg(HSO₄)₂

This means: 1 atom of Magnesium combines with 2 hydrogen sulphate (VI) radicals.

Compounds are formed from chemical reactions. A chemical reaction occurs when atoms of the reactants break free to bond again and form products. A chemical reaction is a statement showing the movement of reactants to form products. The following procedure is used in writing chemical equations:

1. Write the word equation.
2. Write the correct chemical formula for each of the reactants and products.
3. Check if the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.
4. Multiply the chemical formula containing the unbalanced atoms with the lowest common multiple if the number of atoms on one side is not equal. This is called balancing.
5. Do not change the chemical formula of the products or reactants.
6. Assign in brackets the physical state or state symbols of the reactants and products after each chemical formula as:

• (s) for solids
• (l) for liquids
• (g) for gases
• (aq) for aqueous or dissolved in water to make a solution

Practice examples

Write a balanced chemical equation for the following:

1. Hydrogen gas is prepared from reacting Zinc granules with dilute hydrochloric acid.

Procedure:

1. Write the word equation

Zinc + Hydrochloric acid → Zinc chloride + hydrogen gas

2. Write the correct chemical formula for each of the reactants and products

Zn + HCl → ZnCl₂ + H₂

3. Check if the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.

Number of atoms of Zn on the reactant side is equal to product side.

One atom of H in HCl on the reactant side is not equal to two atoms in H₂ on product side.

One atom of Cl in HCl on the reactant side is not equal to two atoms in ZnCl₂ on product side.

4. Multiply the chemical formula containing the unbalanced atoms with the lowest common multiple if the number of atoms on one side is not equal.

Multiply HCl by “2” to get “2” Hydrogen and “2” Chlorine on product and reactant side.

Zn + 2 HCl → ZnCl₂ + H₂

5. Assign in brackets the physical state or state symbols.

Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g)

1. Oxygen gas is prepared from decomposition of Hydrogen peroxide solution to water.

Procedure:

1. Write the word equation

Hydrogen peroxide → Water + oxygen gas

2. Write the correct chemical formula for each of the reactants and products

H₂O₂ → H₂O + O₂

3. Check if the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.

Number of atoms of H on the reactant side is equal to product side.

Two atoms of O in H₂O₂ on the reactant side is not equal to three atoms (one in H₂O and two in O₂) on product side.

4. Multiply the chemical formula containing the unbalanced atoms with the lowest common multiple if the number of atoms on one side is not equal.

Multiply H₂O₂ by “2” to get “4” Hydrogen and “4” Oxygen on reactants.

Multiply H₂O by “2” to get “4” Hydrogen and “2” Oxygen on product side.

When the “2” Oxygen in O₂ and the “2” in H₂O are added on product side, they are equal to the “4” Oxygen on reactants side.

2 H₂O₂ → 2 H₂O + O₂

5. Assign in brackets the physical state or state symbols.

2 H₂O₂(aq) → 2 H₂O(l) + O₂(g)

1. Chlorine gas is prepared from Potassium manganate (VII) reacting with hydrochloric acid to form potassium chloride solution, manganese (II) chloride solution, water, and chlorine gas.

Procedure:

1. Write the word equation

Potassium manganate (VII) + Hydrochloric acid → Potassium chloride + manganese (II) chloride + chlorine + water

2. Write the correct chemical formula for each of the reactants and products

KMnO₄ + HCl → KCl + MnCl₂ + H₂O + Cl₂

3. Check if the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.

Number of atoms of K and Mn on the reactant side is equal to product side.

Two atoms of H in H₂O on the product side is not equal to one atom on reactant side.

Four atoms of O in KMnO₄ is not equal to one in H₂O.

One atom of Cl in HCl on reactant side is not equal to three (one in H₂O and two in Cl₂).

4. Multiply the chemical formula containing the unbalanced atoms with the lowest common multiple if the number of atoms on one side is not equal.

Multiply HCl by “16” to get “16” Hydrogen and “16” Chlorine on reactants.

Multiply KMnO₄ by “2” to get “2” Potassium and “2” manganese, “2 x 4 = 8” Oxygen on reactant side.

Balance the product side to get:

2 KMnO₄ + 16 HCl → 2 KCl + 2 MnCl₂ + 8 H₂O + 5 Cl₂

5. Assign in brackets the physical state or state symbols.

2 KMnO₄(s) + 16 HCl(aq) → 2 KCl(aq) + 2 MnCl₂(aq) + 8 H₂O(l) + 5 Cl₂(g)

(d) Carbon (IV) oxide gas is prepared from Calcium carbonate reacting with hydrochloric acid to form calcium chloride solution, water, and carbon (IV) oxide gas.

Procedure

1. Write the word equation

Calcium carbonate + Hydrochloric acid → Calcium chloride solution + water + carbon (IV) oxide

Calcium chloride solution + water + carbon (IV) oxide

2. Write the correct chemical formula for each of the reactants and products

CaCO₃ + HCl → CaCl₂ + H₂O + CO₂

3. Check if the number of atoms of each element on the reactant side is equal to the number of atoms of each element on the product side.

4. Multiply the chemical formula containing the unbalanced atoms with the lowest common multiple if the number of atoms on one side is not equal.

5. Assign in brackets the physical state or state symbols.

CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

(d) Sodium hydroxide solution neutralizes hydrochloric acid to form salt and water.

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

(e) Sodium reacts with water to form sodium hydroxide and hydrogen gas.

2 Na(s) + 2 H₂O(l) → 2 NaOH(aq) + H₂(g)

(f) Calcium reacts with water to form calcium hydroxide and hydrogen gas.

Ca(s) + 2 H₂O(l) → Ca(OH)₂(aq) + H₂(g)

(g) Copper (II) oxide solid reacts with dilute hydrochloric acid to form copper (II) chloride and water.

CuO(s) + 2 HCl(aq) → CuCl₂(aq) + H₂O(l)

(h) Hydrogen sulphide reacts with Oxygen to form sulphur (IV) oxide and water.

2 H₂S(g) + 3 O₂(g) → 2 SO₂(g) + 2 H₂O(l)

(i) Magnesium reacts with steam to form Magnesium Oxide and Hydrogen gas.

Mg(s) + 2 H₂O(g) → MgO(s) + H₂(g)

(j) Ethane (C₂H₆) gas burns in air to form Carbon (IV) oxide and water.

2 C₂H₆(g) + 7 O₂(g) → 4 CO₂(g) + 6 H₂O(l)

(k) Ethene (C₂H₄) gas burns in air to form Carbon (IV) oxide and water.

C₂H₄(g) + 3 O₂(g) → 2 CO₂(g) + 2 H₂O(l)

(l) Ethyne (C₂H₂) gas burns in air to form Carbon (IV) oxide and water.

2 C₂H₂(g) + 5 O₂(g) → 4 CO₂(g) + 2 H₂O(l)