CHEMICAL COMBINATION Periodic table of the First 20 elements

FIRST TERM E-LEARNING NOTES

SUBJECT: CHEMISTRY

CLASS: SS1

WEEK 8

TOPIC: CHEMICAL COMBINATION

CONTENTS:

(a) Periodic table (First 20 elements)

(b) Types of chemical bonds

(c) IUPAC system of naming inorganic compounds.

PERIOD 1: PERIODIC TABLE (FIRST 20 ELEMENTS)

Chemical combination: The atoms of noble gases e.g. Helium He, Neon Ne are very stable; because their outermost shells are completely filled with electrons. The tendency of the other elements is to attain this stable structure possessed by the noble gases. This is achieved during chemical combination. Thus, chemical bonding is the coming together of atoms of the same or different elements, in order to form a stable structure.

The Periodic Table

Periodic table is the arrangement of chemical elements in order of their atomic numbers. Atomic number, Z, of an element is the number of protons in one atom of that element while, Mass number, A, of an element is the sum of the protons and neutrons in it.

Many scientists have attempted to classify chemical elements based on their properties. They include Newland, Lothar Meyer, Dobereiner and Mendeleev. The modern periodic table is based on Mendeleev’s original idea in 1869. The basic assumption behind the modern periodic table known as Periodic Law which states that “the properties of the elements are a periodic function of their atomic number”

The modern periodic Table

The modern periodic table is divided into vertical columns of elements called Groups and horizontal rows of elements called Periods. There are seven (7) periods and eight (8) groups. The first twenty (20) elements are:

1. Hydrogen 11. Sodium
2. Helium 12. Magnesium
3. Lithium 13. Aluminum
4. Beryllium 14. Silicon
5. Boron 15. Phosphorus
6. Carbon 16. Sulphur
7. Nitrogen 17. Chlorine
8. Oxygen 18. Argon
9. Fluorine 19. Potassium
10. Neon 20. Calcium

Periodic table of elements

EVALUATION

1. Define a periodic table
2. List the first twenty (20) elements
3. State the periodic law
4. How many groups and periods are there in the periodic table?
5. In the periodic table, elements are arranged according to their _______

PERIODS 2 and 3: TYPES OF CHEMICAL BONDS

The attractive force between atoms when they combine chemically is called a chemical bond. There are two main types of chemical bonds namely (i) Strong bonds (ii) Weak bonds.

(i) Strong bonds are: (a). Electrovalent (or Ionic) (b).Covalent (c).Co-ordinate (or Dative) (d). Metallic

(a) Electrovalent (Ionic) bond is defined as the electrostatic force of attraction between oppositely charged ions. It involves the transfer of electrons from one atom, donor atom, (usually metallic) to another atom, acceptor atom, (usually non-metallic). The electrons involved reside in the outermost shells of the atoms and are called Valence electron(s).

IONIC COMPOUNDS are crystal lattices consisting of aggregates of oppositely charged ions. Examples of such compounds are Sodium chlorides, calcium oxides, ammonium chlorides etc. The formation of sodium chloride is illustrated below:

Na = 2, 8, 1 Cl =2, 8, 7

Na⁺= 2, 8 Cl^– = 2, 8, 8

Na⁺ + Cl^– → NaCl

Characteristics of Electrovalent bonds

i. They have high melting and boiling points

ii. They are generally soluble in water[mediator_tech]

iii .They are good conductors of electricity when molten or in solution

iv. They do not conduct electricity when solid

v. The energy needed to separate them is relatively high.

(b) Covalent bonds: This is defined as the bonds formed when two atoms donate equal numbers of electrons and share the donated electrons to attain stable octet structure. In covalent bonding, electrons are shared between atoms of the same or different elements such that each atom contributes the shared electrons so as to attain stable noble gas configuration. During the process, discrete or separate molecules are formed with covalent bonds between the atoms.

If electrons are shared between similar atoms, the donated electrons are equally shared which give rise to a non-polar covalent bond e.g. F₂, H₂, O₂, Cl₂ but if the shared pair of electrons involves two different elements with difference in electronegativity, the electrons are then not equally shared, this results in the formation of polar covalent bond, e.g. the formation of HCl, H₂O, HF..

Characteristics of Covalent bonds

i. Low melting and boiling points.

ii. Energy required for separation is low

iii. Do not conduct electricity in the solid or molten state, or in solution

iv. They generally have a strong, easily noticeable smell

v. They are not easily soluble in water, but are usually soluble in organic solvents

(c) Co-ordinate covalent or Dative bond involves sharing of electrons as in the normal covalent bonding, but the shared pair is donated by only one of the participating atoms. For instance, Ammonia and water molecules possess lone pairs and so readily enter into coordinate covalent bonding.

(d) Metallic bond: The electrostatic force of attraction between the positive nuclei and the sea of mobile electrons is called metallic bond. Metallic bonding, therefore, is the process whereby the positively charged nuclei of metal atoms are simultaneously attracted to the sea (or cloud) of mobile electrons. Metallic bond increase with increase in valence electrons of the metal. For example, in period 3, metallic bond increases from sodium to aluminium.

(ii) Weak bonds: These are intermolecular forces of attraction that hold atoms and covalent molecules together in gases, liquids and solids. The most common ones are: (a). Van der Waals forces (b). Hydrogen bond (c) Dipole-Dipole (Dipolar)

(e) Van der Waals forces: They were first described by J.D. van der Waals, and are known as van der Waals forces. They are weak short-ranged attractive forces formed between covalent molecules. They are the only attractive forces between the atoms of the noble gases and non-polar covalent molecules, and are responsible for the low melting and boiling point of covalent compounds. Due to increase in van der Waals forces, there is gradation in the physical properties of the Halogens: Fluorine and chlorine are gases; bromine is a liquid; and iodine is a solid.

Van der Waal force becomes stronger as the relative atomic mass increases among the non-metals.

(f) Hydrogen bond: This is an intermolecular force which arises when hydrogen is covalently linked to highly electronegative elements like nitrogen, oxygen and fluorine.

The presence of hydrogen bonds between H₂O molecules is responsible for water being a liquid at room temperature and with a high boiling point; if not, it would have been a gas, like hydrogen H₂S. HF is a liquid at room temperature, while HCl is a gas.

EVALUATION

• What is a chemical bond?
• List three (3) general properties of electrovalent compounds.
• Define covalent bond and state its characteristics
• State two (2) differences between covalent and electrovalent compounds.
• Explain the term Metallic bonding

PERIOD 4: IUPAC SYSTEM OF NAMING INORGANIC COMPOUNDS

Chemical compounds are named according to the International Union of Pure and Applied Chemistry (IUPAC) system. In order to understand the basic principles behind the IUPAC system, a good knowledge of the concept of Oxidation Number (ON) is very essential.

Concept of oxidation number

An oxidation number (ON) is a positive or negative number assigned to an atom according to a set of rules. It is sometimes called Oxidation State.

Naming of inorganic compounds

1. Binary compounds. Binary compounds contain two elements only. The metal is named first, followed by the name of the second element ending with –ide. If the metal is one that has variable valencies, the valency exhibited will be written in Roman numeral examples are given below:

Name of compounds

2. Radicals. In naming radicals, the last element is mentioned first with its number of atoms given as mono (1), di (2), tri (3), tetra (4), penta (5), etc. The other element’s name ends with –ate.

EVALUATION

• Define oxidation number and, determine the ON of sulphur in SO₃^2-
• Give the IUPAC name of the following: (i) Al(NO₃)₃ (ii) MnO₂ (III) CuSO₄ .5H₂O
• What is the correct IUPAC name for NO^2-?

GENERAL EVALUATION

OBJECTIVE TEST:

1. Which of these are found in the nucleus of an atom?

A. electrons and protons B. electrons and neutrons C. protons and neutrons D. photons and electrons E. photons and neutrons.

1. The type of bond between two atoms of an element with atomic number 7 is? A. ionic B. covalent C. hydrogen bond D. metallic bond E. coordinate covalent bond.
2. The ON of phosphorus, P in PH₃is? A. +2 B. -3 C. -1 D. +3
3. Give the IUPAC name of the compound NO₂. A. nitrogen dioxide B. nitrogen monoxide C. nitrogen (II) oxide D. nitrogen (iv) oxide[mediator_tech]
4. Atomic number, Z, is the number of __________ in one atom an element. A. protons B. neurons C. electrons D. atoms

ESSAY QUESTIONS

1. By means of a diagram, show the arrangement of electrons in one atom of Sodium.
2. The electronic configurations for the metal calcium, the non-metals silicon and chlorine can be represented as:

Ca : 2, 8, 8, ; Si : 2, 8, 4 ; and Cl : 2, 8, 7. (a) Explain, in terms of electrons, the formation of calcium chloride and silicon chloride. (b) Give two (2) differences in physical properties you would expect between calcium chloride and silicon chloride.

1. Compare the characteristics of ionic with those covalent compounds.

Use this fig. to answer questions 4 and 5.

I II III IV V VI VII VIII

(a) Which of the following pairs of letters denotes elements containing the same number of electrons in their outermost shell?

(b) What letter presents an element that participates in covalent rather than ionic bonding?

WEEKEND ASSIGNMENT:

New School Chemistry for Senior Secondary Schools by Osei Yaw Ababio;

PRE–READING ASSIGNMENT:

Read about States of matter, its kinetic theory and application.