Basic Atomic and Molecular Concepts

Atoms And Molecules (Basic Concepts)

The ancient Indian and Greek philosophers have always wondered about the unknown and unseen form of matter. Based on divisions and subdivisions, they theorised that the smallest particle of matter must be indivisible. Ancient Indian philosopher Maharishi Kanad postulated that if we go on dividing matter (padarth), we shall get smaller and smaller particles. Ultimately, a stage will come when we shall come across the smallest particles beyond which further division will not be possible. He named these particles 'parmanu'.

Another Indian philosopher, Pakudha Katyayama, elaborated this doctrine and said that these particles normally exist in a combined form, which gives us various forms of matter.

Around the same era, ancient Greek philosophers Democritus and Leucippus suggested that if we go on dividing matter, a stage would come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning ‘indivisible’). All these were philosophical considerations, not based on experiments.

The next significant developments in chemistry came in the late 18th century with the work of Antoine L. Lavoisier, who laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. In a closed system, the total mass of the reactants before a chemical reaction is equal to the total mass of the products after the reaction.

Law of Constant Proportions (or Law of Definite Proportions): In a chemical substance, the elements are always present in definite proportions by mass.

These laws paved the way for Dalton's atomic theory, which provided a basis for understanding the composition of matter.

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What Is An Atom?

Atoms are the basic building blocks of matter. According to Dalton's atomic theory (1808):

1. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (Modern science has shown atoms are divisible into sub-atomic particles like electrons, protons, and neutrons, but they remain the smallest particles that participate in chemical reactions).
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.

Atoms are very small, much smaller than anything we can imagine or compare with. Atomic radii are typically measured in nanometres (nm). 1 nm = $10^{-9}$ m.

For example, the radius of a Hydrogen atom is approximately $10^{-10}$ m.

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What Are The Modern Day Symbols Of Atoms Of Different Elements?

Early chemists used pictorial symbols for elements (e.g., Dalton's symbols). However, as more elements were discovered, this became impractical. A systematic way of representing elements was needed.

Berzelius suggested that symbols of elements be made from one or two letters of the name of the element.

Today, the symbols for elements are approved by the International Union of Pure and Applied Chemistry (IUPAC). These symbols are globally recognised.

Rules for writing chemical symbols:

1. The symbol is usually the first letter of the element's name in English. This letter is always written in uppercase. Example: Hydrogen (H), Oxygen (O), Carbon (C).
2. If there is more than one element starting with the same letter, a second letter (usually from the name) is added to the first letter. The second letter is always written in lowercase. Example: Helium (He), Neon (Ne), Cobalt (Co), Chlorine (Cl).
3. Sometimes, the symbols are taken from the Latin, German, or Greek names of elements. Example: Iron (Fe) from Ferrum, Sodium (Na) from Natrium, Potassium (K) from Kalium, Gold (Au) from Aurum, Silver (Ag) from Argentum.

Here are some common elements and their symbols:

Element Name	Symbol
Hydrogen	H
Helium	He
Carbon	C
Oxygen	O
Nitrogen	N
Sodium	Na
Potassium	K
Calcium	Ca
Magnesium	Mg
Aluminium	Al
Iron	Fe
Copper	Cu
Zinc	Zn
Silver	Ag
Gold	Au
Lead	Pb
Chlorine	Cl
Sulphur	S
Iodine	I
Mercury	Hg

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Atomic Mass

It is difficult to determine the mass of a single atom. So, the concept of relative atomic mass is used. Atomic mass is the mass of an atom compared to the mass of a standard atom.

Initially, Hydrogen was used as the standard, with its mass taken as 1 atomic mass unit (amu). Later, Oxygen (taken as 16 amu) was used as a standard. Since 1961, Carbon-12 isotope ($^{12}\text{C}$) has been accepted as the standard reference for measuring atomic masses. Carbon-12 is assigned an atomic mass of exactly 12 atomic mass units (u).

One atomic mass unit (u) is defined as exactly one-twelfth ($1/12$) the mass of one atom of Carbon-12.

$ 1 \text{ u} = \frac{1}{12} \times \text{Mass of one } ^{12}\text{C atom} $

The mass of an atom of any other element is expressed relative to this standard.

Relative atomic mass of an element is the average mass of the atoms of the element, compared to $1/12$th the mass of a Carbon-12 atom.

$ \text{Relative Atomic Mass} = \frac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{Mass of one } ^{12}\text{C atom}} $

The atomic mass of most elements is a fraction because of the existence of isotopes (atoms of the same element with different masses) and the concept of average atomic mass.

Examples of atomic masses (approximately):

• Hydrogen: 1.008 u (often rounded to 1 u)
• Carbon: 12.011 u (often rounded to 12 u)
• Oxygen: 15.999 u (often rounded to 16 u)
• Sodium: 22.990 u (often rounded to 23 u)
• Chlorine: 35.453 u (often rounded to 35.5 u)

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How Do Atoms Exist?

Atoms of most elements are very reactive and do not exist independently in nature. They combine with other atoms (of the same element or different elements) to form molecules and ions.

Atoms combine because they seek stability, usually by achieving a complete outer electron shell (like noble gases).

Only atoms of noble gases (like Helium, Neon, Argon, etc.) are chemically unreactive and exist as free, individual atoms.

Atoms form molecules and ions, which then aggregate to form the matter that we can see, feel, or touch.

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What Is A Molecule?

A molecule is the smallest particle of an element or a compound that is capable of independent existence under ordinary conditions. It shows all the properties of that substance.

Molecules are formed by the combination of two or more atoms that are held together by chemical bonds.

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Molecules Of Elements

The molecules of an element are constituted by the same type of atoms.

The number of atoms constituting a molecule is known as its atomicity.

• Monoatomic: Molecules containing only one atom. Examples: Noble gases like Helium (He), Neon (Ne), Argon (Ar), etc. Their atomicity is 1.
• Diatomic: Molecules containing two atoms. Examples: Hydrogen (H$_2$), Oxygen (O$_2$), Nitrogen (N$_2$), Chlorine (Cl$_2$). Their atomicity is 2.
• Triatomic: Molecules containing three atoms. Example: Ozone (O$_3$). Its atomicity is 3.
• Polyatomic: Molecules containing more than three atoms. Examples: Phosphorus (P$_4$, atomicity 4), Sulphur (S$_8$, atomicity 8).

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Molecules Of Compounds

Atoms of different elements join together in a definite proportion to form molecules of compounds.

The chemical formula of a compound represents the composition of a molecule of that compound, showing the symbols of the constituent elements and the number of atoms of each element.

Examples:

• Water (H$_2$O): A molecule of water contains 2 atoms of Hydrogen and 1 atom of Oxygen.
• Carbon dioxide (CO$_2$): A molecule of carbon dioxide contains 1 atom of Carbon and 2 atoms of Oxygen.
• Ammonia (NH$_3$): A molecule of ammonia contains 1 atom of Nitrogen and 3 atoms of Hydrogen.
• Sulphuric Acid (H$_2$SO$_4$): A molecule of sulphuric acid contains 2 atoms of Hydrogen, 1 atom of Sulphur, and 4 atoms of Oxygen.

The Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass units (u).

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What Is An Ion?

Atoms normally contain an equal number of protons (positively charged particles) and electrons (negatively charged particles), making them electrically neutral.

However, atoms or groups of atoms can gain or lose electrons to achieve a stable electron configuration (like noble gases). When an atom or a group of atoms loses or gains electrons, they acquire a net electrical charge and are called ions.

• Cation: A positively charged ion formed when an atom loses one or more electrons. Example: Sodium atom (Na) loses 1 electron to form a Sodium ion (Na$^+$). Magnesium atom (Mg) loses 2 electrons to form a Magnesium ion (Mg$^{2+}$).
• Anion: A negatively charged ion formed when an atom gains one or more electrons. Example: Chlorine atom (Cl) gains 1 electron to form a Chloride ion (Cl$^-$). Oxygen atom (O) gains 2 electrons to form an Oxide ion (O$^{2-}$).

Ions can also be groups of atoms carrying a net charge. These are called polyatomic ions. Example: Sulphate ion (SO$_4^{2-}$), Nitrate ion (NO$_3^-$), Ammonium ion (NH$_4^+$), Carbonate ion (CO$_3^{2-}$).

Compounds containing ions are called ionic compounds. They are formed by the electrostatic attraction between positively charged cations and negatively charged anions. Example: Sodium Chloride (NaCl) is formed by Na$^+$ and Cl$^-$ ions. Magnesium Chloride (MgCl$_2$) is formed by Mg$^{2+}$ and Cl$^-$ ions.

For ionic compounds, the term Formula unit mass is used instead of molecular mass, as they do not exist as discrete molecules but as a crystal lattice of ions. The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of an ionic compound.

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Writing Chemical Formulae

The chemical formula of a compound is a symbolic representation of its composition. It tells us the elements present in the compound and the number of atoms of each element.

The combining power (or capacity) of an element is known as its valency. Valency can be thought of as the number of bonds an atom of the element can form. Valency of an ion is equal to the charge on the ion (ignoring the sign). While writing chemical formulae, we need to know the valencies of the elements or the charges of the ions involved.

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Formulae Of Simple Compounds

The simplest compounds are binary compounds, which are made up of two different elements.

Steps for writing chemical formulae:

1. Write the symbols of the elements or ions involved.
2. Write the valency (or charge) of each element/ion below its symbol.
3. Criss-cross the valencies/charges. The valency of the first element/ion becomes the subscript for the second, and the valency of the second becomes the subscript for the first. (Do not write the sign of the charge).
4. If the valencies are the same, they cancel out (subscript is 1, which is usually not written).
5. If a polyatomic ion is involved, and you need more than one of them, put parentheses around the polyatomic ion before writing the subscript.

Let's look at some examples:

1. Formula of Hydrogen Chloride:

• Symbols: H, Cl
• Valencies: H has valency 1, Cl has valency 1.
• Criss-cross: H$_1$Cl$_1$
• Formula: HCl

$ \begin{array}{cc} \text{Element:} & \text{H} & \text{Cl} \\ \text{Valency:} & 1 & 1 \end{array} \implies \text{H}_1\text{Cl}_1 \implies \text{HCl} $

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2. Formula of Magnesium Chloride:

• Symbols: Mg, Cl
• Valencies: Mg has valency 2, Cl has valency 1. (Mg forms Mg$^{2+}$ and Cl forms Cl$^-$)
• Criss-cross: Mg$_1$Cl$_2$
• Formula: MgCl$_2$

$ \begin{array}{cc} \text{Element:} & \text{Mg} & \text{Cl} \\ \text{Valency:} & 2 & 1 \end{array} \implies \text{Mg}_1\text{Cl}_2 \implies \text{MgCl}_2 $

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3. Formula of Water:

• Symbols: H, O
• Valencies: H has valency 1, O has valency 2.
• Criss-cross: H$_2$O$_1$
• Formula: H$_2$O

$ \begin{array}{cc} \text{Element:} & \text{H} & \text{O} \\ \text{Valency:} & 1 & 2 \end{array} \implies \text{H}_2\text{O}_1 \implies \text{H}_2\text{O} $

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4. Formula of Carbon Tetrachloride:

• Symbols: C, Cl
• Valencies: C has valency 4, Cl has valency 1.
• Criss-cross: C$_1$Cl$_4$
• Formula: CCl$_4$

$ \begin{array}{cc} \text{Element:} & \text{C} & \text{Cl} \\ \text{Valency:} & 4 & 1 \end{array} \implies \text{C}_1\text{Cl}_4 \implies \text{CCl}_4 $

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5. Formula of Aluminium Oxide:

• Symbols: Al, O
• Valencies: Al has valency 3, O has valency 2. (Al forms Al$^{3+}$ and O forms O$^{2-}$)
• Criss-cross: Al$_2$O$_3$
• Formula: Al$_2$O$_3$

$ \begin{array}{cc} \text{Element:} & \text{Al} & \text{O} \\ \text{Valency:} & 3 & 2 \end{array} \implies \text{Al}_2\text{O}_3 $

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Writing Formulae for Compounds with Polyatomic Ions: When a compound contains a polyatomic ion, the formula is written by first identifying the ions and then applying the criss-cross method, using parentheses around the polyatomic ion if its quantity is more than one.

6. Formula of Sodium Nitrate:

• Ions: Na$^+$, NO$_3^-$
• Charges (Valencies): Na has charge +1 (valency 1), NO$_3$ has charge -1 (valency 1).
• Criss-cross: Na$_1$(NO$_3$)$_1$
• Formula: NaNO$_3$

$ \begin{array}{cc} \text{Ion:} & \text{Na}^+ & \text{NO}_3^- \\ \text{Charge:} & +1 & -1 \end{array} \implies \text{Na}_1(\text{NO}_3)_1 \implies \text{NaNO}_3 $

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7. Formula of Calcium Hydroxide:

• Ions: Ca$^{2+}$, OH$^-$
• Charges (Valencies): Ca has charge +2 (valency 2), OH has charge -1 (valency 1).
• Criss-cross: Ca$_1$(OH)$_2$
• Formula: Ca(OH)$_2$

$ \begin{array}{cc} \text{Ion:} & \text{Ca}^{2+} & \text{OH}^- \\ \text{Charge:} & +2 & -1 \end{array} \implies \text{Ca}_1(\text{OH})_2 \implies \text{Ca(OH)}_2 $

Here, parentheses are used around OH because we need two hydroxide units for every one calcium ion.

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8. Formula of Aluminium Sulphate:

• Ions: Al$^{3+}$, SO$_4^{2-}$
• Charges (Valencies): Al has charge +3 (valency 3), SO$_4$ has charge -2 (valency 2).
• Criss-cross: Al$_2$(SO$_4$)$_3$
• Formula: Al$_2$(SO$_4$)$_3$

$ \begin{array}{cc} \text{Ion:} & \text{Al}^{3+} & \text{SO}_4^{2-} \\ \text{Charge:} & +3 & -2 \end{array} \implies \text{Al}_2(\text{SO}_4)_3 $

Here, parentheses are used around SO$_4$ because we need three sulphate units for every two aluminium ions.

A table of common ions and their valencies can be helpful in writing chemical formulae:

Valency 1 (Charge +1)	Valency 2 (Charge +2)	Valency 3 (Charge +3)	Valency 1 (Charge -1)	Valency 2 (Charge -2)	Valency 3 (Charge -3)
Sodium (Na$^+$)	Magnesium (Mg$^{2+}$)	Aluminium (Al$^{3+}$)	Chloride (Cl$^-$)	Oxide (O$^{2-}$)	Nitride (N$^{3-}$)
Potassium (K$^+$)	Calcium (Ca$^{2+}$)	Iron(III) (Fe$^{3+}$)	Bromide (Br$^-$)	Sulphide (S$^{2-}$)	Phosphate (PO$_4^{3-}$)
Silver (Ag$^+$)	Zinc (Zn$^{2+}$)	Chromium(III) (Cr$^{3+}$)	Iodide (I$^-$)	Sulphate (SO$_4^{2-}$)
Copper(I) (Cu$^+$)	Iron(II) (Fe$^{2+}$)		Hydrogen Carbonate (HCO$_3^-$)	Sulphite (SO$_3^{2-}$)
Ammonium (NH$_4^+$)	Copper(II) (Cu$^{2+}$)		Hydroxide (OH$^-$)	Carbonate (CO$_3^{2-}$)
	Lead(II) (Pb$^{2+}$)		Nitrate (NO$_3^-$)	Chromate (CrO$_4^{2-}$)
	Barium (Ba$^{2+}$)		Nitrite (NO$_2^-$)	Dichromate (Cr$_2$O$_7^{2-}$)

*Note: Some elements like Iron (Fe) and Copper (Cu) show variable valencies.

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