Atoms and Molecules are the building blocks of matter in the universe. So what is the difference between atoms and molecules? While atoms are the basic unit of matter, molecules are just a group of atoms bonded together. Let us learn more about atoms, molecules, their behaviour and properties.
| Table of Content |
Keyterms: Protons, Electrons, Neutrons, nucleons, nucleus, Microscope, Atomic mass, Atomic number, Mass number
Read More: Conservation Laws
Atoms: Definition
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Atoms are the smallest unit of an element. They exhibit properties relative to the element. It is so minute in size that it cannot even be seen under a microscope. The atoms are further subdivided into protons, electrons and neutrons. Some of the important terminologies are given below.
- Protons: Protons are positively charged particles. Protons along with neutrons form the nucleus and together they are called the nucleons.
- Electrons: Electrons are negatively charged particles that revolve around the nucleus. The electrons orbit around the nucleus in a series of levels called energy levels. The mass of the electrons is 1/1836 the mass of protons, thus making it negligible.
- Neutrons: Neutrons are neutral particles present inside the nucleus of the atom. The number of protons in an atom of an element remains constant whereas the number of neutrons can change.
- Atomic number: Atomic number is the total number of protons present in an atom. It is represented by the letter Z.
- Mass number: Mass number is the total number of protons and neutrons present in the nucleus of the atom. It is represented by the letter A.
- Atomic Mass: It is the total mass of the atom in an element. It is roughly equivalent to the mass of the total protons and neutrons in the nucleus as the mass of electrons is negligible. It is expressed in Atomic Mass Unit (amu).
Dalton’s Atomic Theory
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According to Dalton’s atomic theory,
- All matter is made of tiny particles called atoms.
- Atoms are indivisible.
- Atoms can neither be created nor destroyed.
- Atoms of any given element are identical in mass and properties
- Atoms of the same or different elements combine to form molecules and compounds
- Atoms of different elements are different in their mass and chemical properties.
It was later proved that atoms can be further divided into subatomic particles and atoms of the same elements can sometimes differ. These were considered as the major drawbacks of Dalton’s atomic theory.
Video Lecture on Dalton’s Atomic Theory
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| Important Topic Related Links | ||
|---|---|---|
| Laws of Chemical Combination | Displacement Reactions | Mass and Momentum |
Properties Of An Atom
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Atoms are extremely small and their sizes are measured in nanometers ( 1m = 109 nm).
To put it in perspective, one teaspoon of sugar contains as many atoms as there are grains of sand in all the beaches of the world combined. Example: The radius of a hydrogen atom is 10-10m
Atoms have a nucleus around which the electrons orbit. These subatomic particles are held together by nuclear bonds. Atoms are the smallest particle that takes part in a chemical reaction. Atoms can either be stable or unstable depending upon the number of electrons in their outermost energy level.
To bond with other atoms, these outermost electrons are either gained or lost. When an atom gains or loses an electron, it acquires a charge and is called an ion. If an atom loses an electron, it acquires a positive charge and is called a cation. Similarly, if an atom gains an electron to fill up the energy level, it acquires a negative charge and is called an anion.
Molecules: Definition
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Molecules are a group of atoms of the same element or different elements bonded together. Molecules are the smallest unit of a compound. Unlike atoms, they exist in a free state and are stable. The atoms in a molecule are bonded together by covalent chemical bonds.
The molecules might not exhibit the same chemical properties as their constituting atoms. For example, Sodium (Na) is a highly reactive metal and Chlorine (Cl) is a poisonous gas. But when these are combined, they form Sodium Chloride (NaCl) or table salt which is neither a metal nor poisonous.
Read More: Conservation of Energy Formula
Key Differences Between Atoms and Molecules
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| Atoms | Molecules |
|---|---|
| Atoms are the basic unit of matter. | Molecules are the basic unit of a compound. |
| Atoms are made up of Protons, Neutrons and Electrons. | Molecules are made of atoms of the same or different elements. |
| The components of the atom are held together by means of nuclear bonds. | The atoms of a molecule are held together by means of covalent bonds. |
| Atoms exhibit the same properties of its constituting element. | Molecules do not exhibit the same properties as its constituting atoms. |
| Atoms may or may not be stable depending upon the number of electrons in its outermost or valence shell. | Molecules are formed to get stable and they exist in a free state. |
| Except for noble gases, all other atoms of elements possess certain levels of reactivity. | Molecules are less reactive. |
| Eg: H, O, Cl, Na | Eg: H2, NaCl, O2 |
Bonding In Atoms And Molecules
Atomic Bonding: The Coulombic forces exist between the nuclei and the electrons. These are the forces of attraction and repulsion that make up the nuclear bond in the atom. The energy of attraction or repulsion between atoms is called bond energy. The higher the bond energy, the harder it becomes to separate the atoms. This is why it is harder to melt solids than to boil liquids.
Molecular Bonding: The atoms in a molecule are bonded together by an interatomic linkage that arises out of sharing of electrons. This is called a Covalent bond. A single line between molecules indicates that there is a covalent bond between two atoms. Double lines represent double bonding between the molecules (i.e atoms involving two electron pairs) and triple lines represent triple bonding. Covalent bonding is also directional, meaning that the atoms only bond in specific directions giving the molecules definite shapes.
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Things To Remember
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- Atoms are the basic unit of all matter.
- Molecules are the smallest unit of a compound and are made of atoms of the same or different elements.
- Atoms can be subdivided into protons, electrons and neutrons.
- The atom is held together by nuclear bonding and molecules by covalent bonding.
- Depending on the number of electrons in the outermost shell of an atom, it may or may not be stable.
- Molecules are formed when two or more atoms share their electrons and are thus stable.
- Molecules being the smallest particle of a compound, exhibit the chemical properties of the compound and not its substituting atoms.
Sample Questions
Ques: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass? (2 marks)
Ans: The postulate of Dalton’s atomic theory that is the result of the law of conservation of mass is—the relative number and kinds of atoms are constant in a given compound. Atoms cannot be created nor destroyed in a chemical reaction.
Ques: Define Atomic Mass Unit. (2 marks)
Ans: One atomic mass unit is equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.
Ques: Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, NH2, CH3OH. (2 marks)
Ans: The molecular masses are calculated as follows:
H2 → 1 x 2 = 2 amu
O2 → 16 x 2 = 32 amu
Cl2 → 35.5 x 2 = 71 amu
CO2 → 1 x 12 + 2 x 16 = 12+32 = 44 amu
CH4 → 1 x 12 + 4 x 1 = 12 + 4 = 16 amu
C2H6→ 2 x 12 + 6 x1 = 24 + 6 = 30 amu
NH2 → 1 x14 + 3 x 1 = 14 + 3 = 17 amu
CH3OH → 12 + 3 x 1 + 16 + 1 = 32 amu
Ques: Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u. (2 marks)
Ans: The formula unit mass of
(i) ZnO = 65 u + 16 u = 81 amu
(ii) Na2O = (23 u x 2) + 16 u = 46 u + 16 u = 62 amu
(iii) K2C03 = (39 u x 2) + 12 u + 16 u x 3 = 78 u + 12 u + 48 u = 138 amu
Ques: What are polyatomic ions? Give examples. (2 marks)
Ans: The ions which contain more than one atom (same kind or may be of a different kind) and behave as a single unit are called polyatomic ions e.g., OH–, SO42-, CO32-.
Ques: Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate. (2 marks)
Ans: (a) Quick lime (chemical name: Calcium oxide)
Elements → Calcium and oxygen
(b) Hydrogen bromide
Elements → Hydrogen and bromine
(c) Baking powder (chemical name: Sodium hydrogen carbonate)
Elements → Sodium, hydrogen, carbon and oxygen
(d) Potassium sulphate
Elements → Potassium, sulphur and oxygen
Ques: Distinguish between molecular mass and molar mass. (2 marks)
Ans: The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.
Ques. What is meant by the term chemical formula? (2 marks)
Ans: The chemical formula of a compound is a symbolic representation of its composition and the actual number of atoms in one molecule of a pure substance. It may be an atom or a compound. Eg: ZnO, NaCl, NaOH
Ques. Give two drawbacks of Dalton’s atomic theory. (2 marks)
Ans: (i) According to modern theory, the atom is not the ultimate indivisible particle of matter. Today, we know that atoms are divisible, i.e., they are themselves made up of particles (protons, electrons, neutrons, etc.).
(ii) In the case of isotopes of an element, the assumption that the atoms of the same element have the same mass does not hold good.
Ques. How would you differentiate between a molecule of element and a molecule of a compound? Write one example of each type. (2 marks)
Ans: The molecule of an element is made up of only one kind of atom. Examples: O2, H2, Cl2
The molecule of a compound is made up of two or more different kinds of atoms in a fixed ratio. Examples: Na2O, K2CO3, NH3
Ques. Define an Isotope. (2 marks)
Ans: Isotopes are atoms of the same element that has the same atomic number but different mass number due to the difference in the number of neutrons present in the nuclei. Examples of radioactive isotopes include carbon-14, tritium (hydrogen-3), chlorine-36, uranium-235, and uranium-238
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