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Atoms and

Molecules

Law of Chemical Combination

Dalton’s atomic theory

Modern day symbols of Elements

Atomic Mass

Molecule

Chemical Formulae

Molecular Mass

Molar Concept

Topics to be Covered

Atoms are building blocks of all matter. According to modern atomic theory, an atom is the smallest particle of an element which takes part in chemical reaction. Atoms are very small and which can’t be seen even through very powerful microscope. ATOMS

Law of Conservation of Mass Law of Constant Proportions Law of Chemical Combination

Lavoisier and Joseph L. Proust.

Solution Y in flask, Solution X in ignition tube. Weigh flask with both solutions. Tilt flask to mix X and Y. Re-weigh the flask after mixing. Check for any change in mass. Mass remains constant (Law of Conservation of Mass). Activity 3. Set X: Copper sulphate (or barium chloride, or lead nitrate) Set Y: Sodium carbonate (or sodium sulphate, or sodium chloride)

Q. Give an example of this law of conservation of mass when it

applies to physical change.

Q. In a reaction 4.6 g of barium chloride reacted with 3.4 g of

sodium sulphate. The products obtained were 2.8 g of sodium

chloride and 5.2 g of barium sulphate.

The reaction takes place as follows:

Barium chloride + Sodium sulphate → Sodium chloride + Barium

sulphate."

Show that the above observation is in agreement with the law of

conservation of mass.

The elements in a pure chemical compound are always present in the same proportions by mass, regardless of how the compound is created. → It was given by Joseph Proust. Example: (i) 18 gm of H₂O = 2 gm of hydrogen + 16 gm of oxygen ⇒ mass of hydrogen : mass of oxygen = 2:16 = 1: (ii) 36 gm of H₂O = 4 gm of hydrogen + 32 gm of oxygen ⇒ mass of hydrogen : mass of oxygen = 4:32 = 1: (iii) In water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively. Law of Constant Proportions

Q. Calculate the percentage of elements in 1.5 g of calcium

carbonate if Ca = 40%, C = 12%, O = 48%.

If the law of constant proportion is true, what weight of these

elements will be present in another sample?

(Atomic masses: Ca = 40 u, C = 12 u, O = 16 u)

Dalton’s Atomic Theory According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. John Dalton

All matter is made of very tiny particles called

atoms.

Atoms are indivisible particles, which cannot be

created or destroyed in a chemical reaction.

Postulates of Dalton’s Atomic Theory:

No Isobars: Dalton said atoms of different elements have different masses, but isobars have the same mass number. No Whole-Number Ratios Always: Complex compounds like sugar (C12H22O11)) do not always follow simple whole-number ratios. No Allotropes Defined: Allotropes like graphite and diamond have different properties that Dalton's theory can't explain.

Dalton: First scientist to use symbols for elements. Berzelius: Suggested using one or two letters from the element's name for its symbol. Element Naming: Initially, elements were named after their discovery locations (e.g., Copper from Cyprus). Modern Day Symbols of Elements IUPAC: Now responsible for approving element names, symbols, and units. Symbols typically use one or two letters from the element's English name (e.g., H for Hydrogen, Al for Aluminium). Special Cases: Some symbols are derived from Latin, German, or Greek names (e.g., Fe for Ferrum, Na for Natrium, K for Kalium).

Trick to Rememeber First 20 Elements of Periodic Table

1. Hydrogen (H)
2. Helium (He)
3. Lithium (Li)
4. Beryllium (Be)
5. Boron (B)
6. Carbon (C)
7. Nitrogen (N)
8. Oxygen (O)
9. Fluorine (F)
10. Neon (Ne)
    1. Sodium (Na)
11. Magnesium (Mg)
12. Aluminum (Al)
13. Silicon (Si)
14. Phosphorus (P)
15. Sulfur (S)
16. Chlorine (Cl)
17. Argon (Ar)
18. Potassium (K)
19. Calcium (Ca)

Dalton’s Atomic Theory: Introduced the concept of atomic mass,

explaining the law of constant proportions.

Atomic Mass: Mass of an atom of an element.

IUPAC (1961): Adopted the term " atomic mass unit (u) " to express atomic

and molecular masses.

1 atomic mass unit (u) = 1/12 of the mass of a carbon-12 atom.

Example: Hydrogen atom has a mass of 1 u or 1.673 × 10⁻²⁴ grams.

Atomic Mass