Chemistry Atoms and Molecules Overview

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Questions and Answers

What is the unseen form of matter that ancient Indian and Greek philosophers pondered over?

The unseen form of matter that ancient Indian and Greek philosophers pondered over is the smallest particle of matter, known as an atom.

Is there a change in mass when a chemical reaction takes place?

False (B)

What is the name given to the smallest particle of matter that cannot be divided further, as proposed by Democritus?

The smallest particle of matter that cannot be divided further, as proposed by Democritus, is called an atom.

Who established the foundation of chemical sciences by discovering two important laws of chemical combination?

<p>Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination: the Law of Conservation of Mass and the Law of Constant Proportions.</p> Signup and view all the answers

What is the statement of the Law of Conservation of Mass?

<p>The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.</p> Signup and view all the answers

Explain the Law of Constant Proportions.

<p>The Law of Constant Proportions states that a chemical compound always contains the same elements in the same proportion by mass, irrespective of its origin or method of preparation.</p> Signup and view all the answers

Who proposed the atomic theory that explained the Law of Conservation of Mass and the Law of Constant Proportions?

<p>John Dalton proposed the atomic theory that explained the Law of Conservation of Mass and the Law of Constant Proportions.</p> Signup and view all the answers

According to Dalton's Atomic Theory, which of the following statements are true? (Select all that apply)

<p>Atoms combine in the ratio of small whole numbers to form compounds. (A), All matter is made up of very tiny particles called atoms. (C), Atoms of different elements have different masses and chemical properties. (F), Atoms of a given element are identical in mass and chemical properties. (G)</p> Signup and view all the answers

What is an atom? What is its relative size?

<p>An atom is the smallest particle of an element that can exist independently and retains all its chemical properties. Atoms are extremely small, even smaller than the thickness of a sheet of paper. Their size is typically measured in nanometers.</p> Signup and view all the answers

What are modern day symbols for atoms of different elements? Explain using an example.

<p>Modern day symbols for atoms of different elements are usually derived from the first one or two letters of the element's name in English. The first letter is capitalized and the second letter is lowercase. For example, the symbol for hydrogen is 'H', for oxygen is 'O', and for carbon is 'C'.</p> Signup and view all the answers

What is the difference between atomic mass and molecular mass?

<p>Atomic mass refers to the mass of a single atom of an element, while molecular mass refers to the sum of the atomic masses of all the atoms present in a molecule. The units for both are atomic mass units (u).</p> Signup and view all the answers

How do atoms exist? Explain using some examples.

<p>Atoms of most elements tend to combine to form molecules or ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel, and touch. For example, a hydrogen molecule (H2) consists of two hydrogen atoms bonded together, while water (H2O) is formed by the combination of two hydrogen atoms and one oxygen atom. Other examples include sodium chloride (NaCl) or calcium carbonate (CaCO3), all of which consist of ions in a larger arrangement.</p> Signup and view all the answers

Define a molecule. Give examples of molecules of elements and compounds.

<p>A molecule is a group of two or more atoms bonded together, forming the smallest particle of an element or compound that can exist independently and retains all its properties. Examples of elements that exist as molecules include oxygen (O2) and nitrogen (N2). Examples of compounds that exist as molecules include water (H2O) and carbon dioxide (CO2).</p> Signup and view all the answers

What is an ion? Explain using an example.

<p>An ion is a charged particle, which can be negatively charged (anion) or positively charged (cation). Sodium chloride (NaCl) is a classic example of a compound composed of ions. Sodium (Na) forms a positive ion (Na+) by losing one electron, while chlorine (Cl) forms a negative ion (Cl-) by gaining one electron.</p> Signup and view all the answers

Define valency. Explain how valency is used to write the chemical formula of a compound. How do we indicate the valency of an element in a chemical formula?

<p>Valency is the combining capacity of an element, represented by a number, that indicates how many atoms of other elements it can combine with. To write the chemical formula, we use a criss-cross method. We write the symbols of the combining elements and their valencies, then cross-multiply the valencies, placing them as subscripts for each element. For example, in sodium chloride (NaCl), sodium has a valency of +1 and chlorine has a valency of -1, resulting in the formula NaCl.</p> Signup and view all the answers

What is meant by 'formula unit' of a compound?

<p>The formula unit of a compound refers to the smallest representative unit of an ionic compound. It indicates the ratio of the ions present in the compound.</p> Signup and view all the answers

What is the difference between molecular mass and formula unit mass?

<p>Molecular mass is used for covalent compounds, where atoms are bonded together in fixed ratios, while formula unit mass is used for ionic compounds, where the smallest unit is a ratio of ions.</p> Signup and view all the answers

Define a mole. How many entities constitute a mole?

<p>A mole is a unit of amount, defined as the amount of substance that contains the same number of elementary entities (atoms, molecules, ions, etc.) as there are atoms in exactly 12 g of carbon-12. This number is known as Avogadro's number, which is 6.022 x 10^23 entities per mole.</p> Signup and view all the answers

What's the difference between molar mass and atomic mass?

<p>Atomic mass refers to the mass of a single atom of an element, while molar mass refers to the mass of one mole of that element. Atomic mass is expressed in atomic mass units (u), while molar mass is expressed in grams per mole (g/mol).</p> Signup and view all the answers

Given the atomic masses of hydrogen (1 u) and oxygen (16 u), calculate the molecular mass of water (H2O).

<p>The molecular mass of water (H2O) is 18u. This is calculated by summing the atomic masses of two hydrogen atoms (2 x 1 u = 2 u) and one oxygen atom (16 u), which gives us 18 u.</p> Signup and view all the answers

Explain how to calculate the number of moles of a substance given its mass and molar mass.

<p>The number of moles (n) of a substance can be calculated by dividing the given mass (m) of the substance by its molar mass (M). The formula is: n = m / M.</p> Signup and view all the answers

Explain how to calculate the mass of a substance given its number of moles and molar mass.

<p>To calculate the mass (m) of a substance, we multiply its number of moles (n) by its molar mass (M). The formula is: m = n x M.</p> Signup and view all the answers

How many atoms or molecules are present in one mole of any substance?

<p>One mole of any substance contains ­6.022 × 10^23 atoms or molecules. This number is known as Avogadro’s number.</p> Signup and view all the answers

What is the difference between a formula unit and a molecule?

<p>A formula unit is the smallest representative unit of an ionic compound, while a molecule is the smallest unit of a covalent compound that can exist independently. Formula units are used to represent the ratio of ions in an ionic compound, but molecules are actual entities that can exist on their own.</p> Signup and view all the answers

Flashcards

Law of Conservation of Mass

In a chemical reaction, mass is neither created nor destroyed.

Parmanu

The smallest indivisible particle of matter, according to an ancient Indian philosopher.

Atom

The smallest indivisible particle of matter, according to ancient Greek philosophers.

Chemical Reaction

A process where substances change into new substances.

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Dalton's Atomic Theory

A scientific explanation about the structure of matter.

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Study Notes

Atoms and Molecules

  • Ancient Indian and Greek philosophers pondered the divisibility of matter
  • Around 500 BC, Maharishi Kanad postulated the existence of smallest particles (Parmanu)
  • Pakudha Katyayama suggested these particles exist in combined forms.
  • Democritus and Leucippus proposed indivisible particles (atoms).
  • Lavoisier established the Law of Conservation of Mass
  • Proust established the Law of Constant Proportions (Definite Proportions).
  • Dalton's atomic theory explained these laws. All matter is made of atoms. Atoms are indivisible. Similar atoms have the same mass.Atoms of different elements have different masses. Atoms combine in whole number ratios.

Law of Conservation of Mass

  • Mass is neither created nor destroyed in a chemical reaction.

Law of Constant Proportions

  • Compounds always contain the same elements in the same proportion by mass.

Dalton's Atomic Theory

  • All matter consists of tiny, indivisible particles called atoms.
  • Atoms of the same element are identical in mass and properties.
  • Atoms of different elements have different masses and properties. Atoms combine in simple whole-number ratios to form compounds.

Atomic Mass

  • A relative scale comparing the mass of atoms to a standard (carbon-12)
  • The mass of one atom of an element in atomic mass units (u).

Atoms

  • The smallest unit of an element. Indestructible fundamental particles of an element or a compound.
  • Cannot exist independently in most cases they form molecules or ions.

Molecules

  • Made up of two or more atoms bonded together
  • The smallest unit of a substance that can exist independently.
  • Show all the properties of the substance.

Atomicity

  • Number of atoms present in a molecule of an element.
  • Monatomic (Argon), Diatomic (Hydrogen), Polyatomic (Phosphorus).

Molecular Mass

  • Calculated by adding the atomic masses of all the atoms in a molecule

Formula Unit Mass

  • Calculated from the formula of the ionic compounds, i.e. adding atomic masses of all atoms in the formula

Mole Concept

  • A mole is the amount of substance that contains the same number of particles as there are atoms in exactly 12 g of carbon-12.
  • 1 mole = 6.022 × 1023 particles
  • Molar mass (g/mol): The mass of one mole of a substance, numerically equal to its atomic or molecular or formula mass.

Chemical Formulae

  • Symbolic representation of composition of compounds.
  • Metal symbol comes first except for some cases
  • Polyatomic ions are represented in brackets if more than one of the same ion is present like calcium hydroxide (Ca(OH)2).
  • Valencies or charges are balanced for a neutral formula.

Ions

  • Charged particles.
  • Cation: Positive ion; Anion: Negative ion

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