Chemistry Atoms and Molecules Overview

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

False

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

<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

<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

<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

<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

<p>Atoms combine in the ratio of small whole numbers to form compounds.</p> Signup and view all the answers

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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

<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