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Atoms are the fundamental building blocks of matter. They are incredibly small particles that make up everything we see around us, from the air we breathe to the metal in a car. Each atom consists of a nucleus, containing protons and neutrons, surrounded by a cloud of electrons.
The arrangement and number of these subatomic particles determine the properties of each atom.
According to Dalton's atomic theory, each element is made up of identical atoms, differing from those of any other element. For example, all carbon atoms are the same, but they are different from oxygen atoms.

• Protons have a positive charge.
• Neutrons have no charge.
• Electrons have a negative charge.

The unique combination and arrangement of protons, neutrons, and electrons in an atom give each element its distinct characteristics.

Elements are substances that consist entirely of one type of atom. Each element is defined by its atomic number, which is the number of protons in its nucleus. For example, hydrogen has one proton, carbon has six, and oxygen has eight.
According to Dalton's theory, elements are pure substances. Every element's atoms are identical and have specific properties.
There are over 100 known elements, and they are listed in the Periodic Table. Elements can exist in different forms known as isotopes, which have the same number of protons but different numbers of neutrons.

• Pure gold contains only gold atoms.
• In nature, oxygen usually exists as O2 molecules.
• Carbon can exist as graphite or diamond, depending on the arrangement of its atoms.

Chemical reactions involve the rearrangement of atoms to form new substances. According to Dalton's atomic theory, atoms are neither created nor destroyed in a chemical reaction.
Instead, they are simply rearranged. For example, when hydrogen reacts with oxygen to form water, the atoms rearrange but do not disappear.
This principle is called the Law of Conservation of Mass.

• Reactants are substances that start a chemical reaction.
• Products are substances formed by the chemical reaction.
• Breaking and forming bonds between atoms dictate chemical reactions.

Consider the reaction: 2H2 + O2 → 2H2O Hydrogen and oxygen molecules (reactants) rearrange to form water molecules (products). The total number of each type of atom stays the same before and after the reaction.

Compounds are substances formed when atoms of two or more different elements chemically bond together. According to Dalton's atomic theory, these atoms combine in fixed ratios to form compounds.
This is why water always has two hydrogen atoms for every oxygen atom, giving it the chemical formula H2O.
Compounds have unique properties that are different from the elements they are made of. For example:

• Salt (NaCl) is formed from sodium and chlorine.
• Carbon dioxide (CO2) is formed from carbon and oxygen.
• Glucose (C6H12O6) is a sugar formed from carbon, hydrogen, and oxygen.

In each example, the ratio in which atoms combine determines the compound’s unique characteristics and properties.

Atomic mass is the mass of an atom, usually expressed in atomic mass units (amu). This mass is mainly due to the protons and neutrons in the nucleus, as electrons contribute very little to the overall mass.
Each proton and neutron roughly has a mass of 1 amu.
The atomic mass of an element is often a weighted average of all the isotopes of that element. For instance:

• Carbon-12 has an atomic mass of 12 amu.
• Carbon-14 has an atomic mass of 14 amu.
• The average atomic mass of carbon is approximately 12.01 amu due to the relative abundance of its isotopes.

Understanding atomic mass helps in calculating molecular masses of compounds and plays a crucial role in stoichiometry, which is the study of the quantitative relationships in chemical reactions.