Atoms: The Building Blocks of Matter

Study Notes

Atoms are the fundamental units of matter, composed of a dense nucleus surrounded by a cloud of negatively charged electrons.
The nucleus contains protons (positively charged) and neutrons (electrically neutral).
Atoms are incredibly small, with dimensions on the order of 10⁻¹⁰ meters (Angstroms) and masses around 10⁻²⁶ kilograms.

The nucleus is composed of nucleons: protons and neutrons.
Protons and neutrons have nearly equal masses.
The nucleus is positively charged due to the presence of protons.
The electric charge of a proton is called the elementary charge (e), which is 1.6 x 10⁻¹⁹ Coulombs.

The electronic cloud is composed of electrons, which have a negative charge equal to -e (-1.6 x 10⁻¹⁹ Coulombs).
The mass of an electron is significantly smaller than that of a proton or neutron, approximately 2000 times less.

Atomic Number (Z): Represents the number of protons in the nucleus of an atom. It determines the element's identity.
- In a neutral atom, the number of protons equals the number of electrons.
- For example, nitrogen (Z = 7) has 7 protons and 7 electrons.
Mass Number (A): Represents the total number of protons and neutrons in the nucleus.
- A = Z + N, where N is the number of neutrons.
- For example, a carbon atom with a mass number of 12 (A = 12) contains 6 protons (Z = 6) and 6 neutrons (N = 6).
Symbolic Representation:
- An atom is symbolized as AX, where X is the element symbol, A is the mass number, and Z is the atomic number. This generally refers to the nucleus of the element.

A nuclide is defined by its specific combination of protons (Z) and neutrons (N).
Different isotopes of the same element have the same atomic number (Z) but different numbers of neutrons (N).

Mass Defect: The difference between the mass of the nucleons and the mass of the nucleus.
- This mass difference is converted into energy, known as binding energy, which holds the nucleus together.
Atomic Molar Mass (M): The mass of one mole of atoms of a particular element.
- It is expressed in grams per mole (g/mol).

The molar mass of a molecule is the sum of the molar masses of its constituent atoms.
- For example, the molar mass of H₂SO₄ is (2 x 1 g/mol) + 32 g/mol + (4 x 16 g/mol) = 98 g/mol.
Avogadro's Constant (Nₐ): The number of entities (atoms, ions, molecules, etc.) in one mole of a substance.
- Nₐ = 6.022 x 10²³ entities per mole.
Isotopes: Atoms of the same element with the same number of protons (Z) but different numbers of neutrons (N). This results in different atomic masses.
Ions: Atoms that have gained or lost electrons, resulting in a net charge.
- Cations are positively charged ions (lost electrons).
- Anions are negatively charged ions (gained electrons).