Atoms and Elements: Structure, Properties, and Periodic Trends

Atoms and Elements

Element Names and Symbols

Elements are the fundamental substances that make up all matter. Each element is represented by a unique chemical symbol, which is a one- or two-letter abbreviation of its name.

• Chemical symbols are standardized abbreviations (e.g., H for hydrogen, Na for sodium).

• When given the name of an element, you should be able to write its correct symbol, and vice versa.

• Examples: O for oxygen, Fe for iron, Cl for chlorine.

The Periodic Table: Organization and Classification

The periodic table arranges elements by increasing atomic number and groups elements with similar properties together.

• A period is a horizontal row on the periodic table.

• A group is a vertical column containing elements with similar chemical properties.

• Special groups include:

  • Group 1A (1): Alkali metals

  • Group 2A (2): Alkaline earth metals

  • Group 7A (17): Halogens

  • Group 8A (18): Noble gases

• Metals are found to the left of the heavy zigzag line; nonmetals are to the right; metalloids (except aluminum and tennessine) are along the zigzag line.

Subatomic Particles and Atomic Structure

An atom is the smallest particle of an element that retains its chemical properties. Atoms are composed of three types of subatomic particles:

• Protons: Positive charge (+), located in the nucleus.

• Neutrons: No charge (neutral), located in the nucleus.

• Electrons: Negative charge (−), located outside the nucleus in electron clouds.

Atomic Number, Mass Number, and Isotopes

The atomic number and mass number describe the composition of an atom:

• Atomic number (Z): Number of protons in the nucleus; defines the element.

• Mass number (A): Total number of protons and neutrons in the nucleus.

• In a neutral atom, the number of protons equals the number of electrons.

Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons.

• The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

Example: Carbon has two common isotopes: 12C and 13C.

Calculating Atomic Mass

• Atomic mass is calculated using the formula: $\text{Atomic mass} = \sum (\text{isotope mass} \times \text{percent abundance})$

Energy Levels, Sublevels, and Orbitals

Electrons in atoms occupy specific energy levels, sublevels, and orbitals:

• Energy levels (n): Main shells where electrons reside (n = 1, 2, 3, ...).

• Each energy level contains one or more sublevels (s, p, d, f).

• Each sublevel contains a specific number of orbitals:

  • s: 1 orbital (holds 2 electrons)

  • p: 3 orbitals (holds 6 electrons)

  • d: 5 orbitals (holds 10 electrons)

  • f: 7 orbitals (holds 14 electrons)

• Each orbital can hold a maximum of two electrons with opposite spins.

Electromagnetic radiation (such as visible light) is related to the energy changes of electrons in atoms. When electrons absorb energy, they move to higher energy levels; when they release energy, they fall to lower levels, emitting light of specific wavelengths.

Electron Configurations and Orbital Diagrams

The arrangement of electrons in an atom is described by its electron configuration and can be represented by orbital diagrams.

• Electrons fill orbitals in order of increasing energy (Aufbau principle).

• Within a sublevel, electrons occupy empty orbitals singly before pairing (Hund's rule).

• Electron configuration notation lists the number of electrons in each sublevel (e.g., 1s2 2s2 2p6 3s2 3p2 for silicon).

• Abbreviated configurations use the symbol of the previous noble gas in brackets (e.g., [Ne] 3s2 3p2).

• The periodic table is divided into s, p, d, and f blocks, corresponding to the filling of these sublevels.

Example: The orbital diagram for silicon shows paired and unpaired electrons in the 3p orbitals.

Periodic Trends and Valence Electrons

The properties of elements are influenced by their valence electrons (electrons in the outermost energy level).

• Elements in the same group have similar valence electron arrangements.

• The Lewis symbol represents valence electrons as dots around the element symbol.

• Atomic size increases down a group and decreases across a period (left to right).

• Ionization energy (energy required to remove a valence electron) decreases down a group and increases across a period.

• Metallic character increases down a group and decreases across a period.

Summary Table: Periodic Trends

Property	Down a Group	Across a Period (Left to Right)
Atomic Size	Increases	Decreases
Ionization Energy	Decreases	Increases
Metallic Character	Increases	Decreases

Example: Sodium (Na) is more metallic and larger than chlorine (Cl), which is in the same period but further to the right.

Additional info: The periodic trends are explained by the effective nuclear charge and the shielding effect, which influence how strongly the nucleus attracts valence electrons.