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What is an **electron configuration**?
The distribution of electrons of an atom in atomic orbitals. For example, the configuration of carbon is $1s^2 2s^2 2p^2$.
What is an **atom**?
The smallest unit of an element that retains the chemical properties of that element. It consists of a nucleus of protons and neutrons, surrounded by electrons.
What is the **atomic number**?
The number of protons in the nucleus of an atom, which determines the chemical properties of an element and its place in the periodic table.
Define **isotopes**.
Atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different atomic masses.
What is the **mass number**?
The total number of protons and neutrons in an atom's nucleus.
Explain the **Bohr model** of the atom.
A model in which electrons orbit the nucleus in specific energy levels or shells, with energy quantization explaining atomic spectra.
Define the **Pauli exclusion principle**.
No two electrons in an atom can have the same set of four quantum numbers, meaning an orbital can hold a maximum of two electrons with opposite spins.
What is **ionization energy**?
The energy required to remove an electron from a gaseous atom or ion.
Describe **atomic radius** trend on the periodic table.
Atomic radius decreases across a period from left to right due to increased nuclear charge, and increases down a group due to additional electron shells.
What is **electronegativity**?
A measure of an atom's ability to attract and hold electrons when bonded to another atom.
What is the **Heisenberg uncertainty principle**?
It is impossible to simultaneously know both the exact position and exact momentum of a particle, such as an electron.
What are **quantum numbers**?
Numbers that describe the properties of atomic orbitals and the electrons in those orbitals: principal ($n$), azimuthal ($l$), magnetic ($m_l$), and spin ($m_s$) quantum numbers.
Define **valence electrons**.
Electrons in the outermost shell of an atom that are involved in forming bonds with other atoms.
What is the **photoelectric effect**?
The emission of electrons from a metal when it is exposed to light of sufficient energy. Demonstrated by Einstein, supporting the particle nature of light.
Explain **Hund's rule**.
Electrons will fill degenerate orbitals (orbitals with the same energy) singly before filling them in pairs, to maximize total spin.
What is **shielding effect**?
The reduction in effective nuclear charge on the electron cloud, due to differences in attraction forces among electrons in different shells.
What does the **Aufbau principle** state?
Electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.
Define **effective nuclear charge** ($Z_{eff}$).
The net positive charge experienced by valence electrons. Calculated as the atomic number minus the shielding effect of inner electrons.
What is a **covalent bond**?
A chemical bond formed by the sharing of one or more pairs of electrons between atoms.
What is a **metallic bond**?
A bond formed by the attraction between positively charged metal ions and the electrons around them. Electrons are delocalized.
Explain the **quantum mechanical model** of the atom.
A model that describes electrons as wave functions, providing probabilities of finding an electron in a particular region around the nucleus.
What is the **periodic law**?
The properties of elements are periodic functions of their atomic numbers. Elements with similar properties appear at regular intervals.
Define **ionization energy trend** on the periodic table.
Ionization energy generally increases across a period and decreases down a group due to increased nuclear charge and electron shielding.
What is the **octet rule**?
Atoms tend to form compounds in ways that give them eight valence electrons, achieving a noble gas electron configuration.
Explain the concept of **electron affinity**.
The change in energy (usually released) when an electron is added to a neutral atom to form a negative ion. Increases across a period and decreases down a group.
What are **transition metals**?
Elements found in the d-block of the periodic table, characterized by partially filled d orbitals and the ability to form various oxidation states.
Define **paramagnetism**.
A form of magnetism that occurs in materials with unpaired electrons that align with external magnetic fields, making the material magnetic.
What is an **alloy**?
A mixture of two or more elements, where at least one is a metal, designed to have enhanced properties like strength or corrosion resistance.
Explain **inert pair effect**.
The tendency of the s-electrons in the outermost electron shell to remain non-ionized or unshared in compounds, common in heavier p-block elements.
What is a **noble gas**?
Elements in Group 18 of the periodic table, known for their lack of chemical reactivity due to having full valence electron shells.
Describe **lanthanide contraction**.
The gradual decrease in the atomic and ionic radii of the lanthanide series elements from La to Lu, caused by poor shielding of nuclear charge by f-electrons.
What is a **covalent radius**?
Half of the distance between the nuclei of two identical atoms joined by a covalent bond in a molecule.
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