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Dmitri Mendeleev

Russian chemist (1869) who published the first real periodic table, arranging elements by atomic mass and leaving spaces for undiscovered elements.

Periodic Law (Mendeleev’s version)

Properties of elements are a periodic function of their atomic mass

Moseley’s Periodic Law (modern version)

Properties of elements are a periodic function of their atomic number

Periods

The horizontal rows on the periodic table; elements in the same period have the same number of energy levels

Groups

The vertical columns on the periodic table; elements in the same group have similar chemical properties due to having the same number of valence electrons

Alkali Metals (Group 1)

Highly reactive metals, soft, and explosive in water. They are stored under kerosene and form salts with nonmetals

Alkaline Earth Metals (Group 2)

Less reactive than alkali metals, form basic compounds, and are harder and denser. Common in sea salts

Transition Metals (Groups 3-12)

Hard metals with high melting points, forming colored compounds, and conducting heat and electricity well

Metalloids

Elements with properties of both metals and nonmetals, found on the staircase line (B, Si, As, Te, At, Ge, Sb)

Halogens (Group 17)

Highly reactive nonmetals, form salts with metals, and are not found free in nature

Noble Gases (Group 18)

Nonreactive gases, previously thought to be inert, but some (Kr, Xe, Rn) can form compounds

Lanthanides

Rare earth metals (f-block) used in alloys and to produce the glow in TV screens; shiny and reactive.

Actinides

Radioactive f-block elements, mostly synthetic, with uranium used as nuclear fuel and in colored glass

Coulombic Attraction

The force of attraction between positively charged protons in the nucleus and negatively charged electrons. It affects trends in atomic size, ionization energy, and electronegativity.

Atomic Radius

The distance from the nucleus to the outermost electron in an atom, measured in picometers (pm)

Electronegativity

The ability of an atom to attract electrons in a covalent bond

Ionization Energy

The energy required to remove the most loosely held electron from an atom

Ionic Radius

The size of an ion; cations (positive ions) are smaller than their neutral atoms, and anions (negative ions) are larger due to increased electron-electron repulsion

Isoelectronic Species

Atoms or ions that have the same electron configuration

Shielding

The effect where inner electron shells reduce the attraction between the nucleus and the outermost electrons, influencing trends like ionization energy and atomic size