Download Periodicity

Chemistry
with Mr. Torre
Periodicity
History of the Periodic Table
In the mid-1800’s about 70 elements had been described. Dmitri Mendeleev (1834-1907), a Russian Chemist, listed
the elements in vertical columns in order of increasing atomic mass. He noticed a regular, or “Periodic,” recurrence of the
physical and chemical properties of the elements in the list. Mendeleev arranged the elements into columns so that
elements with the most similar properties were side by side. He left empty spaces for elements that had not yet been
discovered. Later those elements were found to fit right in the spaces of Mendeleev’s table.
In 1913, Henry Moseley (1887-1915), a British physicist, determined the nuclear charge (atomic number) of the
atoms of the elements. Moseley arranged the elements in a table by order of atomic number.
The Periodic Table
- Horizontal rows are called periods.
- Vertical columns are called groups or families
- The Periodic Law states, “When elements are arranged in order of increasing atomic number, there is a periodic
pattern in their physical and chemical properties.”
Periodic Trends (p. 243)
Atomic Radius is the distance from the nucleus of an atom to its valence electrons.
greater atomic radius = greater size of the atom
- Moving left to right across a period (from Potassium to Krypton, for example), atomic radius decreases. More
protons in the nucleus means greater attraction to the valence electrons, so the electrons are pulled closer to the
nucleus.
- Moving down a group (Hydrogen to Francium, for example), atomic radius increases. Each atom in the column
has one more energy level than the one before. Each energy level “shields” the next by separating it from the
nucleus and reducing the attraction of the valence electrons. So the valence electrons could be further away,
making the atom larger.
Ionization Energy is the energy required to remove an electron from an atom. The first ionization energy is to remove
the first electron; the second ionization energy is to remove the second electron; etc.
- Moving left to right across a period (from Potassium to Krypton, for example), ionization energy increases. More
protons in the nucleus means greater attraction to the valence electrons, so more energy is required to pull the
electrons away.
- Moving down a group (Hydrogen to Francium, for example), ionization energy decreases. Increased shielding
reduces the attraction of the valence electrons. So less energy is required to remove valence electrons.
Electronegativity is how strongly the nucleus of an atom attracts the electrons of another atom in a bond.
- Moving left to right across a period (from Potassium to Krypton, for example), electronegativity increases. More
protons in the nucleus means greater attraction to the valence electrons of another atom.
- Moving down a group (Hydrogen to Francium, for example), electronegativity decreases. Increased shielding
reduces the attraction of the valence electrons of another atom.