Periodic Trend

• Law of Triades
when elements are arranged by atomic weight and then grouped in 3’s, the middle element has properties that are an average of the other two members (Dobreiner)
• Law of Octaves
pairs similar elements which differed by some multiple of 8 in atomic weight, any given element will exhibit analogous behavior to the eighth element following it in the table when the table is arranged by atomic weight
• Mendeleev’s Periodic Table
– organized by increasing atomic weight in horizontal rows and columns, and if he left blanks (suggesting elements had not yet been discovered) a pattern was apparent, elements with similar chemical properties appeared at regular intervals periodically
• Modern Periodic Table
arranged by atomic number (number of protons and electrons) which the properties of the elements depend on
Modern Periodic Law
• Physical and chemical properties of the elements are periodic functions of increasing atomic number
• Metal characteristics
1. Metallic luster (shine
2. Generally solids at room temperature
3. Malleable (beaten up into sheets)
4. Ductile-drawn into wires
5. Good conductors of heat and electricity
• Non-metal
1. Rarely have metallic luster
2. Gasses or soft solids are at room temperature
3. Not malleable or ductile
4. Poor conductors of heat and electricity
5. Molecules in their elemental form
6. Combine with other non-metal to form covalent compounds
7. Generally form negative ions, anions
• Metalloids
1. Generally look metallic but are britle (not malleable or ductile)
2. Neither good conductors or insulators, instead they are semi-conducotrs
• Atomic Radius
1. Half of the distance between the centers of two atoms of that element are just touching each other
2. The atomic radius decreases across a period from left to right because as we move from left to right across a period the atomic number increases (the nuclear force of attraction increases since the atomic number gives the number of protons) while the number of occupied shells remains the same (because they are in the same period). The nucleus Is pulling stronger while the cloud remains the same size as we move across the period which causes the electrons to be pulled closer to the nucleus and the atomic radius therefore increases
3. The atomic radius increases down a group because the nuclear force of attraction increases and the number of shells increase, however the latter is dominant so the atomic radius increases
Ionic Radius
1. The ionic radius is the radius of anion or cation, when neutral atoms are ionized there is no change in their sizes
Cation Mg and Mg+2 – Mg has 12 e and 12 p where Mg+2 has 12p and 10 e Mg+2 will be smaller because the protons can exert more energy on the electrons pulling them closer, also a decreased electron-electron repulsion and increased nuclear force of attraction (vice versa for anions- anions ionic radius increases)
Ionization Energy
1. The minimum energy required to remove an electron from the ground state of the isolated gaseous atom (measures how strongly an atom holds its electrons)2. The smaller the radi the higher the ionization energy3. In a period the ionization energy increases as the atomic radius decreases and the nuclear force of attraction increases4. In a group ionization energy decreases as the atomic radius increase and the nuclear force of attraction decreases5. First Ionization Energy- energy needed to remove the first electron from the atom (Na+energy? Na+ + 1e-)6. I1
Electron Affinity
1. The energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion
2. Increases across a period and decreases down a group because of the atomic radius and nuclear force of attraction
Example: F+ e- ?F – change in heat=-328 KJ/mol *exothermic process
1. Atom’s ability to attract a pair of electrons in chemical bond
Melting Point
1. The melting point for metals generally decreases as you go down a group
2. The melting point for non-metal generally increases as you go down group
1. Metals: Period- reactivity decreases Group- reactivity increases (easier for electrons to be taken away towards the left of the period or towards the bottom of the group)
2. Non-metals: Period reactivity increase Group reactivity decreases (higher electronegativity right of the period and upper part of the group)
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