Chemistry Lesson 1
Atomic and Molecular Structure (Grades 9-12)
Connection Among the Location in the Table, the Atomic Number, and Mass | How to Identify Metals, Semimetals, Nonmetals, and Halogens | How to Identify Alkaline Metals, Alkaline Earth Metals, and Transition Metals | Lanthanide, Actinide, Transactinide, and Transuranium Elements | Ionization Energy, Electronegativity, Relative Sizes | How Many Electrons Can Bond? | Size and Mass | Location and Quantum Electron Configuration | Summary
|IONIZATION ENERGY, ELECTRONEGATIVITY, RELATIVE SIZES|
The amount of energy required to pull an electron off a neutral atom is called the ionization energy. Since each successive electron shell is larger than the previous one, the electrons in the shells further from the nucleus require less energy to be pulled off. In other words, the larger the shell number or the further down in the Periodic Table of the Elements, the lower the ionization energy.
As we go from left to right on the Periodic Table of the Elements, there are more and more protons in the nucleus. The greater number of protons pulls stronger at the valence electrons (the electrons in the outermost shell). This causes an increase in the ionization energy.
To summarize, moving down the table causes a decrease in the ionization energy. Moving to the right causes an increase in the ionization energy. See the figure for the trend.
Electronegativity is the measure of how easy it is to place another electron in the neutral atom. It is almost the opposite of ionization energy, except that some elements don’t accept another electron and it becomes impossible to measure a value of electronegativity. As the shells get larger, it is harder to put an electron in the atom.
As more protons get added to the nucleus, the easier it is to add an electron. The noble gases don’t want to get another electron, so it is almost impossible to put an extra one in the atom.
To summarize, moving down the table causes a decrease in electronegativity. Moving to the right causes an increase in electronegativity. The arrows in the figure show increasing electronegativity.
Atomic size is based on the distance between two atoms. One method uses the distance between different atoms that are bonded together; another method uses the distance between like atoms of the element in a solid. Since the atom is incredibly small, scientists use small units to measure the size of an atom, either Angstroms (Å, 1 x 10–10 m), nanometers (nm, 1 x 10–9 m), or picometers (pm, 1 x 10–12 m).
When a neutral atom gains electrons (becomes an ion), its size increases because there are no longer the same numbers of protons as electrons; when there are fewer protons than electrons in an atom, there is less positive force to pull the electrons in tighter so the atom’s radius becomes larger. When a neutral atom loses an electron, it is left with more protons than electrons and so there is more positive force pulling the remaining electrons in tighter, with the atomic radius becoming smaller.
The size of the neutral atoms increases when moving down a group (column). This is because there are electrons added to the next larger shell. The size of the atoms decreases when moving from left to right (along a period). This is because there are more protons in the nucleus that pull on the electrons. The pull increases, the electrons get closer and the size decreases.
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