Chapter 2 part 2.notebook
Atoms and the Periodic Table
Chemists define, and measure, an atom’s size in terms of its radius. The radius of an atom is the distance from its nucleus to the approximate outer boundary.
September 18, 2014
Atomic Size (Radius)
There are two general trends for atomic size:
1.As you go down each group in the periodic table, the size of an atom increases.
i. the valence electrons occupy an energy level that is further from the nucleus.
ii. Electrons experience less attraction for the nucleus.
iii. Inner energy levels block, or shield the valence electrons from the attractionof the nucleus
2.As you go across a period, the size of an atom decreases
i. positive charge on the nucleus also increases across a period. Greater attraction for electrons.
ii. without additional energy the electrons are restricted to their outer energy level.
Chapter 2 part 2.notebook
September 18, 2014
METAL IONS ARE ___________________ THAN THEIR NEUTRAL ATOM BECAUSE:
NONMETAL IONS ARE __________________ THAN THEIR NEUTRAL ATOM BECAUSE:
Rank the following atoms/ions in order of increasing size: Na, Na+1, Ar, Ca, N­3, B+3, Si, Br
Chapter 2 part 2.notebook
September 18, 2014
Rank the following atoms/ions in order of increasing size: Mg, Mg+2, Rb+, Ar, F­, S­2, S
Ionization Energy
An Ion is a charged atom.
An atom that gains electrons becomes a negatively charged anion.
An atom that loses electrons becomes a positively charged cation.
Main group metals tend to lose electrons
Non­metals tend to gain electrons
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It takes energy to overcome the attractive force of a nucleus and pull an electron away from a neutral atom. The energy that is required to remove an electron from an atom is called IONIZATION ENERGY.
The ionization energy that is needed to remove one electron from the outer energy level of the atoms of the main­group elements is called the 1st Ionization Energy.
It is measured in units of kJ/mol. A kilojoule (kJ) is a unit of energy. A mole (mol) is an amount of a substance.
Atoms that give up electrons easily have low ionization energies.
Chapter 2 part 2.notebook
There are two general trends for ionization energy.
1.Ionization energy tends to decrease down a group.
i. Electrons in the outer energy level are farther from the positive force of the nucleus.
2.Ionization energy tends to increase across a period.
i.the attraction between the nucleus and the electrons in the outer energy level increases. Thus, more energy is needed to pull an electronaway from its atom.
September 18, 2014
Chapter 2 part 2.notebook
Electron Affinity
Some atoms have a low attraction for electrons and others a greater attraction. Electron affinity is a measure of the change in energy that occurs when an electron is added to the outer energy level of an atom to form a negative ion.
If energy is released when an atom of an element gains an electron, the electron affinity is expressed as a negative integer.
large negative number:
• high electron affinity (wants e­)
• lots of energy is released!
positive OR small negative number:
• low electron affinity (doesn't want e­)
• must add energy
September 18, 2014
When energy is absorbed when an electron is added, electron affinity is low, and is expressed as a positive integer.
Metals have very low electron affinities
Although the trends are not as apparent, in general the trends for electron affinity are:
1.Electron affinity tends to decrease down a group.
2.Electron affinity tends to increase across a period.
Chapter 2 part 2.notebook
September 18, 2014
Chapter 2 part 2.notebook
September 18, 2014