3.3.3 Using atomic theory, bonding, and the repeating pattern of electron configurations, explain the trends of atomic radii across periods and down columns.
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There are many experiments in our daily life which confirm the presence of electric charges and forces between them (either attractive or repulsive). For example, when we run a comb through our hair, and then place the comb near a piece of paper, the comb attracts the piece of paper. Attractive forces between the comb and piece of paper are even strong to suspend the paper.
Charges which are same type repel each other and charges which are of opposite type attract each other.
Electric charge can be defined as the characteristic of a unit of matter which reflects the number ofelectrons (negatively charged particles) compared to the number of protons (positively charged particles).Lets look at some examples!
Coulombic attraction is a fancy name for the attraction that occurs between oppositely charged particles. In chemistry, we talk about the attraction between protons (+) in the nucleus to the electrons (-) surrounding the nucleus. Because electrons are negatively charged, they are attracted to nuclei, which are positively charged.
Many properties of atoms depend on their electron configurations and on how strongly their outer electrons are attracted to the nucleus. Using the Bohr model can prove to be a helpful tool in predicting the properties of an element.
The effective nuclear chargeof an atom is the net positive charge “felt” by the VALENCEelectron. This means taking into account the attractive forces felt between the protons and electrons as well as the replusive forces felt between the valence electrons and the core (inner) electrons. Electrons intervening between the nucleus and the outer (valence) electrons are said to sheild the outer electrons from the nucleus so that the outer electron does not experience the full attractive pull of the positive nuclear charge. Effective nuclear charge increase more steadily going across a period (more protons but not more shells) than going down a group (more protons but also more shells).
In general :
Across a Period (row)
Trend:Coulombic attraction increases
Reasoning: There are the same number of energy levels/shells, but there are more protons resulting in a greater attraction of outer electrons to the nucleus
Down a Group (column)
Trend:Coulombic attraction decreases
Reasoning:There more shells/energy levels, more distance between outer electrons resulting in a weaker attraction of outer electrons to the nucleus
Chlorine and magneiusmhave the same number of energy shells, the same number of inner (core) electrons and the outer electrons are the same distance away from the nucleus.
Chlorine has 17 protons and magnesium has 12 protons. Therefore, since chlorine”s nucleus has a greater positive charge, the outer electrons will experience more of that positive charge (increase effective nuclear charge) which causes the chlorine atom to have a smaller atomic radius.
For each set of elements below, identify which element will have the stronger attractive forces between their outer (valence) electron(s) and the nucleus. Explain your reasoning.
a. Mg or Ar b. K or Cs c. Li or Xe
Periodic Trends: Atomic Radii
Atomic radii is useful for determining many aspects of chemistry such as various physical and chemical properties. The periodic tablegreatly assists in determining atomic radius and presents a number of trends.Atomic radiusis defined as being the total distance from the center of an atom’s nucleus to the outermost electron orbital.
As you begin to move across or down the periodic table, trends emerge that help explain how atomic radii change.
Across a Period (row)
Trend: In general, the size of an atom will decrease as you move from leftto the right of a certain period.
Reasoning: More protons, more attraction, increase effective nuclear charge, smaller radius
Down a Group (column)
Trend: In general, the size of an atom will increase as you move down a group
Reasoning:More shells/energy levels, more distance between outer electrons in decrease effective nuclear charge = less attraction, larger radius
Need a visual??? Check out this quick video
What influences the atomic size of an atom?Which atom is larger, K or Br?Which atom is larger, Na or Cl?Which atom is smaller, Be or Ba?Put in order of largest to smallest F, Ar, Sr, Cs.