Effective Nuclear Charge, Shielding, and Trends on the Periodic Table Vocabulary: effective nuclear charge (Zeff) shielding atomic radius ionization energy electronegativity Apollo and Diana Attacking the Children of Niobe, Jacques-Louis David, 1772 (French)

Trend What is a trend? a pattern or direction Starbucks Stock History

Periodic trends Nonmetals Metaloids Metals The elements have trends, too. For example: Look at the zig-zag line on your periodic table. Metals are on the left, nonmetals on the right, and the metalloids are adjacent to the line. Nonmetals Metaloids Metals

Periodic Trends We’re going to look at trends on the periodic table for physical properties of elements. These are: atomic radius electronegativity ionization energy

Periodic Trends You can make predictions about certain behavior patterns of an atom and its electrons based on the position of the atom in the periodic table. All periodic trends can be understood in terms of 2 basic rules that describe the relationship between protons and electrons in an atom, which is referred to as “Effective Nuclear Charge, Zeff”.

1. Attractions Electrons are attracted to protons in the nucleus of the atom. The closer an electron is to the nucleus, the more strongly it is attracted, increasing effective nuclear charge. The more protons in a nucleus, the more strongly an electron is attracted, increasing effective nuclear charge.

Effective Charge (Zeff) A good estimate for the effective charge (Zeff) across a period attracting a valence electron can be calculated using the following equation: Zeff = Z – core electrons

Zeff of Period 4 K Ca Sc Ti V Cr The number of protons increases as you go across a period. More protons = more attraction of the electrons to the nucleus. Increased Zeff as you go across the period. Calculate Zeff for K, Ca, and Sc.

2. Electron Shielding and Repulsion Electrons repel other electrons. Electrons are repelled by other electrons in an atom, decreasing effective nuclear charge. Effective nuclear charge decreases when electron levels exist between the valence level and the nucleus of the atom. This is commonly referred to as electron shielding.

Shielding in the alkali metals Shielding: the blocking of the nuclear charge to the valence electrons by the core electrons. More energy levels = more shielding. (Valence electrons are circled.) Increased shielding Decreased attraction of valence electrons

Trends: Atomic Radius The atomic radius of an atom is the approximate distance from the nuclei of an atom to its valence electrons. Typically measured as half the distance between the nuclei of identical bonded atoms. From the online lab, what trend in atomic radius was observed when going left to right across a period on the periodic table? Why? From the online lab, what trend in atomic radius was observed when going top to bottom in a group on the periodic table? Why?

Trends: Ionization Energy Electrons are attracted to the nucleus of an atom, so it takes energy to remove an electron. The energy required to remove the outermost electron from an atom is called the first ionization energy. The energy required to remove the next furthest electron is called the second ionization energy, and so on.

Trends: Ionization Energy What trend do you predict in ionization energy when going left to right across a period on the periodic table? Why? What trend do you predict in ionization energy when going top to bottom in a group on the periodic table? Why?

Trends: Electronegativity Electronegativity refers to how strongly the nucleus of an atom attracts the electrons of other atoms in a bond (noble gases are not measured based on electronegativity). The scale is from 0-4 with 0 meaning no attraction for valence electrons and 4 as the highest attraction. When an atom gains or loses an electron, the atom is called an ion.

Periodic Table Trends

Explaining Trends LEFT TO RIGHT Moving from left to right on the periodic table, protons are added to the nucleus and electrons are added to the valence level. Due to the addition of protons to the nucleus of an atom Zeff increases (although electron repulsion increases slightly with additional electrons there is no increase in electron shielding) . This causes atomic radius to decrease, and all other trends to increase. TOP TO BOTTOM Moving from top to bottom on the periodic table, new shells (energy levels) of electrons are added to the atom. This pushes the valence electrons further away from the nucleus and increases the electron shielding in the atom (protons are also being added, but the shielding far outweighs this attraction). This results in a decreases in Zeff causing atomic radius to increase, and all other trends to decrease.

Connecting Trends and Atomic Properties Explain why the atomic radius of neon is… Less than a neutral magnesium atom. Greater than the common ion formed by magnesium. Which element, potassium or calcium, has a higher first ionization energy? Which has a higher second ionization energy? Justify your answers. Explain why the electronegativity of oxygen is higher than the electronegativity of chlorine.