PERIODIC PROPERTIESChapter 6

Atomic structure and periodic trends

• The atomic number (number of protons) defines the element and is now the order in which elements are arranged

• The columns are called groups and were originally arranged because elements in the same group had similar properties, they are also called famiiles.

• The rows are called periods , • the elements on the left side of the period are more metallic and those on the right nonmetallic

Electron arrangement and periodic table

• Electrons are the most important to chemists because they are the particles involved in chemical reactions.

• The number of electrons = number of protons in a neutral atom.• As the atomic number increases, the number of electrons

increases. For the first 20 elements, the period on the periodic table matches Bohr’s energy levels in the atom

Electron structure

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• In period 1 there are two elements : Hydrogen (Z=1) and Helium (Z=2), the maximum number of electrons in the first energy level is 2

• In period 2 there are eight more elements ending at Neon, this period can hold 8 electrons

• In period 3 there are eight more elements ending at Argon, this period can hold 8 electrons

Subdivisions of the periodic table and electron arrangement• The Bohr energy level can be divided into sublevels: s, p, d,

and f. • Each sublevel differs slightly in energy • These match the sections of the periodic table

Valence electrons and the periodic tableElectrons fill the lowest energy locations in an atom first.

For the first 20 elements the last electrons in the atom are placed in the highest energy level . These electrons are called the valence electrons. The valence electrons are the ones most likely involved in chemical reactions In the first two columns the valence electrons are in the s-sublevel.In the last six columns the valence electrons are in the p-sublevel.

Valence electrons continued• There are only 8 valence electrons in any atom• By knowing the location of an element you can determine the

location of the valence electrons in an element

Periodic trends in physical properties• Atomic Radius is the term used to describe the size of an atom• For elements on the right side of the periodic table (nonmetals) it is

defined as half the distance between two covalently bonded atoms

Trends in atomic radii• Atomic radii is a major factor in explaining many periodic trends

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Trend: Atomic radius increases top to bottom in a group and decreases left to right in a period

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Cause of trends in Atomic radius• There are 3 primary factors:• 1) the energy level of the valence electrons. The

higher the energy level, the greater distance of the valence electron from the nucleus

• 2) Number of protons in the nucleus. The more protons the greater the attraction of the nucleus for the negatively charged electrons

• 3) The shielding of the valence electrons from the nucleus due to the repulsion between the inner energy shell electrons

Causes of trends continued

An effective way to consider the effect of the nucleus on a valence electron is to introduce the idea of the effective nuclear charge ( Z effective)

This is calculated by adding the total charge of the protons to the charge of the core electronsFor example the Z effective for Na is ((+11)+ (-10)) = +1

Trend in atomic radius in a group

• As you go down a group the atomic radius increases.

• Why?• 1) Each element’s valence electrons are in a higher energy

shell so they are farther from the nucleus• 2) The Z effective is about the same because the atomic

number increases but the number of core electrons also increases

• This causes the force of attraction between the nucleus and the valence electron to weaken, allowing the cloud of electrons to expand.

Physical Properties• Define the terms first ionization energy and electron affinity

• The first ionization energy is the minimum energy needed to remove one mole of electrons from one mole of gaseous ions at 25 C and 1 atm ( standard thermodynamic conditions)

• This is the energy involved in forming cations ( positive ions) from neutral atoms and is part of the process of forming ionic bonds.

• Electron affinity is the energy released when an atom attracts an electron forming an anion ( a negative ion).

First Ionization Energy trends

• Ionization energy decreases down a group because:• 1) The valence electrons are in higher energy levels, which are

farther from the nucleus, weakening the attraction of the nucleus

• 2) The effective nuclear charge is essentially the same for each element in the group because another energy shell of core electrons is added.

• 3) The electron-electron repulsion increases with additional electrons.

• Since the force between nucleus and electrons weakens with distance( which is increasing) and increases with effective nuclear charge( which is constant) The energy to remove an electron decreases

Trend in atomic radius in a period

• 3.2.3 Describe and explain the trends in atomic radius, first ionization energy and electronegativity for elements across period 3

• As you go left to right in a period the atomic radius decreases.• Why?• 1) Each element’s valence electrons are in the same energy

shell so they start at the same distance from the nucleus• 2) The Z effective increases because the atomic number

increases but the number of core electrons stays the same• This causes the force of attraction between the nucleus and

the valence electron to increase, causing the cloud of electrons to contract.

Trends in first ionization energy

• The first energy increases moving to the right in the period.• What are the three factors that cause this??

Trends in electron affinity• 3.2.4 Compare the relative electron affinity values of two or

more elements based on their positions in the periodic table• Electron affinity values follow the same trend as ionization

energy• What are the reasons???