Periodic Trends
Dec 31, 2015
Periodic Trends
Introduction to ions
Atoms are electrically neutral because they have equal numbers of protons and electrons
When an electron is either gained or lost by an atom, it is no longer electrically neutral and it becomes an ION or a charged atom.
Example: Lithium
3 Protons +33 Electrons -3Overall Charge: 0 - neutral
Positively charged ions: CATIONS
Loss of an electron causes the sodium atom to have morepositive charge in the nucleus than negative charge. Losingone electron gives it a 1+ charge.
Negatively charged ions: ANIONS
Gain of an electron gives chlorine more negative chargethan positive, so it becomes a 1- ion.
Ionization Energy
To remove an electron from an atom to make it a cation involves energy.
This is called Ionization Energy and it is measured in kJ/mol.
Ionization Energy The energy needed to remove an electron
from an atom or ion in the gaseous state. The higher the ionization energy, the harder it is to remove the electron.
Ionization Energy
Ionization energy for Fr (87)_____________________ kJ/mol
Ionization energy for F(9) ______________________kJ/mol
From which atom is it easier to remove an electron?_________From which atom is it harder to remove an electron?_________
Periodic Trends
Ionization Energy – Down a group As you move down a group, the
size of the atom increases because it adds more and more orbitals.
The further away the electrons are from the nucleus and its attractive forces, the easier it is to remove.
Lithium
Sodium
Potassium
Rubidium
Periodic Trends
Ionization Energy – Down a group Electron Shielding also plays a role as you
move down a group. The inner electrons shield the outermost
electrons from the attractive pull of the nucleus. This causes the electrons to be held less tightly
by the nucleus. This means it will be easy to remove them from
the atom.
Periodic Trends
Ionization Energy – Across a Period One electron and one proton is added to the
atom If the electron is added to the same energy
level, the higher nuclear charge (from the additional proton in the nucleus) attracts the outer electrons more strongly.
Thus the energy required to remove and electron becomes larger.
Li Be B C N O F
Periodic Trends
Ionization Energy Graph – Across a Period
Example:Li has 3 electrons and 3 protons. Neon has 10 electrons and 10 protons. This large number of protons holds the electrons more tightly than Li’s 3 protons can.
Periodic Trends
Atomic Radius It’s hard to measure the size of one atom alone. One method to measure an atom is to calculate
the bond radius, which is the length that is half the distance between nuclei of two bonded atoms.
Periodic Trends
Down a group
Increases as you move down a period due to increase in the number of orbitals
As a result of this, electrons are not pulled close to the nucleus and are easily pulled off
Atomic Radius
Across a period Decreases
because the added protons pull the electrons closer, making the atom physically smaller
Graph of Atomic Radius
Periodic Trends
Electronegativity
A measure of the ability of an atom to attract electrons. The larger the electronegativity, the more the atom attracts electrons.
Electronegativity
Across a periodIncreases because more protons are added to the nucleus, which allows it to attract negatively charged electrons.
Electronegativity
Down a group:
Decreases because the larger size of the atom decreases the pull from the nucleus so electrons are not attracted to it.
Electronegativity Graph
Ionic Radii
When metal atoms lose electrons, they lose an entire energy level.
This makes the atom’s radius smaller.
Normal size
Decreased ionicsize
Ionic Radii
When atoms gain an electron, it makes them larger because the electrons repulse (push away from) each other.
Normal size
Increased ionic size