Periodic Trends

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