Trends in the PT

Explanations

Trends in Atomic Radius

• Atomic Radius: Half the distance between nuclei in two bonded atoms

– Down a group, atomic radius increases– Across a group, atomic radius decreases

Trend in Atomic Radius: Down a Group• This is Lithium• It has 3 protons• It has 3 electrons, 2 on the

first energy level and 1 on the second energy level

• This is Potassium• It has 19 protons• It has 19 electrons, 2 on the

first energy level, 8 on the second energy level, 8 on the third energy level and 1 on the fourth energy level

There is a pretty strong attraction between the nucleus and the outer electron because there is only one energy level between it and the nucleus. This keeps the electron close and the atom small.

There is a weak attraction between the nucleus and the outer electron because the electrons in the 3 energy levels between ‘shield’ the outer electron from the nucleus. This allows the outer electron to drift far from the nucleus making the atom larger.

Trend in Atomic Radius: Across a Period• This is Lithium• It has 3 protons• It has 3 electrons, 2 on the

first energy level and 1 on the second energy level

• This is Fluorine• It has 9 protons• It has 9 electrons, 2 on the

first energy level, 7 on the second energy level

There is a pretty strong attraction between the nucleus and the outer electron because there is only one energy level between it and the nucleus. This keeps the electron close and the atom small.

There is an even stronger attraction between the nucleus and the outer electron because the ‘nuclear charge’ or number of protons have increased, but the number of energy levels stays the same. The number of electrons between the outer electrons and the nucleus stay the same while the nucleus gets stronger. This brings the outer electrons closer, making the atom smaller.

Trends in Ionization Energy

• The energy needed to remove the outer electron

– Down a group, Ionization Energy decreases– Across a period, Ionization Energy increases

Trend in Ionization Energy: Down a Group• This is Lithium• Think about how easy it would be to

remove the outer electron • The nucleus has a pretty good grip

on it so it would be harder to remove the outer electron

• This is Potassium• Think about how easy it would be

to remove the outer electron• The attraction of the electron to

the nucleus is shielded by the electrons in between so it would be easier to remove the outer electron

IE Li > K

Trend in Ionization Energy: Across a Period• This is Lithium• It has a nuclear charge of 3

without many electrons between it and the outer electron, so it would take a less energy to remove the outer electron than for fluorine

• This is Fluorine• It has a nuclear charge of 9

without many electrons between it and the outer electrons so it would take much more energy to remove the outer electron

IE Li < F

Trends in Electronegativity

• An atom’s attraction for electrons while it’s in a bond

– Down a group, electronegativity decreases– Across a period, electronegativity increases

Trend in Electronegativity: Down a Group• This is Lithium• It has a nuclear charge of 3 without

many electrons between it and electrons of another atom, so it could attract the electrons of the other atom better than potassium

• This is Potassium• It has a nuclear charge of 19

which seems powerful, but the electrons in the inner energy levels shield the nucleus from attracting electrons from other atoms

EN Li > K

Trend in Electronegativity: Across a Period• This is Lithium• It has a nuclear charge of 3

without many electrons between it and electrons of another atom, so it is weaker in attracting electrons than fluorine

• This is Fluorine• It has a nuclear charge of 9

without additional energy levels between it and the electrons of another atom, so it would attract the electrons even more powerfully

EN Li < F