Atomic theory and the periodic table

Atomic theory and the periodic table

Key points

• The structure of atoms can be represented using simple bohr diagrams.

• Elements in chemical groups have similar valence electron arrangements.

• The periodic table shows how properties of elements change in predictable ways.

Concept 1: The structure of atoms can be represented using simple diagrams.

Atomic Theory

• The atom: Smallest unit of an element that has the properties of that element

Atomic Theory

• Each atom has a tiny, dense nucleus with neutrons and protons

• Nucleus is surrounded by electrons, which exist in specific electron energy shells

• Most of the mass of the atom is in the nucleus

Bohr Diagrams

• Bohr diagrams represent the electron arrangements of atoms using energy shells

• Show how many electrons occupy each specific energy level or shell

Energy shells

• First energy shell: maximum two electrons

• Second and third energy shell: maximum eight electrons (for the first 20 elements)

• After the third shell atoms can hold more than 8 electrons

Energy Shells

• Valence shell (outermost energy shell): occupied by valence electrons (electrons in the outermost occupied energy shell of an atom)

Concept 2: Elements in chemical groups have similar electron arrangements.

Key patterns in the periodic table

• Atoms in the same group have the same number of valence electrons

• Atoms in the same period have the same number of occupied energy shells

Groups and Valence Electrons

• Group 1: One valence electron

• Group 2: Two valence electrons

• Groups 13-18: 3, 4, 5, 6, 7, 8 valence electrons

• Exception: Helium has 2 valence electrons (other noble gases have 8 valence electrons)

Period and Energy Shells

• First period (hydrogen and helium): One occupied energy shell

• Second period: Two occupied energy shells

• Third period: Three occupied energy shells

Noble Gas Stability: A Full Valence Shell

• Noble gases are stable (unreactive) because they have full valence shells

• Their atoms do not tend to gain, lose, or share electrons

Open Ended

Explain what is happening in this picture. Why is it that the balloons nearest to helium and neon are more buoyant where as xenon and krypton are less buoyant?

How Other Elements Achieve Full Valence Shells

• Other elements can achieve a full valence shell by gaining or losing electrons during a chemical reaction

• When a neutral atom gains or loses an electron, it becomes an ion

How Other Elements Achieve Full Valence Shells

• Loses an electron: becomes positively charged ion

• Gains an electron: becomes a negatively charged ion

Reactivity

• Reactivity of an element is linked to how close it is to having a full valence shell

• Most reactive elements: Groups 1 and 17 (elements are only one electron away from a full valence shell)

Reactivity

• Example: Sodium (group 1) easily gives up an electron, since it exposes the full valence shell underneath

• Example: Fluorine (group 17) readily gains an electron, since it completes their valence shell

Concept 3: The periodic table shows how properties of elements change in predictable ways.

Periodic trend

• A regular variation in the properties of elements based on their atomic structure

• Periodic table can analyze these trends because it can help you compare variations in groups and periods

Atomic Size Trends

• Atomic size increases moving down a group

• As you move down a group, elements have atoms with increasing numbers of energy shells

Atomic Size Trends

• The greater the number of shells, the farther the valence electrons are from the nucleus, and the larger the atom

Atomic Size Trends

• Atomic size decreases moving left to right across a period

• Elements have increasing numbers of electrons across a period

Atomic Size Trends

• Number of occupied valence shells stay the same, but the number of protons in the nucleus increases

• How does this result in decreasing atomic size across a period?

Atomic Size Trends

• Attraction between valence electrons and the nucleus increases because a greater positive charge on the nucleus pulls more strongly on the electrons

• Therefore, the electrons are pulled more tightly towards the nucleus, leading to decreasing atomic size

Metal Reactivity and Atom Size

• Group 1 metals: Potassium is more reactive than sodium

• Both have one valence electron

• Potassium atom is larger than sodium

Metal Reactivity and Atom Size

• Pull of the positive charge on the nucleus is weaker

• Valence electron is easier to remove (less energy is needed to remove the electron)

Open Ended

Why are Alkali metals more reactive than Alkaline earth metals in the same period?

Multiple Choice

How many valence electrons does Boron have?

• 1

• 2

• 3

• 4

Multiple Choice

Choose the most reactive element

Multiple Choice

Why does the atomic size increase when you go down groups?

• more electron shells

• more protons

• more mass