Comprehensive Guide to Atomic Structure and Periodic Table Trends and Properties

Atomic Structure and the Periodic Table

• The Periodic Table is organized such that atoms of the elements increase in mass as you progress from left to right (starting with hydrogen) and downwards through the table. For instance, a hydrogen atom has less mass than a sodium atom.

• Each element is defined by an atomic number, which indicates the number of protons contained within an atom of that element. The atomic number increases by exactly one for every subsequent element in the table.

• Each element possesses a mass number, which represents the total sum of protons and neutrons within each atom of that element.

• Atoms are composed of subatomic particles with specific charges:

  • Protons: Positive charge.

  • Electrons: Negative charge.

  • Neutrons: No charge (neutral).

• An atom has no overall electrical charge because the number of protons is equal to the number of electrons, resulting in a balanced state.

• The relationship between these values is expressed by the formula: $\text{Mass Number} = \text{Protons} + \text{Neutrons}$.

• Case Study: Lithium ($Li$)

  • Atomic number: $3$

  • Mass number: $7$

  • Number of protons: $3$

  • Number of electrons: $3$

  • Number of neutrons: $4$ (Calculated as $\text{Mass Number} - \text{Atomic Number} = 7 - 3 = 4$)

• Periodic Table Layout:

  • Periods: Horizontal rows. The period number corresponds to the number of electron shells an atom possesses (e.g., Period $1$ has $1$ shell, Period $2$ has $2$ shells, etc.).

  • Groups: Vertical columns.

  • Metals and Non-metals: The table distringuishes between these two categories, with metals generally on the left and non-metals on the right.

Properties of Materials and Density

• The mass of an atom is directly related to its mass number. For example, Hydrogen has an atomic mass of $1$ and Carbon has an atomic mass of $12$, meaning one carbon atom has $12$ times more mass than one hydrogen atom.

• Density Definition: Density is the measure of the mass for a fixed volume of a substance.

• Density Unit: The standard unit provided is $g/cm^3$.

• Comparison of Sodium and Iron:

  • A piece of sodium measuring $1\,cm^3$ has a mass of $0.97\,g$. Its density is $0.97\,g/cm^3$.

  • A piece of iron measuring $1\,cm^3$ has a mass of $7.87\,g$. Its density is $7.87\,g/cm^3$.

  • Iron is more dense than sodium because it has more mass for the same volume.

The Electronic Structure of Atoms

• The model of the atom has evolved through the contributions of several scientists:

  • J. J. Thompson and Ernest Rutherford: Developed early models illustrating the structure of the atom.

  • Niels Bohr: In $1913$, the Danish scientist advanced Rutherford's model. He provided evidence that electrons move in specific electron shells (also known as energy levels) around the nucleus. He was awarded the Nobel Prize for this discovery.

• Electronic Arrangement Rules:

  • Electrons are held in place around the nucleus by electrostatic forces.

  • The first electron shell can hold a maximum of $2$ electrons.

  • The second and third electron shells can hold up to $8$ electrons each.

  • Electrons always fill the inner shells first before moving to the next shell.

• Electronic Structure Notation: The arrangement is written as a sequence of numbers separated by commas. For example, Boron (atomic number $5$) is written as $2,3$, indicating $2$ electrons in the first shell and $3$ in the second.

• Examples of Atomic Structure:

  • Lithium ($Li$): Atomic number $3$, Mass number $7$. Contains $3$ protons, $4$ neutrons, and $3$ electrons arranged as $2,1$.

  • Sodium ($Na$): Atomic number $11$, Mass number $23$. Contains $11$ protons, $12$ neutrons, and $11$ electrons arranged as $2,8,1$.

  • Potassium ($K$): Atomic number $19$, Mass number $39$. Contains $19$ protons, $20$ neutrons, and $19$ electrons arranged as $2,8,8,1$.

Group 1: The Alkali Metals

• Group $1$ elements, known as the Alkali Metals, include Lithium ($Li$), Sodium ($Na$), and Potassium ($K$).

• Trends Moving Down Group 1:

  • The atomic number increases.

  • The mass number increases.

  • The physical size of the atom increases.

  • The melting point decreases.

• Physical Properties Comparison:

  • Lithium ($Li$): Atomic number $3$, Mass number $7$, Melting point $180\,^{\circ}C$, Boiling point $1360\,^{\circ}C$.

  • Sodium ($Na$): Atomic number $11$, Mass number $23$, Melting point $98\,^{\circ}C$, Boiling point $900\,^{\circ}C$.

  • Potassium ($K$): Atomic number $19$, Mass number $39$, Melting point $63\,^{\circ}C$, Boiling point $777\,^{\circ}C$.

• Predictions: Based on the decreasing trend, the next element, Rubidium, would be expected to have a melting point lower than $63\,^{\circ}C$.

• Structural Similarity: All Group $1$ metals have exactly one electron in their outer shell ($2,1$; $2,8,1$; and $2,8,8,1$). This similarity is why they are categorized as Group $1$.

Group 7: The Halogens

• Group $7$ elements are known as the Halogens. This group includes Fluorine ($F$), Chlorine ($Cl$), and Bromine ($Br$).

• Physical States and Appearance:

  • Fluorine ($F$): A pale yellow gas at room temperature.

  • Chlorine ($Cl$): A yellowish green gas at room temperature.

  • Bromine ($Br$): A brown-red liquid at room temperature.

• Trends Moving Down Group 7:

  • Melting points increase (Fluorine: $-220\,^{\circ}C$, Chlorine: $-101\,^{\circ}C$, Bromine: $-7\,^{\circ}C$).

  • Boiling points increase (Fluorine: $-188\,^{\circ}C$, Chlorine: $-34\,^{\circ}C$, Bromine: $59\,^{\circ}C$).

  • Density and mass number increase.

  • Reactivity decreases: Fluorine is the most reactive, followed by chlorine, with bromine being the least reactive of the three.

• Halogen Data Table:

  • Fluorine ($F$): Atomic number $9$, Electronic structure $2,7$, Mass number $19$.

  • Chlorine ($Cl$): Atomic number $17$, Electronic structure $2,8,7$, Mass number $35.5$.

  • Bromine ($Br$): Atomic number $35$, Mass number $80$. (The full electronic structure for bromine includes $7$ electrons in the outer shell).

Questions & Discussion

• How many electrons are there in an atom of carbon? Carbon has an atomic number of $6$, therefore it has $6$ electrons.

• How many protons are there in an atom of beryllium? Beryllium has an atomic number of $4$, meaning it has $4$ protons.

• How many neutrons are there in an atom of boron? Boron has a mass number of $11$ and an atomic number of $5$. $\text{Neutrons} = 11 - 5 = 6$.

• Draw a diagram to show the structure of an atom of magnesium. Magnesium (Atomic number $12$) has a structure of $2,8,2$.

• What is the name of the element that has the electronic structure 2,8,3? The total number of electrons is $2 + 8 + 3 = 13$. The element with atomic number $13$ is Aluminium.

• Draw a labelled atomic diagram of the element fluorine. Fluorine (Atomic number $9$) has a structure of $2,7$.

• How is the model of the atom we use today different from the Rutherford model? The modern model (Bohr model) includes electrons moving in specific electron shells or energy levels, whereas Rutherford's model did not specify these defined paths.

• Where in the Periodic Table do you find the metals? Metals are found on the left and center of the Periodic Table.

• What is the trend in the boiling points of Group 1 elements? The boiling points decrease as you go down the group.

• What prediction can you make about the boiling point of rubidium? It will be lower than that of potassium ($777\,^{\circ}C$).

• How many more electrons than lithium does sodium have? Lithium has $3$ electrons and Sodium has $11$. $\text{Difference} = 11 - 3 = 8$.

• What happens to the size of the atoms as you go down Group 1? The size of the atoms increases.

• What similarity is there in the structure of Group 1 atoms? They all have one electron in their outer shell.

• Suggest why Group 1 is called Group 1. They are named after the single electron in their outermost shell.

• Are the halogens metals or non-metals? They are non-metals.

• What is the trend in colour in Group 7? The colors become darker as you go down the group (pale yellow to yellowish green to brown-red).

• What would you predict about the boiling point and melting point of iodine? Iodine, being below bromine in Group $7$, would have a higher melting point and boiling point than bromine.

• Would you expect iodine to be more or less reactive than bromine? Less reactive, as reactivity decreases down Group $7$.

• How many electrons do you predict will be in the outer shell of bromine? Seven, because it is in Group $7$.