3 — atomic structure

Question 1

Atom

Answer 1

An atom is the smallest particle that can still have the chemical characteristics of an element.

Question 2

Relative mass of an electron

Answer 2

1/1840 unit mass of a proton or neutron
Negligible

Question 3

Nucleon number

Answer 3

Nucleon number or relative atomic mass (A) = number of protons + number of neutrons in the nucleus (N)

Question 4

Isotopes

Answer 4

Isotopes are atoms of the same element with the same number of protons and electrons but different number of neutrons.

Question 5

Characteristics of isotopes

Answer 5

1. Similar chemical properties because chemical reactions only involve the outermost electrons and not the protons and neutrons. (Same kind of chemical reactions can occur)
2. Different physical properties because physical properties are affected by mass. (Eg density, melting or boiling point)

Question 6

First electron shell

Answer 6

1. Closest to the nucleus and corresponds to the lowest energy level
2. Can hold max of 2 electrons
3. Always filled first

Question 7

2nd and third electron shell

Answer 7

1. Have increasing energy levels
2. Usually hold up to a max of 8 electrons each
3. Filled in order, 2nd electron shell must be fully filled first before third electron shell is filled.

Question 8

Valence shell

Answer 8

Valence shell of an atom refers to the shell that is furthest away from the nucleus of the atom.

Question 9

Valence electrons

Answer 9

Valence electrons refer to the electrons in the outermost shell of an atom.

Question 10

Groups and periods in the periodic table

Answer 10

1. Elements in the same group have the same number of valence electrons. Hence they tend to exhibit similar chemical properties.
2. Elements in the same period hav the same number of electron shells.

Question 11

Ions

Answer 11

An ion is a charged particle formed from an atom or a group of atoms by the loss or gain of electrons.

Question 12

Noble gases

Answer 12

Atom of noble gases (group 18) do not form ions. They have stable electronic configuration since their valence shell is completely filled with electrons. Hence, they are unreactive.

Question 13

Why are atoms reactive?

Answer 13

Atoms form ions to achieve stable electronic configuration.

Question 14

Explain why helium and argon are chemically similar

Answer 14

Both helium and argon are chemically unreactive and they do not lose, gain or share electrons. Helium has a completely filled first electron shell and argon has a completely filled third outermost electron shell. Thus, both have stable electronic configurations.

Question 15

Suggest the trend in the melting points of elements oxygen, Sulfur and zinc. Explain. [3]

Answer 15

Lowest: oxygen < Sulfur < zinc (highest)

Both oxygen and Sulfur have a simple molecular structure in which weak intermolecular forces of attraction exist. A small amount of energy is needed to overcome these forces of attraction and hence they have a lower melting point as compared to zinc.
As Sulfur has a larger molecular size than oxygen, more energy is needed to overcome the stronger electrostatic forces of attraction. Thus, Sulfur has a higher melting point than oxygen. Zinc has a giant metallic structure where strong electrostatic forces of attraction exist betw zinc ions and a “sea of delocalised mobile electrons”. These forces of attraction require a lot of energy to overcome so zinc has the highest melting point.

Question 16

Discuss the differences betw the way that bonds are formed in MgO and O2.

Answer 16

Mg loses 2 2valence e- readily to oxygen to form Mg2+ and O2-. Electrostatic forces of attraction between oppositely charged ions result in the formation of an ionic bond.
Two oxygen atoms share four electrons which results in the formation of a covalent bond.

Question 17

Describe the similarities and differences betw the bonding and structures in diamond and graphite [4]

Answer 17

1. Both diamond and graphite are made up of carbon atoms covalently bonded tgt.
2. Both diamond and graphite have giant covalent structures.

Differences:
1. The carbon atoms in diamond are arranged in a tetrahedral structure while the carbon atoms in graphite r arranged in layers of hexagons held tgt by weak intermolecular forces of attraction.
2. In diamond, each carbon atom is bonded to four other carbon atoms but in graphite, each carbon atom is bonded to three other carbon atoms covalently, leaving a free electron in its valence electron shell.

Question 18

Explain why graphite and carbon dioxide have different properties. [4]

Answer 18

1. Graphite has a high melting and boiling point as there r strong covalent bond betw the carbon atoms in its giant covalent structure. A large amount of heat energy is needed to overcome the strong covalent bonds.
2. CO2 has a lower melting and boiling point as it has a simple molecular structure made up of small carbon dioxide molecules held tgt by weak intermolecular forces of attraction. A small amt of heat energy is needed to overcome the weak intermolecular forces of attraction between the CO2 atoms.
3. For graphite, each carbon atom has a mobile outer electron that is not involved in bonding, allowing graphite to conduct electricity.
4. In CO2, all outer electrons of the carbon atoms r used in bonding w oxygen, thus there r no free electrons to conduct electricity.

Question 19

Suggest why the radius of a lithium atom decreases when it forms a lithium ion.

Answer 19

A lithium ion loses its valence electron in order to form a lithium ion. When its valence electron is lost, the number of electron shells in lithium decreases from 2 to 1, thus atomic radius decreases.

Question 20

Compare the bonding and structures of the poly(propene), silicon dioxide and propanol [4]

Answer 20

Bonding:
Poly(propene): covalent bonding in C—C and C—H
Propanol: covalent bonding in C—C, C—H, C—O and O—H
Silicon dioxide: covalent bonding betw Si and O atoms

Structure:
Poly(propene): Long-chained molecules that intertwine tgt
Propanol: simple molecules of C3H7OH
Silicon dioxide: a silicon atom is bonded to 4 oxygen atoms in a tetrahedral manner that extends to form a giant molecular structure.

Question 21

Explain why the melting points of the 3 compounds differ from each other. [4] (Silicon dioxide, propanol and poly(propene)

Answer 21

Propanol has the lowest melting point because very small amount of energy is required to overcome the very weak intermolecular forces of attraction betw atoms.

Silicon dioxide has the highest melting point because very large amount of energy is required to break the strong covalent bonds betw atoms.

Melting point of poly(propene) is higher than propanol because more energy is required to overcome the intermolecular forces of attraction betw long-chained molecules than that betw simple propanol molecules.

Question 22

Explain why neither the ‘dot-and-cross’ diagram nor the ball and stick model is an accurate representation of an ethanol molecule. [3]

Answer 22

1. The ‘dot-and-cross’ diagram does not show the length of bond betw the atoms.
2. The ‘dot-and-cross’ diagram does not show the 3 dimensional arrangement of the atoms in the ethanol molecule.
3. The ball and stick model does not show the differences in atomic radii of the atoms.
4. The ball and stick model does not show the valence electrons which r not involved in bonding in the oxygen atom.

Question 23

Using ideas about structure and bonding to explain the differences in melting point and electrical conductivity of lithium, graphite and oxygen. [5]

Answer 23

Melting points:
1. O2 has the lowest melting point as it is a simple molecule with weak intermolecular forces of attraction which require little energy to overcome.
2. Lithium has a metallic structure that has strong electrostatic forces of attraction betw positively charged cations and ‘sea of delocalised mobile electrons’ which require more energy to overcome
3. Graphite has a macro molecular structure with strong covalent bonds betw its atoms. Since a lot of energy is needed to overcome the forces of attraction, it has very high melting point.

Electrical conductivity:
1. There r no mobile electrons in oxygen to conduct electricity. Hence, it does not conduct electricity in any state.
2. There is a ‘sea of delocalised mobile electrons’ to conduct electricity in lithium
3. Each carbon atom in graphite is covalently bonded to 3 carbon atoms, leaving 1 mobile electron per carbon atom to conduct electricity.

Question 24

Conduction of electricity can have a different effect on metals and on solutions of ionic compounds. Describe this difference.

Answer 24

When electricity is passed thru, solutions of ionic compounds can be decomposed into their constituent elements but metals will not be affected.

Question 25

Electrical conductivity of molten ionic compounds

Answer 25

Depends on the number of mobile ions and not the number of delocalised mobile electrons.