The table below shows the properties and location where the subatomic particles are found. It is important to know the properties of the subatomic particles and its location. °C
|Proton||+ 1||1||Nucleus of Atom|
|Electron||– 1||1/1840||Electron Shell / Orbital|
What you see in the periodic table for an element will be denoted as:
where a is the proton (atomic) number, b is the mass (nucleon) number and X is the chemical symbol of the element.
Proton (Atomic) Number is the number of protons the element has in the nucleus of an atom.
Mass (Nucleon) Number is defined as the total number of protons and neutrons in the nucleus of an atom.
As an atom is electrically neutral (having an overall charge of zero), the number of electrons must be equal to the number protons in an atom.
An element only has one proton number. The number of protons gives the identity of the element. Therefore, to identify the element of a particle (atom or ion), look at the number of protons the particle has.
A common mistake is mixing up between proton number and mass number. Therefore, when referring to the periodic table, check the legend too to see you are looking at the right number. The periodic table is your best friend in chemistry examinations apart from your calculator.
The protons and neutrons of an atom is found in the nucleus of the atom, which is surrounded by electron shell(s) containing electrons. An example of a lithium atom is shown below:
Question: Which statement about an atom is incorrect?
- Atoms of an element can have different nucleon (mass) number.
- If a proton is added to the nucleus of the atoms of an element, the element changes into a different element.
- The nuclei of all atoms contain both protons and neutrons.
- The proton number is always less than or equal to the nucleon (mass) number.
|a) True. Some elements have isotopes (atoms of the same element with same proton number but different mass number).
b) True. An element only has one proton number. If the number of protons in the atom changes, the identity of the atom changes as well, according to the proton number.
c) False. Refer to the periodic table that hydrogen has a mass number of 1, and proton number of 1. Thus, its mass number is due only that 1 proton, and 0 neutrons.
d) True, as shown in the periodic table. Except for hydrogen where proton number is equal to its mass number, the rest of the elements has its proton number lower than their respective mass number.
Isotopes are atoms of the same element carrying the same proton number but different mass number. Thus, they have different number of neutrons in the nucleus of the isotopes. For example, carbon is made up of isotopes of carbon-12, carbon-13 and carbon-14. The relative atomic mass of carbon in the periodic table is the average mass of all the carbon isotopes according to their relative abundance in nature.
Question: Do isotopes of the same element have different reactions?
The answer is no, they have the same reactions. This is because reactivity is dependent on the number of valence electrons. For isotopes, they have the same proton number, thus they have the same number of electrons, and hence, the same chemical reactivity.
Question: Deuterium, , is an isotope of hydrogen. Which statement about deuterium is false?
- Its diatomic molecules diffuse at a faster rate than that of hydrogen.
- It has a higher density as hydrogen.
- It has the same number of protons but different number of neutrons as hydrogen.
- It undergoes the same chemical reactions as hydrogen.
|a) False. Deuterium is heavier than hydrogen. As speed of diffusion decreases with increasing mass of gas, deuterium molecules diffuse slower than that of hydrogen.
b) True. As density is equals to mass / volume, for the same volume of deuterium and hydrogen, the density of deuterium is higher than hydrogen as it is heavier.
c) True. The definition of isotopes are atoms of the same element with same proton number but different mass number, and thus different number of neutrons.
d) True, isotopes undergo the same chemical reactions.
Chemical Bonding: Melting Point
Since covalent compounds have low melting point, is it right to conclude that covalent bonds are weak?
The answer is no. A common misconception that students have is that the covalent bonds are weak since covalent compounds have low melting point, which is false. Covalent bonds are very strong due to the sharing of electrons between atoms. The reason for the low melting point of covalent compounds is due to the weak intermolecular forces of attraction between molecules, known as Van der Waals forces.
When a question requires an explanation relating to melting or boiling points of substances, answer using SAEC (Structure – Attraction – Energy – Conclusion) format. Below is a table showing a summary on how to tackle a question asking to explain the high melting point of different compounds.
|Substance||Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
Structure the substance adopts.
|Giant ionic lattice||Simple covalent structure||Extensive/ Giant network of covalent bonds||Giant metallic lattice|
Strong or weak attractive forces between atoms or ions etc.
|Strong electrostatic forces of attraction between oppositely charged ions||Weak intermolecular forces of attraction (Van der Waals) between molecules||Strong covalent bonds between atoms||Strong electrostatic forces of attraction between positive ions and sea of delocalised electrons|
Energy required to overcome the attractive forces.
|A lot of energy is required to overcome these strong electrostatic forces of attraction.||Less energy is required to overcomes these weak Van der Waals forces between molecules.||Large amount of energy is required to overcome these strong covalent bonds between atoms.||Large amount of energy is required to overcome these strong electrostatic forces of attraction.|
Linking it back to the question.
|Thus, (name of substance) has a high/low melting/boiling point.|
Sometimes, the question may ask to compare the melting points of two different substances. Similarly, SAEC format needs to be used to explain the difference in melting points.
Question: Explain why MgO have a higher melting point than H2O.
Common error that students make would be just stating that MgO is an ionic compound while H2O is a simple covalent molecule, without explaining further.
To obtain full marks, elaborate further;
MgO is an ionic compound with strong electrostatic forces of attraction between Mg2+ and O2- ions in its ionic lattice structure, while H2O is a simple covalent molecule with simple covalent structure and have weak intermolecular forces of attraction between molecules. As a larger amount of energy is required to overcome the strong electrostatic forces of attraction in MgO than the weak intermolecular forces in H2O, MgO thus has a higher melting point than H2O.
Chemical Bonding: Electrical Conductivity
|State||Can it conduct electricity?|
|Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
|Liquid or Molten||Yes||No||No||Yes||Yes|
|Aqueous (Dissolved in water)||Yes||Most covalent compounds are insoluble except for *hydrogen halides. Therefore, hydrogen halides dissolved in water (aqueous hydrogen halide) can conduct electricity.|
*In the table above, most simple covalent molecules are insoluble in water – except for hydrogen halides. Aqueous solutions of hydrogen halides are known as hydrohalic acids (hydrochloric acid, hydrofluoric acid etc).
Note: Pure water does not conduct electricity. However, presence of impurities may affect its electrical conductivity. For example, if the impurity is an ionic salt, then the impure water can conduct electricity. On the other hand, if the impurity is a simple covalent substance that is not a hydrogen halide, then the water is still not able to conduct electricity.
When a question tests the concept of electrical conductivity of substances, the keywords examiners are looking out for are shown below:
|Electrical conductors||Presence of mobile charge carriers||
|Non-electrical conductors||Absence of mobile charge carriers||
Question: Silicon carbide has the same structure as diamond. Predict the electrical conductivity of silicon carbide and explain.
Silicon carbide has a tetrahedral structure where all carbon and silicon atoms use all of its valence electrons for covalent bonding. As there are no free electrons that can act as mobile charge carriers, silicon carbide is not able to conduct electricity.