Summary of what I learnt ( Periodic Table)

In the periodic table, the elements are arranged in order of increasing proton (atomic) number, and are classified according to groups and periods.

Group – a vertical set of elements

Period – a horizontal row of elements

Group:

The groups in the Periodic Table are numbered from I to IV and then Group 0. Some of these groups have names too:

Group I: Alkali metals

Group II: Alkaline earth metals

Group VII: Halogens

Group 0: Noble gases

Elements between Group II and III are known as transition metals or transition elements. Element in the same group will have similar chemical properties and will undergo the same type of chemical reactions.

Period:

Each period is numbered, 1, 2, 3 etc. Elements in the 1^st period will only have their 1^st shell fully/partially occupied with electrons. Elements in the 2^nd period will have their 1^st shell fully occupied with electrons, and their 2^nd shell fully/ partially occupied with electrons.

Patterns in the Periodic Table:

Electronic Structure

Down each group, the number of valence electrons is the same for each element and is equal to the group number.

Example: Group I Elements

Li = 2.1

Na = 2.8.1

K = 2.8.8.1

Metals and Non-metals

Across the period, the properties of elements change from metallic to non-metallic. Generally, elements with small number of electrons in the valence shells (eg. Group I and Group II) are metals. Elements with large number of electrons in the valence shells (eg. Group VII and 0) are non-metals.

The line that divides metals from non-metals runs diagonally through the Periodic Table. Elements found beside this dividing line are known as metalloids. Metalloids have the same properties of non-metals and metals.

Charges on ions

[Refer to the pictures above]

Charges on the ions formed are related to the group number and number of valence electrons. Elements are on the left side of the Periodic Table lose their valence electrons to form cations with charges corresponding to their group number. Elements on the right side of the Periodic Table gain electrons to form anions. The charges on the anions are corresponding to the number of electrons gained to fill their valence shells with 8 electrons.

Summary of what I learnt (Atomic Structure)

What is an atom?

An atom is the smallest unit of an element, having the properties of that element. Each element consists of only one particular type of atom. In other words, the atoms of one element will be different from the atoms of another element. Atoms are not like solid balls as proposed by Dalton in 1803. In fact, atoms are made up of even smaller particles known as subatomic particles.

Subatomic particles:

These subatomic particles comprise of:

-proton (positively charged)

-neutron (neutral – no charge)

-electron (negatively charged)

Relative Mass:

-proton = 1

-neutron = 1

-electron = 1/186

Take note that the mass of proton, neutron and electron are too small and inconvenient to work with. By using relative mass, we do not have to remember the exact value of the various mass.

Counting subatomic particles:

The number of protons in an atom is called the proton number. Proton number is also known as the Atomic number.

Nucleon number is the number of protons and neutrons in the nucleus of an atom. It is also called the Mass number.

Seeking for proton number and nucleon number:

Proton number and nucleon number of an element can be found together with the symbol of the element in the Periodic Table. (Refer the pictures above) The carbon atom has 6 protons, 6 electrons, and 6 neutrons.

Atomic Model:

The centre of an atom is called the nucleus which contains the protons and neutrons. The electrons in an atom are arranged in shells (orbits) at different distances from the nucleus. The shell nearest to the nucleus is numbered 1, the second nearest shell to the nucleus is numbered 2 and so on. The nucleus is positively charged as it contains protons and neutrons. Protons are positively charged while neutrons are electrically neutral.

Note: Shells are also called energy levels.

Each shell can hold a certain maximum number of electrons.

-1^st shell: 2 electrons

-2^nd shell: 8 electrons

For the 1^st 20 elements, the maximum number of electrons that can go into the third shell is 8. For elements after calcium in the 4^th period, their 3^rd shell can hold up to 18 electrons.

How to differentiate the atoms of 1 element from the atoms of another element:

All atoms of the same element have the same number of protons while those of different elements contain different number of protons.

[Advanced] Isotopes:

Isotopes are atoms of the same element with different numbers of neutrons. All atoms of the same element have the same proton number but different nucleon number. For instance, carbon has 3 isotopes: carbon-12, carbon-13, carbon-14

Arrangement of electrons in atoms:

Electronic structure and electronic configuration

Nitrogen-14 atom has 7 electrons. 2 of its electrons will go into the 1^st shell, the remaining electrons will go into the 2^nd shell. (Refer to picture). With 7 electrons, nitrogen has the electronic configuration of 2.5.

Argon-40 atom has 18 electrons. 2 of its electrons will go into the 1^st shell, 8 electrons will go into the 2^nd shell, and the remaining 8 electrons will go into the 3^rd shell. (Refer to picture). With 18 electrons, argon has the electronic configuration of 2.8.8.

Valence shell

The shell which is farthest from the nucleus and occupied by electrons is called the valence shell (outer shell).The electrons in the valence shell are known as valence electrons (outer electrons). In a chemical reaction, only these valence electrons are involved chemical bonding between atoms.

Take note that if the question only asks you to draw the outer electronic structure, you are to only draw the valence electrons in the electronic structure.

Formation of ions:

During chemical reactions, some atoms might lose/gain electron(s). Atom becomes an ion (charged particle) when it gains or loses electron(s). An atom is electrically neutral because the number of electrons is the same as the number of protons, thus when it gain/loses electrons, the positive and negative charges are not balanced. Therefore, the atom becomes a charged particle.

Formation of cation:

[Refer to the pictures above]

When an atom loses 1 or more electrons, it becomes a positively charged particle called cation. In a lithium atom, there are 3 protons and 3 electrons. In a lithium ion, there are 3 protons and 2 electrons. Therefore, the lithium ion carries an overall positive charge of 1+ and is written as Li⁺.

Formation of anion:

[Refer to the pictures above]

When an atom gains 1 or more electrons, it becomes a negatively charged particle called anion. In a fluorine atom, there are 9 protons and 9 electrons. In a fluoride ion, there are 9 protons and 10 electrons. Therefore, the fluoride ion carries an overall positive charge of 1- and is written as F^-.

*Note that metals generally form cations while non-metals generally form anions.

Summary of what I learnt (Chemical Bonding)

Why do atoms bond?

Noble gases, such as helium, neon and argon, are monoatomic because their valence shells are fully occupied by electrons. Thus, noble gases are stable and do not undergo bonding with other atoms. Since atoms with electronic configuration of noble gases are stable, atoms bond to achieve electronic configuration of a noble gas. Atoms do so by transfer or sharing of electrons with other atoms.

By having an electronic configuration of noble gases, atoms will achieve stability.

-When you have 2 electrons in the 1^st shell, you have a duplet structure

-When you have 8 electrons in the rest of the shells, you have an octet structure

Chemical bonds:

There are three ways of forming chemical bonds between atoms:

1) Ionic bonding

2) Covalent bonding

3) Metallic bonding (To be covered in Sec. 3)

Ionic bonding:

Ionic bonding is the electrostatic force of attraction between oppositely charged ions in an ionic compound. It is usually formed between metals and non-metals.

Naming of ionic compounds:

(name of cation) (space) (name of anion)

Formation of ionic bonds:

1) Using the formation of sodium chloride (refer to the pictures above) as an example:

The sodium atom loses 1 electron to form a positive ion (cation) in order to obtain an octet structure. The chlorine atom gains the electron from sodium atom to form a negative ion (anion) in order to obtain an octet structure. The ions have opposite charges and therefore attract each other to form an ionic compound known as sodium chloride. This attraction force is called ionic bond.

Sodium atom = 11 protons and 11 electrons

Sodium ion = 11 protons and 10 electrons

Therefore, the sodium ion carries an overall positive charge of 1+ and is written as Na⁺.

2) Using the formation of magnesium chloride (refer to the pictures above) as an example:

The magnesium atom gives up 2 electrons to form a magnesium ion, Mg²⁺. These 2 electrons are transferred to 2 chlorine atoms to form 2 chloride ions, Cl^-. The magnesium chloride has the formula MgCl_2-.

Chlorine atom = 17 protons and 17 electrons

Chloride ion = 17 protons and 18 electrons

Therefore, the chloride ion carries an overall positive charge of 1- and is written as Cl^-.

Steps to draw ‘dot-and-cross’ diagrams of ionic compounds:

1) Draw the valence electrons of the elements using ‘Dot and Cross’ only.

2) Electron(s) is/are transferred from the valence shell of the metal to the valence shell of the non-metal.

3) The anion that had gained electron(s) from the cation will now have 2 types of electrons: 1 originally from its valence shell, another 1 from the cation, and the 2 different type of electrons are differentiated by ‘dots’ and ‘crosses’.

4) Indicate the charges of the ions

Examples of ‘dot-and-cross’ diagram can be found in the pictures above.

Covalent bonding:

A covalent bond is a bond formed by the sharing of a pair of electrons. Some atoms form bonds by sharing electrons to gain electronic configuration of a noble gas. The bonds formed are known as covalent bonds. Generally, covalent bonds are formed between atoms of non-metal. It can also be formed between atoms of same elements or between atoms of different elements. Compounds which contain covalent bonds are known as covalent compounds.

Single covalent bond- 1 pair of shared electrons between 2 atoms

Double covalent bond – 2 pairs of shared electrons between 2 atoms

Triple covalent bond – 3 pairs of shared electrons between 2 atoms

Covalent bonding in elements:

1) Hydrogen molecules, H₂ (Refer to the pictures above)

A hydrogen atom has 1 valence electron. It requires 1 more electron to obtain the electronic configuration of a noble gas.

In order to obtain the electronic configuration of helium, 2 hydrogen atoms can share a pair of electrons between themselves to form hydrogen molecules, H₂. In order to differentiate the 2 electrons, the electron of 1 atom is represented by a ‘cross’ while the other electron of another atom is represented by a ‘dot’.

2) Oxygen molecules, O₂ (Refer to the pictures above)

In order to obtain the electronic configuration of a noble gas, each oxygen atom requires 2 more electrons. Instead of sharing a pair of electrons, 2 oxygen atoms can share 2 pairs of electrons to form a double bond and obtain an octet structure.

Covalent bonding in compounds:

[Refer to the pictures above]

The carbon atom has 4 valence electrons and it needs 4 more electrons to gain an octet structure. The carbon atom can share its 4 electrons with 4 other hydrogen atoms, forming a single covalent bond with each of the hydrogen atom.

Valency:

The number of electrons an atom uses to form bonds is called its valency.

Hydrogen: 1 valency

Oxygen: 2 valencies

Aluminum: 3 valencies

Carbon: 4 valencies