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.