Atomic Structure

What is an atomic model ? An atomic model is a model that was first developed to describe the building blocks of matter. Various atomic models where created by many scientists to describe the structure of an atom as well as its composition. A combination of the two latest atomic models(Bohr and quantum model) is used at A-level physics for simplicity, where an atom is comprised by a nucleus and negatively charged electrons orbiting the nucleus.

Atomic structure The atomic structure is made out of two parts as seen in figure 1 below.

Nucleus: The nucleus is a very small and dense portion of the atom containing a very large proportion of the atom s mass. It is comprised of protons and neutrons, which are subatomic particles made of quarks, that are held together by a strong interaction force. Protons are positively charged particles while neutrons have a neutral charge. Therefore, the nucleus has a positive charge.

Electrons are negatively charged particles with very small mass, which are revolving around the nucleus in a circular orbit. Each orbital or shell usually contains a fixed number of electrons, which depends on the total number of shells/ orbital.

How many orbiting electrons are in an atom?

Equation 1 below, can be used to calculate the total number of orbiting electrons in an atom, where n is the total number of shells/orbitals in an atom. Orbiting electrons can occupy more than one shell, only if the previous one is filled.

E = 2n²

Equation 1. Orbiting electrons in an atom

Example 1:

A. 1. What is the maximum number of orbiting electrons in an atom with 1 shell ?

E = 2n2 = 2*1² = 2 max orbiting electrons

A. 2. What is the maximum number of orbiting electrons in an atom with 2 shells ?

B. E = 2n2 = 2*2² = 8 max orbiting electrons

What is the maximum number of electrons in each shell?

There is fixed maximum number of orbiting electrons for each shell as seen in example 1. Specifically example 1a shows that the first shell can only hold two electrons. Since orbiting electrons can occupy more than one shell, only if the previous one is filled, then the maximum number of orbiting electrons for each shell will depend on the number of electrons in the previous shell . More specifically, the number of orbiting electrons in the previous shells will be subtracted from the total orbiting electrons in the atom as shown in steps in example 2.

Example 2:

1^st shell : 2 electrons .

2^nd shell : 6 electrons .

Find total orbiting electrons in the atom (8) , then find the orbiting electrons in previous shell (2). Then subtract 8-2=6.

3^rd shell : 10 electrons.

Find total orbiting electrons in an atom with 3 shells (18) ,then find electrons in 1^st shell(2) and electrons in 2nd shell(6). Finally subtract 18-2-6=10.