atom 1222516 1920 Atomic Structure

Atomic Structure


An atom is the smallest particle of matter that cannot be broken down by chemical means. Each atom consists of a nucleus and a number of particles called electrons that orbit around it. The nucleus is itself a cluster of two kinds of particles, protons and neutrons. The diagram below shows how an atom looks like, magnified many millions of times. The electrons are shown in blue, protons in red and neutrons in green.


Subatomic particles

Electron 1/1840 negative (-1) shells
Proton 1 positive (+1) nucleus
Neutron 1 neutral (0) nucleus

The protons and neutrons cluster together in the centre, forming the nucleus; this is the heavy part of the atom. the electrons circle very fast around the nucleus, at different energy levels from it; these energy levels are called shells. Since electrons are so light, their mass is usually taken to be zero. Since they make up the atom, protons, neutrons and electrons are often called sub-atomic particles.

Proton number and nucleon number

Every atom has a different number of protons. You can identify an atom by the number of protons in it. The number of protons in an atom is called its proton number and is given symbol Z. Every atom has an equal number of protons and electrons. So atoms have no overall charge. Protons and neutrons form the nucleus, so are called nucleons.

The total number of protons and neutrons in an atom is called its nucleon number and is given symbol A.

The nuclide notation is represented as follows:
Nuclide whereby X is the chemical symbol of the element.

nucleon number (A) = proton number (Z)+ number of neutrons (N)


Electronic configuration

Electrons are arranged in shells around the nucleus. The first shell carries a maximium of 2 electrons, the second shell a maximum of 8 electrons and the third shell a maximum of 8 electrons as long as the proton number does not exceed 20.

Electronic Configuration

Hydrogen, the simplest atom, has 1 electron and so it has 1 shell.

Here are the first 20 elements, shown as a list:

Element list

Element Symbol Proton number Number of electrons Electron structure
Hydrogen H 1 1 1
Helium He 2 2 2
Lithium Li 3 3 2,1
Beryllium Be 4 4 2,2
Boron B 5 5 2,3
Carbon C 6 6 2,4
Nitrogen N 7 7 2,5
Oxygen O 8 8 2,6
Fluorine F 9 9 2,7
Neon Ne 10 10 2,8
Sodium Na 11 11 2,8,1
Magnesium Mg 12 12 2,8,2
Aluminium Al 13 13 2,8,3
Silicon Si 14 14 2,8,4
Phosphorus P 15 15 2,8,5
Sulfur S 16 16 2,8,6
Chlorine Cl 17 17 2,8,7
Argon Ar 18 18 2,8,8
Potassium K 19 19 2,8,8,1
Calcium Ca 20 20 2,8,8,2

The Periodic Table

The Periodic Table is a vital tool used by chemists to predict the way in which elements react during chemical reactions. It is a method of categorising elements according to their properties.

The period number tells you how many shells there are. All the elements in a group have the same number of electrons in their outer shells. So Group I elements have 1, Group II have 2, and so on. These outer-shell electrons are also called the valency electrons. The group number is the same as the number of outer-shell electrons. The valency electrons dictate how an element reacts. So the elements in Group I all have similar reactions, for example.

Group Number
1 2 3 4 5 6 7 8
Period 1 11H 24He
Period 2 37Li 49Be 511B 612C 714N 816O 919F 1020Ne
Period 3 1123Na 1224Mg 1327Al 1428Si 1531P 1632S 1735.5Cl 1840Ar
Period 4 1939K 2040Ca


Some atoms of the same element can contain different numbers of neutrons and so have different nucleon numbers. Atoms of the same element which have the same proton number but different neutron numbers are called isotopes.

Some common isotopes
Chlorine-35 17 17 18 35
Chlorine-37 17 17 20 37
Carbon-12 6 6 6 12
Carbon-14 6 6 8 14

As you can see,

  • Cl-35 and Cl-37 are isotopes of element Cl.
  • C-12 and C-14 are isotopes of element C.

Since isotopes have the same number of electrons, they have the same proton number and the same chemical properties. Carbon-12 and carbon-14 react the same way. However they have diferent physical properties i.e. Chlorine-37 is heavier than Chlorine-35 for the same volume.


The nuclide notations of elements X and Y are given below.

2311X and 3517Y

(i) State the number of protons and neutrons in element X. [2] (ii) Give the nuclide notation for a possible isotope of Y. [2] (iii) Write the electronic configuration of element X. [1]


  1. 11 electrons 12 neutrons
  2. 3717Y
  3. 2.8.1


An ion is an electrically charged particle. When an atom loses one or more electrons it becomes a positively charged ion. For example, during the chemical reactions of potassium, each atom loses an electron to form a positive ion, K+