# 2.3.1 The Structure of Atom

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## The Modern Atomic Model

According to the modern atomic model,
1. The central nucleus consists of protons and neutrons. It containing almost all the mass of the atom.
2. the nucleus of an atom is very small compared to the size of the atom
3. the electrons are orbiting outside the nucleus in the electron shells
4. the electrons are moving in electron shells at a very high speed and we cannot determine the position of the electrons at a particular time

## The Subatomic Particles of an Atom​

1. Atoms are made up of tiny particles called subatomic particles.
2. An atom contains three types of subatomic particles:
1. proton,
2. neutron and
3. electron,
3. The proton and neutron form the nucleus at the centre of an atom. They are also called the nucleon of an atom.
4. The electron moves around the nucleus at a very high speed.
5. The nucleus is positively charged because of the presence of protons, which are positively charged. The neutrons are neutral.
6. The symbols, charge and relative masses of proton, neutron and electron are as below.
 Particle Symbol Relative charge Relative mass Proton p +1 1 Neutron n 0 1 Electron e -1 1/1840

## The Charge of Particles

1. A neutral atom contains the same number of electrons as the protons.
2. The positive and negative charges of the protons and electrons respectively neutralise each other, for example, (+4) + (-4) = 0
3. If the number of protons is greater than the number of electrons, the particle is positively charged.
4. If the number of protons is greater than the number of electrons, the particle is positively charged.

Example:

 Number of protons Number of electrons Charge 3 3 0 5 2 +3 9 10 -1 11 10 +3 16 18 -2 17 18 -1 20 18 +3

## Proton Number and Nucleon Number

1. Proton number = the number of protons
2. Nucleon number = Number of protons + Number of neutrons

### Proton Number

1. The proton number (Z) represent the number of protons found in the nucleus of an atom.
2. Proton number = the number of protons
3. The proton number is also known as the atomic number.
4. In an atom of neutral charge, the number of electrons also equals the atomic number.
5. Hence, the proton number of an atom can also represent the number of electrons.

### Nucleon Number

1. The nucleon number (A), also called atomic mass number or mass number, is the number of protons plus the number of neutrons in an atomic nucleus. (Nucleon number = Number of protons + Number of neutrons)
2. The nucleon number of an atom is about the same as the mass of the atom because the mass of an electron is very small and can be ignored.
 Atom Proton Number Nucleon Number Amount of Proton Amount of Neutron Helium 2 4 2 2 Oxygen 8 16 8 8 Sodium 11 23 11 12 Chlorine 17 35 17 18
[Notes: In ion, the amount of protons IS NOT equal to the amount of electrons]