2.2 Atomic Structure

Historical Development of Atomic Models
■ This video contains information about historical development of atomic models.

■ Atomic models introduced by scientists:

Scientist & Atomic model Explanation ► Suggested that all matter is composed of atoms, indivisible and indestructible particles. ► All atoms of a particular element are identical. ► Atoms of different element are different in size and mass. ► When elements react, their atoms combine in a simple whole-number ratios. ► Suggested that the model of an atom as a sphere of positively charged matter with negatively charged electrons surrounding them. ► Suggested that most of the mass of the atom is contained in the small nucleus, and that the rest of the atom is mostly empty space. ► Electrons circled the nucleus at high speed and forming a negatively charged cloud. ► This video contains information about Rutherford's model of atom. ► Proposed that electrons travel around the nucleus in fixed orbits or shells. ► Each orbit is at a fixed distance from the nucleus in the circle shape. ► This video contains information about Neils Bohr's atomic atom. ► He discovered electrically neutral subatomic particles which are called neutrons. ► Neutrons always reside in the nucleus of atoms and they are about the same size as protons. ► All nuclei contain neutrons and protons except for hydrogen, which contain protons only.

Subatomic Particles
■ Tiny particles that made up atoms.
■ There are three types of subatomic particles:

	(a)	Proton
	(b)	Neutron
	(c)	Electron

■ This video contains information about subatomic particles.

■ Comparison between subatomic particles.

	►	The proton and neutron form the nucleus at the centre of an atom.
	►	The electron moves around the nucleus at a very high speed.
	►	The mass of the atom is concentrated at the nucleus.
	►	The animation below shows the characteristics comparison of subatomic particles.

■ For a neutral atom,

	►	Number of protons = Number of electrons
	►	Number of positive charges = Number of negative charges

The Changes in the States of Matter
■ Definition: Proton number is the number of protons in the atom of an element.

	(a)	Proton number = number of protons
	(b)	The number of protons and electrons of atom of different elements is different.

■ Definition: Nucleon number is the total number of protons and neutrons in the nucleus of an element's atom.

	(a)	Nucleon number = number of protons + number of neutrons.
	(b)	Nucleon number = proton number + number of neutrons.

■ Symbol of atom for an element:

	(a)	The standard representation for an atom shows the proton number and the nucleon number.
	(b)	he animation below shows proton number, nucleon number and symbol of an atom.

✍ Worked-example 2.2(a)
Complete the following table on the relationship between number of of subatomic particle with proton number and nucleon number.

Symbol	Nukleon number	Proton number	Number of protons	Number of electrons	Number of neutrons
$\begin{array}{c}1\\ 1\end{array}H$	1	1	1	1	0
$\begin{array}{c}4\\ 2\end{array}\mathrm{He}$	4	2	2	2	2
$\begin{array}{c}12\\ 6\end{array}C$	12	6	6	6	6

✍ Worked-example 2.2(b)
Choose true [✔] or false [✘] for the statement given.

Statement
Proton and neutrons form the nucleus at the centre of the atom.	✔
An electron is of the same mass as a proton and a neutron.	✘
The proton number is the amount of protons in an atom.	✔
The nucleon number is the amount of neutrons in an atom.	✘

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