Atomic Structure
ICSE Class 8 · Chemistry · Chapter 4
What this chapter covers
- Atoms: the building blocks of matter
- Discovery of subatomic particles
- Electrons, protons and neutrons
- Nucleus and extranuclear part
- Atomic number
- Mass number
- Valency and the hydrogen atom
Key definitions
Atomic structure is the arrangement of the smaller particles, namely electrons, protons and neutrons, that make up an atom.
Subatomic particles are the particles smaller than an atom from which every atom is built.
Key points
- Atoms are the building blocks of all matter; the atom of an element takes part in chemical reactions.
- An atom is made of three subatomic particles: electrons (-1), protons (+1) and neutrons (no charge).
- J. J. Thomson discovered the electron (1897); Goldstein observed positive rays (1886) and Rutherford identified the proton; Rutherford discovered the nucleus (1911); Chadwick discovered the neutron (1932).
- Protons and neutrons each have a mass of about 1 unit; an electron's mass is about (1)/(1840) of that.
- The nucleus is a tiny, dense, positive centre containing protons and neutrons.
- The extranuclear part is the region outside the nucleus where electrons move in shells; it makes up almost all of the atom's size.
- Atomic number Z = number of protons = number of electrons in a neutral atom.
- Mass number A = number of protons + number of neutrons; number of neutrons = A - Z.
- The short form AZX shows the mass number at the top left and the atomic number at the bottom left.
- Valency is the number of hydrogen atoms that combine with one atom of an element or one radical.
Questions answered
Name the three fundamental subatomic particles and give the charge on each.
Electron, charge -1; proton, charge +1; neutron, no charge (0).
Which scientist discovered the electron, and what did his experiment show about cathode rays?
J. J. Thomson. He showed that cathode rays bend towards a positive plate, so they are made of negatively charged particles, and that these particles are much lighter than any atom.
In the gold foil experiment, most particles passed straight through the foil. What did this tell Rutherford about the atom?
Most of the space inside an atom is empty. The few particles that bounced back showed that there is a tiny, heavy, positive nucleus at the centre.
Why was the neutron the last of the three particles to be discovered?
It has no charge, so it is not bent by charged plates and does not show up in the electrical experiments that revealed electrons and protons.
State one difference between the nucleus and the extranuclear part of an atom.
The nucleus contains protons and neutrons and is positively charged; the extranuclear part contains electrons and is negatively charged. (Also accept: the nucleus holds almost all the mass but takes up very little space.)
Define atomic number. What is the atomic number of an atom with 12 protons?
The atomic number is the number of protons in the nucleus of an atom. An atom with 12 protons has atomic number 12 (magnesium).
Why does a neutral atom have equal numbers of protons and electrons?
Each proton has a charge of +1 and each electron -1. The atom has no overall charge only when the positive and negative charges cancel, which needs equal numbers of each.
An atom of calcium is written 4020Ca. Find the number of protons, electrons and neutrons.
Protons = 20, electrons = 20, neutrons = 40 - 20 = 20.