All elements are arranged in the periodic table of the elements in order according to their atomic number;
The atomic number equals the number of protons in the nucleus, which is the same as the number of electrons in an electrically balanced (neutral) atom;
For example, hydrogen has an atomic number of 1 and helium has an atomic number of 2;
In their normal (or neutral) state, all atoms of a given element have the same number of electrons as protons;
The positive charges cancel the negative charges, and the atom has a net charge of zero;
Atomic numbers of all the elements are shown on the periodic table of the elements in Figure 1–3;
Niels Bohr proposed that the electrons in an atom circle the nucleus in different orbits, similar to the way planets orbit the sun in our solar system;
The Bohr model is often referred to as the planetary model;
Another view of the atom called the quantum model is considered a more accurate representation, but it is difficult to visualise;
For most practical purposes in electronics, the Bohr model suffices and is commonly used because it is easy to visualise;
An atom is the smallest particle of an element that retains the characteristics of that element;
Each of the known 118 elements has atoms that are different from the atoms of all other elements;
This gives each element a unique atomic structure;
According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons, as illustrated in Figure 1–1;
The nucleus consists of positively charged particles called protons and uncharged particles called neutrons;
The basic particles of negative charge are called electrons;
Each type of atom has a certain number of electrons and protons that distinguish it from the atoms of all other elements;
For example, the simplest atom is that of hydrogen,
which has one proton and one electron, as shown in Figure 1–2(a);
As another example, the helium atom, shown in Figure 1–2(b), has two protons and two neutrons in the nucleus and two electrons orbiting the nucleus;
All matter is composed of atoms;
All atoms consist of electrons, protons, and neutrons except for normal hydrogen which does not have a neutron;
Each element in the periodic table has a unique atomic structure;
All atoms of a given element have the same number of protons;
At first, the atom was thought to be a tiny indivisible sphere;
Later it was shown that the atom was not a single particle but was made up of a small dense nucleus around which electrons orbit at great distances from the nucleus, similar to the way planets orbit the sun;
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