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# Structure of the Atom, and the Periodic Table: Atomic Number and Mass Number

### 1.0 Structure of the Atom, and the Periodic Table

1.2 Atomic Number and Mass Number

Atomic number (A)

Each atom has protons in its nucleus. The number of protons in an atom is called atomic number (symbol = A). Atomic number is the number of protons in the nucleus of an atom. It is the atomic number that makes elements different. Atoms of the same element have the same number of protons or atomic number.

Questions 1.2

1. State the atomic numbers of the atoms A to E (not the actual symbols) in Figure 1.2.1.
2. Figure 1.2.1: Atomic numbers

3. Group together the atoms which belong to (or represent) the same element. Use the letters only.

Mass number (M)

Protons, neutrons and electrons are extremely small and light. Their masses are too small to measure directly using a weighing balance. Because they have nearly the same mass, each proton and neutron is assigned a mass of 1 unit. But, being much lighter, an electron is assigned a mass of 0 unit; so its mass is ignored.

Table 1.2.2: Assigned (relative) masses of sub-atomic particles

Because the actual mass of an atom depends on the number of protons and neutrons in it, we can conveniently represent the mass with the number of protons and neutrons, and call it mass number. Mass number is the sum of number of protons and neutrons in an atom. An atom with one proton and one neutron, for example, has a mass number of 2; that is, 1+1.

Questions 1.3(a)

1. An atom X has 8 protons and 10 neutrons. What is its mass number?
2. Element Y has 4 neutrons and a mass number of 7. Find its atomic number.