A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Theory and principles
CSCA Chemistry · Lesson 12 of 21Atoms and the periodic table
Atomes et tableau périodique · الذرات والجدول الدوري
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Everything is made of atoms. The periodic table is organised by atomic number, and an element's position tells you how it behaves.
Key ideas
- An atom has protons and neutrons in the nucleus, and electrons around it.
- Atomic number Z = number of protons. Mass number A = protons + neutrons.
- Isotopes have the same Z but different numbers of neutrons.
- Elements in the same group (column) have the same number of outer electrons and similar properties.
Formulas to know
- Neutrons = A − Z
- A neutral atom has as many electrons as protons
- Group 1: alkali metals · Group 17: halogens · Group 18: noble gases
Worked example
Carbon-14 has Z = 6. How many neutrons does it have?
- Mass number A = 14.
- Neutrons = A − Z = 14 − 6.
Answer: 8 neutrons
Practice questions
1. An atom has 11 protons and 12 neutrons. Its mass number?
11 · 12 · 23 ✓ · 1
A = 11 + 12 = 23 (this is sodium-23).
2. Which element is a halogen?
Sodium · Chlorine ✓ · Argon · Calcium
Chlorine is in group 17.
3. How many electrons does a neutral oxygen atom (Z = 8) have?
6 · 8 ✓ · 16 · 2
Neutral atom: electrons = protons = 8.