Atomic Structure
ICSE Class 7 · Chemistry · Chapter 4
What this chapter covers
- Atoms: the building blocks of matter
- Molecules
- Atomicity
- The first 20 elements: names and symbols
- Valency: combining capacity
- Radicals
- Valency and the periodic table
Key points
- Every substance is made of tiny particles called atoms; atoms are the building blocks of matter.
- An atom is the smallest particle of an element; most atoms cannot exist on their own, except those of the noble gases.
- The properties of an element depend upon the atoms that make it up.
- A molecule is the smallest particle of an element or a compound that can exist independently and shows all the properties of the substance.
- Molecules of elements contain one kind of atom; molecules of compounds contain atoms of different elements.
- Atomicity is the number of atoms in one molecule: monoatomic (He), diatomic (O2), triatomic (O3, H2O) and polyatomic (P4, CH4).
- A symbol has a capital first letter and, if needed, a small second letter; Na and K come from Latin names.
- The first 20 elements, in order, are H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K and Ca.
- Valency is the combining capacity of an element: the number of hydrogen atoms that one atom combines with or replaces.
- A radical is a charged single atom or group of atoms that acts as one unit; basic radicals are positive and acid radicals are negative.
- The valency of a radical equals the size of its charge, and it can be found from the hydrogen atoms it combines with or replaces.
- The periodic table groups elements with similar properties; elements in one group usually share a valency, which runs 1, 2, 3, 4, 3, 2, 1, 0 across the second row (Li to Ne) and again across the third row (Na to Ar).
Questions answered
Define an atom. What do the atoms of most elements do, since they cannot exist on their own? Which atoms are the exception?
An atom is the smallest particle of an element that can take part in a chemical change. The atoms of most elements join with other atoms to form molecules. The atoms of the noble gases, such as helium, neon and argon, are the exception: they stay single.
What is the difference between H and H2?
H stands for one atom of hydrogen, which does not exist on its own. H2 stands for one molecule of hydrogen gas, made of two hydrogen atoms, which can exist independently.
Define a molecule and give one example each of a molecule of an element and a molecule of a compound.
A molecule is the smallest particle of an element or a compound that is capable of independent existence and shows all the properties of that substance. Oxygen, O2, is a molecule of an element; water, H2O, is a molecule of a compound.
State the atomicity of each and classify it: (a) Cl2 (b) CO2 (c) Ne (d) CH4.
(a) 2, diatomic. (b) 3, triatomic. (c) 1, monoatomic. (d) 5, polyatomic.
Write the symbols of sodium, potassium, chlorine, calcium and magnesium. Which two come from Latin names?
Na, K, Cl, Ca and Mg. Sodium (Na, from natrium) and potassium (K, from kalium) come from Latin names.
A student wrote the symbol for cobalt as CO. Explain the mistake.
The second letter of a symbol must be small, so cobalt is Co. CO means one atom of carbon joined to one atom of oxygen, which is carbon monoxide.
Define valency in terms of hydrogen. Find the valency of nitrogen from ammonia, NH3.
Valency of an element is the number of hydrogen atoms that combine with, or are replaced by, one atom of that element. In NH3 one nitrogen atom combines with 3 hydrogen atoms, so the valency of nitrogen is 3.
Magnesium reacts with dilute hydrochloric acid to form MgCl2. What is the valency of magnesium, and why?
2. One magnesium atom replaces two hydrogen atoms (it takes the place of the hydrogen in two units of HCl), so it holds two chlorine atoms.