Chemistry Revision - 1

The mass number of an element indicates the amount of protons + neutrons in the element. The mass number is the sum of the number of protons and neutrons in the nucleus of an atom. Electrons are not included in the mass number because they have a negligible mass compared to protons and neutrons. Nucleons refer to both protons and neutrons, so the answer "protons + nucleons" is incorrect.

When a substance reacts with oxygen, it forms an oxide. Oxides are compounds that contain oxygen and another element. This reaction occurs when oxygen combines with a metal, non-metal, or other elements. Oxides can have various properties and can be solid, liquid, or gas depending on the specific substance and conditions. They play important roles in many chemical reactions and are commonly found in nature as minerals or as components of various materials.

In a chemical reaction, the substances that are present before the arrow are called reactants. Reactants are the starting materials that undergo a chemical change to form new substances, which are called products. Bases and acids are types of substances that can be reactants or products in a chemical reaction, but they are not the general term used to describe the substances before the arrow. Therefore, the correct answer is reactants.

All acids contain hydrogen. When an acid dissolves in water, it releases hydrogen ions (H+), which are responsible for the acidic properties of the substance. Therefore, hydrogen is the element that occurs in all acids. Chlorine, oxygen, and sulphur can be present in certain acids, but they are not present in all acids.

Elements in the same group have the same amount of valence electrons. Valence electrons are the electrons in the outermost energy level of an atom and they determine the chemical properties of an element. Elements in the same group have similar chemical properties because they have the same number of valence electrons, which determines how they interact with other atoms to form compounds. Therefore, valence electrons are the correct answer.

The reaction "hydrogen + oxygen → water" indicates that both hydrogen and oxygen are involved in the reaction. Hydrogen and oxygen combine to form water, so they are the reactants in this reaction.

The elements on the Periodic Table are arranged according to increasing atomic number. This means that the elements are ordered based on the number of protons in their nucleus, which is unique to each element. As you move from left to right across a period, the atomic number increases by one, indicating the addition of one proton to the nucleus. This arrangement allows for a systematic organization of elements and helps to identify their properties and trends.

Chlorine is the correct answer because it naturally occurs as a diatomic molecule, meaning it exists as a molecule composed of two atoms of chlorine bonded together. Chlorine gas (Cl2) is a greenish-yellow gas that is commonly found in nature. In contrast, helium, phosphorous, and sodium do not naturally occur as diatomic molecules.

Bromine and iodine are both halogens, which are a group of elements on the periodic table. Halogens are known for their high reactivity and tendency to form salts. They are located in Group 17 of the periodic table and include elements such as fluorine, chlorine, bromine, iodine, and astatine. Therefore, the correct answer is halogens.

Metals lie to the left of the zigzag line on the Periodic Table. The zigzag line separates the metals from the non-metals and metalloids. Metals are typically found on the left side of the periodic table and are known for their shiny appearance, good conductivity of heat and electricity, and malleability. They make up the majority of the elements on the periodic table.

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Water is a compound made up of two elements, hydrogen and oxygen. It is not considered an element itself. Hydrogen, mercury, and calcium are all elements on the periodic table.

The correct answer is CO3 because the part that is the same in all formulas of carbonates is the carbonate ion (CO3). Carbonates are compounds that contain this specific polyatomic ion, which consists of one carbon atom bonded to three oxygen atoms. This ion is responsible for the characteristic properties and behavior of carbonates.

Elements in the same period have the same amount of main energy levels. In the periodic table, periods are the horizontal rows, and each period represents a different energy level. The number of the period indicates the number of energy levels that the elements in that period have. Therefore, elements in the same period have the same amount of main energy levels.

Group 18 elements on the Periodic Table are known as noble gases because they have a full outer electron shell, making them stable and unreactive. They are characterized by their low boiling points, low densities, and lack of color, odor, and taste. These elements include helium, neon, argon, krypton, xenon, and radon. They are called noble gases because they were once believed to be completely inert and noble in their behavior.