Electrons occupy different energy levels around the nucleus, and those in the outermost shell influence how an element interacts with other atoms. Which term refers to the electrons found in the outermost electron shell of an atom?

A pure substance has a consistent composition and characteristic properties throughout the sample. Which statement best defines a pure substance?

Earth appears blue when viewed from space because a large portion of its surface is covered by water. Approximately what percentage of Earth's surface is covered by water?

A neutral atom has no overall electrical charge because its positively charged protons balance its negatively charged electrons. If a neutral atom contains 13 protons, how many electrons must it contain?

Gold consists of only one kind of atom, while water contains hydrogen and oxygen chemically combined in definite proportions. Which explanation correctly distinguishes gold from water?

An image of Earth taken from space shows extensive blue areas and smaller visible landmasses. Which explanation best accounts for Earth's predominantly blue appearance?

Aluminum-27 has an atomic number of 13 and a mass number of 27, and its atom is electrically neutral. How many protons, neutrons, and electrons does an aluminum-27 atom contain, respectively?

A neutral atom contains 17 electrons, which are arranged into shells as 2–8–7 according to the simplified shell model. How many valence electrons does this atom have?

A student attempts to separate water into hydrogen and oxygen using ordinary filtration, but the process does not separate the chemically combined elements. Which method would be more appropriate for separating the elements in water?

A neutral oxygen atom has an atomic number of 8, which identifies the number of protons in its nucleus. How many protons are present in the nucleus of this oxygen atom?

A science project uses a 200-square-centimeter model to represent Earth's surface, and land accounts for approximately 29% of the total area. How many square centimeters should represent land in the model?

A farmer observes that crops are growing poorly because the soil lacks sufficient nutrients and the plants receive inadequate water. Which two Earth spheres are most directly involved in  addressing the plants' immediate needs for soil and water?

A learner calculates the particle counts of a neutral atom with an atomic number of 15 and a mass number of 31, reporting 15 protons, 16 neutrons, and 16 electrons. Which part of the learner’s calculation contains an error?

Three elements have the following simplified electron arrangements: Element P is 2–1, Element Q is 2–8–1, and Element R is 2–8–7. Which comparison correctly identifies the elements that belong to the same main group?

The electron arrangements of sodium and oxygen are 2–8–1 and 2–6, respectively, in the simplified shell model. Which statement correctly distinguishes their positions in the periodic table?

A laboratory contains gold (Au), oxygen (O), water (H₂O), and carbon dioxide (CO₂), all of which are examples of substances. Which classification correctly separates the elements from the compounds?

A mixture of sand and salt can be separated using appropriate physical methods, whereas water must undergo a chemical process to decompose into hydrogen and oxygen. Which comparison BEST explains the difference between these separation processes?

A forest ecosystem contains trees, soil, flowing water, and air that interact to support living organisms. Which classification correctly identifies the four components?

A learner labels a diagram with 71% land and 29% water, reversing the proportion presented in the lesson. Which correction best identifies the learner's mistake?

A laboratory report describes a neutral atom with 20 protons, 20 neutrons, and 20 electrons, while its mass number is recorded as 40. Which judgment best evaluates the consistency of the report?

An unknown element has seven valence electrons and belongs to the same main group as other elements with seven valence electrons. Which prediction is best supported by the relationship between valence electrons and chemical properties?

A learner argues that two atoms must belong to the same element if they have the same number of neutrons, even when their proton counts differ. Which evaluation is scientifically justified?

A school wants to reduce freshwater consumption because only a limited amount of Earth's freshwater is readily available for human use. Which proposed action would BEST support this goal?