Element Examples in Chemistry: 67 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Element.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A pure substance consisting entirely of atoms with the same number of protons (same atomic number), which cannot be broken down into simpler substances by chemical means.

A pure substance that can't be broken down chemically. Gold is gold, oxygen is oxygen.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Element starts by naming the element, charge, and relevant protons, neutrons, or electrons.

Common stuck point: Students often know a formula related to element but skip the recognition step: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using particle counts, nuclear charge, mass number, electron arrangement, or isotope notation to describe an atom or ion?

Worked Examples

Example 1

easy

Explain why gold (Au) and oxygen (O) are considered elements, while water (

\text{H}_2\text{O}

) is not.

Answer

Elements contain only one type of atom; compounds contain two or more.

First step

An element is a pure substance made of only one type of atom.

Full solution

2
Gold contains only gold atoms and oxygen contains only oxygen atoms, so both are elements.
3
Water contains two different types of atoms (hydrogen and oxygen) chemically bonded, making it a compound, not an element.

Elements are the simplest pure substances and cannot be broken down into simpler substances by chemical means. There are currently 118 known elements organized in the periodic table.

Example 2

medium

Iron (Fe) has atomic number 26. How many protons and electrons does a neutral iron atom have? If iron forms the

\text{Fe}^{3+}

ion, how many electrons does it have?

Example 3

medium

A neutral atom of an element has

17

protons. Identify the element, its group, and its likely ionic charge.

Example 4

medium

Element X has

20

protons. Write its electron configuration and identify which group and period it belongs to.

Example 5

medium

Chlorine occurs naturally as two isotopes:

^{35}\text{Cl}

(

75.77\%

, mass

34.97

) and

^{37}\text{Cl}

(

24.23\%

, mass

36.97

). Compute the atomic mass.

Use the table to compute the weighted-average atomic mass of chlorine.

Example 6

hard

An element X reacts with oxygen to form

\text{X}_2\text{O}_3

. If

5.40\,\text{g}

of X combines with

4.80\,\text{g}

of oxygen, what is the molar mass of X? Use

M(\text{O})=16

Example 7

hard

0.500\,\text{mol}

sample of an element has a mass of

32.0\,\text{g}

. Identify the element and its standard form at room temperature.

Example 8

hard

An oxide of element E has composition

80.0\%

E and

20.0\%

O by mass, with empirical formula EO. Find the molar mass of E. Use

M(\text{O})=16

Example 9

challenge

0.600\,\text{g}

sample of pure metal M is dissolved in excess acid, producing

0.560\,\text{L}

\text{H}_2

at STP from the reaction

\text{M} + 2\text{HCl} \rightarrow \text{MCl}_2 + \text{H}_2

. Identify M. (At STP,

1\,\text{mol}

gas

= 22.4\,\text{L}

Example 10

challenge

Element T has two stable isotopes with masses

62.93

and

64.93

. Its observed atomic mass is

63.55

. Find the percent abundance of each isotope.

Observed atomic mass = 63.55 u. Find the percent abundance of each isotope.

Example 11

easy

Sort into elements and compounds:

\text{Fe}

\text{H}_2

\text{CO}_2

\text{NH}_3

\text{Ne}

Sort each substance: element or compound?

Example 12

medium

A 50.0 g sample contains 25.0 g of element A and 25.0 g of element B, chemically bonded. Is it an element or compound?

Example 13

hard

A scientist isolates a substance that decomposes into 4.00 g of element A and 16.00 g of element B when heated, and confirms it has a fixed composition. Was the original substance an element?

Example 14

hard

Use mass conservation:

24\,\text{g}

of magnesium combines completely with oxygen to form

40\,\text{g}

of magnesium oxide. How many grams of oxygen reacted, and is MgO an element?

How many grams of oxygen reacted? Is MgO an element?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

Classify each as an element or compound: (a)

\text{Na}

, (b)

\text{CO}_2

, (c)

\text{Ne}

, (d)

\text{NaCl}

Classify each substance: element or compound?

Example 2

medium

Sample A has atoms with 8 protons and 8 electrons. Sample B has atoms with 8 protons and 10 electrons. Are they the same element? Explain.

Example 3

easy

What is a pure substance made of only one kind of atom called?

Example 4

easy

Is water (

\text{H}_2\text{O}

) an element or a compound?

Example 5

easy

Which of these is an element: table salt (

\text{NaCl}

), oxygen (

\text{O}_2

), or sugar?

Example 6

easy

How many elements are represented in a sample of pure nitrogen gas (

\text{N}_2

Example 7

easy

Are all elements solid at room temperature? Give a non-solid example.

Example 8

easy

Can an element be broken down into simpler substances by chemical means?

Example 9

easy

What property do all atoms of the same element share?

Example 10

easy

Gold can be hammered thin but stays gold. Does this physical change turn it into a different element?

Example 11

medium

Carbon dioxide (

\text{CO}_2

) can be separated into carbon and oxygen. What does this tell you about

\text{CO}_2

Example 12

medium

Element X exists as

\text{X}_4

molecules (like phosphorus,

\text{P}_4

). Is X an element?

Example 13

medium

A substance has atoms with proton counts of both 11 and 17. Is it an element or a compound?

Example 14

medium

There are about 118 known elements. Why can these few elements make millions of substances?

Example 15

medium

Diamond and graphite are both made only of carbon atoms. Are they the same element?

Example 16

medium

Air is mostly nitrogen and oxygen gases mixed together. Is air an element, a compound, or a mixture?

Example 17

medium

An unknown shiny solid conducts electricity and cannot be broken down chemically. What category does it fit?

Example 18

medium

Hydrogen peroxide (

\text{H}_2\text{O}_2

) breaks down into water and oxygen. How many elements are present in hydrogen peroxide?

Example 19

medium

Brass is copper and zinc blended together with no chemical bonding between them. Is brass an element?

Example 20

challenge

A 100 g sample is fully decomposed into 60 g of element A and 40 g of element B with nothing left over. Was the original sample an element, and what does mass conservation tell you?

Mass conservation: 60 + 40

Example 21

challenge

Substance 1: only atoms with

Z=8

. Substance 2: atoms with

Z=1

and

Z=8

. Classify each and name the elements involved.

Which is an element and which is a compound?

Example 22

challenge

Why is the periodic table organized by atomic number rather than by atomic mass?

Example 23

easy

Which of the following is a pure element? (a)

\text{H}_2\text{O}

, (b)

\text{CO}

, (c)

\text{Cu}

, (d)

\text{NH}_3

Example 24

easy

Sort the following into elements vs compounds:

\text{O}_2

\text{H}_2\text{SO}_4

\text{Fe}

\text{CH}_4

\text{He}

Sort each substance into elements vs. compounds.

Example 25

easy

How many different elements are present in glucose,

\text{C}_6\text{H}_{12}\text{O}_6

Example 26

easy

Which group of elements on the periodic table is known for being chemically inert?

Example 27

medium

Two atoms have

6

protons each, but one has

6

neutrons and the other has

7

neutrons. Are they the same element?

Example 28

medium

The molar mass of magnesium is

24.31\,\text{g/mol}

. How many moles are in a

48.62\,\text{g}

sample of pure Mg?

Example 29

medium

An element forms a

+2

cation by losing two electrons. If the resulting ion has

18

electrons, what is the element?

Example 30

medium

Calculate the percent by mass of nitrogen in ammonium nitrate,

\text{NH}_4\text{NO}_3

. Use

M(\text{N})=14

M(\text{H})=1

M(\text{O})=16

Example 31

medium

A neutral atom contains

15

protons,

16

neutrons, and

15

electrons. Identify the element and its mass number.

Example 32

medium

Which two of these are allotropes of the same element: graphite, ozone (

\text{O}_3

), diamond, table salt?

Example 33

hard

Element Q has an electron configuration ending in

\ldots 3d^{10}4s^{2}4p^{3}

. To which group and period does Q belong?

Example 34

hard

An unknown element forms an oxide with the empirical formula MO. If

4.00\,\text{g}

of M reacts with

1.60\,\text{g}

of O, find M. Use

M(\text{O})=16

Example 35

hard

Determine the number of atoms in

11.5\,\text{g}

of pure sodium (Na,

M = 23.0\,\text{g/mol}

). Use

N_A = 6.022 \times 10^{23}\,\text{mol}^{-1}

Example 36

hard

Of the elements Na, Mg, Al, and Si, which has the largest first ionization energy and why?

Example 37

easy

Which of these is a pure element:

\text{H}_2\text{O}

\text{Cu}

\text{NaCl}

\text{CH}_4

Example 38

easy

About how many elements are currently known?

Example 39

easy

Name an element that is a liquid at room temperature.

Example 40

easy

Is sulfur in

\text{S}_8

form considered an element?

Example 41

easy

Two atoms have 6 and 7 protons. Are they the same element?

Example 42

medium

Diamond and graphite both contain only carbon. Are they the same element? Are they the same substance?

Example 43

medium

Air contains nitrogen, oxygen, argon, and traces of others. Is air an element, compound, or mixture?

Example 44

medium

Stainless steel is made of iron, chromium, nickel, and carbon blended together. Is it an element?

Example 45

medium

Element X has atomic number 17 in one sample and atomic number 17 in another, but the two samples have masses 35 u and 37 u. Are they the same element?

Example 46

medium

Sodium (Na) reacts vigorously with chlorine (

\text{Cl}_2

) to form NaCl. Was a new element created?

Example 47

medium

An unknown shiny solid conducts electricity, can be hammered into sheets, and cannot be broken down chemically. What is it likely to be?

Example 48

hard

Why is hydrogen (H) sometimes placed in group 1 and sometimes in group 17 on the periodic table?

Example 49

hard

Element X has

Z = 79

. List its proton count, its likely conductivity, and one common use.

Example 50

hard

Element Q exists as a colorless gas that does not react with anything under normal conditions. To which family does it likely belong?

Example 51

hard

Compare an element (e.g.,

\text{O}_2

) and a compound (e.g.,

\text{O}_3

+ ozone) — both contain only oxygen. Why is

\text{O}_3

still considered an element?

Example 52

challenge

Helium (

\text{He}

) is the second-most abundant element in the universe but rare in Earth's atmosphere. Suggest one reason.

Example 53

challenge

Why is the periodic table organized by atomic number (

Z

) rather than by atomic mass?

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Related Concepts

Atom Periodic Table Compound

Background Knowledge

These ideas may be useful before you work through the harder examples.

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