Atomic structure and the periodic table (P1)

What is an atom?

An atom is the smallest part of an element that can exist. All substances are made of atoms.

How are elements represented?

Each element is represented by a chemical symbol, e.g., O for oxygen and Na for sodium.

How many elements are there?

There are about 100 different elements, which are shown in the periodic table.

What is a compound?

A compound is a substance formed when two or more elements chemically combine in fixed proportions

How are compounds formed?

Compounds are formed through chemical reactions, which create new substances and often involve a detectable energy change.

How can compounds be separated?

Compounds can only be separated into their elements by chemical reactions, not by physical means.

How are chemical reactions represented?

Chemical reactions can be shown using word equations or symbol equations with chemical formulae.

What should students be able to do with the periodic table?

1. Use the names and symbols of the first 20 elements, Group 1 and Group 7 elements, and others in the specification.

2. Name compounds from given formulae or symbol equations.

3. Write word equations for reactions.

4. Write formulae and balanced chemical equations.

What are half equations and ionic equations?

Half equations show electron transfer in reactions, while ionic equations focus on ions involved in chemical changes.

What is a mixture?

A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged.

What processes can mixtures be separated by? (5)

filtration, crystallisation, simple distillation, fractional distillation and chromatography

Why do scientific models change over time?

New experimental evidence may lead to a scientific model being changed or replaced as our understanding improves.

What was the atomic model before the discovery of the electron?

Before the electron was discovered, atoms were thought to be tiny, indivisible spheres.

What is the plum pudding model?

The plum pudding model suggested that an atom was a ball of positive charge with negative electrons embedded in it, like raisins in a pudding.

What experiment led to the rejection of the plum pudding model?

The alpha particle scattering experiment showed that most particles passed through, but some were deflected. This led to the discovery that an atom’s mass is concentrated in a small, positively charged nucleus.

How did the nuclear model differ from the plum pudding model?

• The plum pudding model had electrons embedded in a positively charged sphere.

• The nuclear model proposed a small, dense, positively charged nucleus with electrons orbiting around it.

How did Niels Bohr refine the nuclear model?

Bohr suggested that electrons orbit the nucleus at specific distances (energy levels). His calculations matched experimental observations.

What discovery led to the identification of protons?

Later experiments showed that the nucleus’s positive charge could be divided into smaller particles with equal positive charge, called protons.

What was James Chadwick’s contribution to atomic theory?

About 20 years after the nucleus was accepted, Chadwick provided evidence for the existence of neutrons, neutral particles found in the nucleus.

Why did the alpha scattering experiment change the atomic model?

It showed that atoms have a small, dense, positively charged nucleus instead of a diffuse positive charge, leading to the nuclear model.

What are the relative electrical charges of subatomic particles?

• Proton: +1 (positive)

• Neutron: 0 (neutral)

• Electron: -1 (negative)

Why do atoms have no overall charge?

an atom, the number of electrons (-1 charge) is equal to the number of protons (+1 charge), so the charges cancel out, making the atom neutral.

What is the atomic number of an element?

The atomic number is the number of protons in an atom. All atoms of the same element have the same number of protons.

How do atoms of different elements differ?

Atoms of different elements have different numbers of protons, which determines their identity as an element.

How small is an atom?

Atoms have a radius of about 0.1 nm (1 × 10⁻¹⁰ m).

How small is the nucleus compared to the atom?

The nucleus has a radius of about 1 × 10⁻¹⁴ m, which is less than 1/10,000 of the atom’s total size.

Where is most of an atom’s mass located?

Almost all of an atom’s mass is concentrated in the nucleus.

What are the relative masses of subatomic particles?

• Proton: 1

• Neutron: 1

• Electron: Very small (≈1/1836 of a proton)

What is the mass number of an atom?

The mass number is the sum of protons and neutrons in an atom’s nucleus.

What are isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

How can atoms be represented?

Atoms are represented using symbols that show their atomic number (number of protons) and mass number (protons + neutrons), e.g., ₁²³X.

How can you calculate the number of protons, neutrons, and electrons in an atom or ion?

• Protons = Atomic number

• Neutrons = Mass number - Atomic number

• Electrons (in a neutral atom) = Atomic number

• Electrons (in an ion) = Atomic number ± charge

What is the relative atomic mass of an element?

The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

Where do electrons occupy in an atom?

Electrons occupy the lowest available energy levels (innermost shells) first.

How can the electronic structure of an atom be represented?

The electronic structure can be shown using numbers (e.g., sodium = 2,8,1) or diagrams with electrons in shells.

What does the electronic structure 2,8,1 mean?

It means the atom has:

• 2 electrons in the first energy level (closest to the nucleus)

• 8 electrons in the second energy level

• 1 electron in the third energy level

Can energy levels be called shells?

Yes, energy levels are also referred to as shells in atomic structure discussions.

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How are elements arranged in the periodic table?

Elements are arranged in order of atomic (proton) number and in groups (columns) where elements have similar properties.

Why is it called the periodic table?

It is called the periodic table because elements with similar properties appear at regular intervals.

What do elements in the same group have in common?

Elements in the same group have the same number of outer shell electrons, giving them similar chemical properties.

How is an element’s position in the periodic table related to its electron arrangement?

Period (row) = Number of electron shells

• Group (column) = Number of outer shell electrons

• Atomic number increases from left to right across a period.

How can the periodic table be used to predict reactions?

An element’s group number helps predict its reactivity:

• Group 1 (alkali metals) become more reactive down the group.

• Group 7 (halogens) become less reactive down the group.

How were elements classified before the discovery of subatomic particles?

Scientists arranged elements in order of atomic weights before protons, neutrons, and electrons were discovered.

What were the problems with early periodic tables?

• They were incomplete.

• Some elements were placed in incorrect groups if arranged strictly by atomic weight.

How did Mendeleev improve the periodic table?

• Left gaps for undiscovered elements.

• Rearranged some elements to fit properties rather than strictly following atomic weight.

How was Mendeleev’s periodic table proven correct?

Elements matching Mendeleev’s predictions were later discovered, filling the gaps in his table.

How did the discovery of isotopes explain Mendeleev’s table?

Isotopes have different atomic weights but the same chemical properties, explaining why atomic weight order did not always match element properties.

What key steps led to the modern periodic table?

1. Elements first arranged by atomic weight.

2. Mendeleev left gaps and rearranged some elements.

3. New elements discovered, confirming his predictions.

4. Discovery of isotopes explained inconsistencies in atomic weight order.

What type of elements form positive ions?

Metals form positive ions when they react.

What type of elements do not form positive ions?

Non-metals do not form positive ions.

Where are metals found in the periodic table?

Metals are found on the left and towards the bottom of the periodic table.

Where are non-metals found in the periodic table?

Non-metals are found on the right and towards the top of the periodic table.

What are the key differences between metals and non-metals?

Metals:

• Good conductors of heat and electricity

• High melting and boiling points

• Malleable and ductile (can be shaped and stretched)

• Tend to lose electrons in reactions

Non-Metals:

• Poor conductors (insulators)

• Lower melting and boiling points

• Brittle (if solid)

• Tend to gain or share electrons in reactions

How does atomic structure relate to metal and non-metal positions in the periodic table?

• Metals have fewer outer electrons and tend to lose them, making them reactive and forming positive ions.

• Non-metals have more outer electrons, so they gain or share electrons in reactions.

How do an element’s reactions relate to its atomic structure?

The arrangement of electrons in an atom determines how it reacts:

• Group 1 metals lose 1 electron easily (very reactive).

• Group 7 non-metals gain 1 electron easily (very reactive).

• Noble gases (Group 0) have full outer shells and are unreactive

What are the noble gases and why are they unreactive?

Noble gases (Group 0) have stable electron arrangements. They typically have eight electrons in their outer shell (except helium, which has two), making them unreactive and unlikely to form molecules.

How do the boiling points of the noble gases change within the group?

The boiling points of noble gases increase with increasing relative atomic mass as you go down the group.

What characterizes the alkali metals (Group 1) in terms of electron configuration?

Alkali metals have a single electron in their outer shell, which makes them highly reactive.

How does the reactivity of alkali metals change down Group 1?

The reactivity increases going down the group due to a decrease in ionization energy, making it easier to lose the outer electron.

What are typical reactions of the first three alkali metals?

• With oxygen: They form oxides.

• With chlorine: They form chlorides.

• With water: They react vigorously to produce hydroxides and hydrogen gas.

Students should be able to describe these reaction types in more detail.

What defines the halogens (Group 7) regarding their electron configuration and molecular form?

Halogens have seven electrons in their outer shell and exist as diatomic molecules (e.g., Cl₂, Br₂, I₂), which gives them similar chemical reactivity.

How do the physical properties of halogens change down the group?

As you move down Group 7, the relative molecular mass, melting point, and boiling point increase.

How does the reactivity of halogens vary within Group 7?

The reactivity decreases going down the group. A more reactive halogen can displace a less reactive one from an aqueous solution of its salt.

What types of compounds do halogens form when reacting with metals and non-metals?

• With metals: Halogens typically form ionic compounds (halides).

• With non-metals: They form covalent compounds.

Students should be able to describe the nature of these compounds and the underlying reactions.

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What are transition elements?

Transition elements are metals that have similar properties to each other but differ from Group 1 metals. They generally have higher melting points, greater densities, and are harder and stronger compared to Group 1 metals.

What physical property differences should you know between transition elements and Group 1 metals?

• Melting Points & Densities: Transition elements have much higher melting points and densities than Group 1 metals.

• Strength & Hardness: Transition metals are stronger and harder.

• Reactivity: Group 1 metals are very reactive (e.g., with water and oxygen) while transition elements are less reactive under the same conditions.

How do transition elements react with oxygen, water, and halogens compared to Group 1 metals?

• Group 1 Metals: React vigorously with water and oxygen, forming oxides and hydroxides rapidly.

• Transition Elements: React more slowly with oxygen, water, and halogens; their reactions often require higher temperatures or catalysts.

Which elements are common examples of transition elements, and what are their general properties?

Examples include chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu).

Students should know that these elements demonstrate high melting points, high densities, and significant strength and hardness, setting them apart from Group 1 metals.

What does it mean that transition elements have ions with different charges?

Transition elements can form ions in more than one oxidation state. For example, an element like manganese (Mn) can form Mn²⁺, Mn⁴⁺, or Mn⁷⁺ ions. This versatility influences their chemical reactivity and the types of compounds they form.

Why do transition elements form colored compounds?

Transition metal compounds are often colored because of d–d electron transitions. When light hits these compounds, electrons in the d orbitals absorb certain wavelengths, resulting in vivid colors. For instance, copper (Cu²⁺) compounds often appear blue.

How are transition elements useful as catalysts?

Their ability to exist in multiple oxidation states and form complexes makes transition elements excellent catalysts. This allows them to facilitate chemical reactions by providing alternative reaction pathways with lower activation energies

What are some examples of transition elements that illustrate these properties? (6)

• Chromium (Cr): Forms colorful compounds like chromium oxide used in pigments and catalysts.

• Manganese (Mn): Exhibits multiple oxidation states; permanganate (MnO₄⁻) is a strong oxidizing agent with a deep purple color.

• Iron (Fe): Known for compounds like rust (iron oxides) and as a catalyst in industrial processes.

• Cobalt (Co): Cobalt(II) chloride is well known for its distinct blue color in hydrated form.

• Nickel (Ni): Forms green-colored compounds and is used in catalytic processes, such as in hydrogenation reactions.

• Copper (Cu): Copper(II) sulfate is a classic blue compound and copper catalysts are widely used in organic reactions.

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