GCSE Chemistry: Atoms, Elements, Compounds, Balancing Equations, History of the Atom, Isotopes, and Mixtures

Comprehensive practice flashcards covering atoms, elements, compounds, chemical balancing, the historical evolution of the nuclear atom, subatomic particles, isotopes, relative atomic mass calculations, and physical separation techniques for mixtures.

What is the definition of an atom?

The smallest part of an element that can exist.

Approximately how many atoms are contained within a standard 50p coin?

Around $77,400,000,000,000,000,000,000$ atoms.

What labelled components comprise the nuclear model of an atom?

A central nucleus containing protons and neutrons, surrounded by electrons orbiting in electron shells.

What is the radius of an atomic nucleus, and how does it compare to the overall size of an atom?

The radius is $1 \times 10^{-14}\,m$, which is about $\frac{1}{10\,000}$ the size of an atom.

Where is nearly all of the mass of an atom concentrated, and why?

In the nucleus, because electrons have very little mass compared to protons and neutrons.

Why do atoms have no overall electrical charge?

The number of protons always equals the number of electrons, so the positive and negative charges cancel each other out.

What is an element?

A substance made up of only one type of atom (e.g., pure carbon or silver).

What fundamental property determines the identity of an element?

The number of protons in the nucleus of its atoms (for example, all carbon atoms have $6$ protons).

What is a compound?

A substance formed when two or more elements are held together by chemical bonds.

How many atoms of each element are present in aluminium sulphate, $\text{Al}_2(\text{SO}_4)_3$?

$2$ aluminium atoms, $3$ sulphur atoms, and $12$ oxygen atoms.

How many atoms of each element are present in ammonium sulphate, $(\text{NH}_4)_2\text{SO}_4$?

$2$ nitrogen atoms, $8$ hydrogen atoms, $1$ sulphur atom, and $4$ oxygen atoms.

Why must small subscript numbers never be changed when balancing a chemical equation?

The small subscript numbers define how atoms are chemically bonded together; altering them changes the substance's chemical identity into a different or non-existent compound (for example, changing $\text{H}_2\text{O}$ to $\text{H}_2\text{O}_2$ produces toxic hydrogen peroxide).

How is the reaction between magnesium and oxygen balanced: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$?

$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$

How is the reaction between aluminium and chlorine gas balanced: $\text{Al} + \text{Cl}_2 \rightarrow \text{AlCl}_3$?

$$2\text{Al} + 3\text{Cl}_2 \rightarrow 2\text{AlCl}_3$$

What is a diatomic molecule?

A molecule composed of only two atoms, such as $\text{O}_2$, $\text{H}_2$, and $\text{Cl}_2$ (all halogens).

What atomic concept was proposed by Democritus around 400 BC?

He proposed that all matter is made of indivisible, indestructible particles called atoms ('atomos' meaning indivisible).

How did John Dalton describe atoms in the 1800s?

Dalton believed that atoms were solid spheres and that different elements consisted of spheres of different sizes.

What atomic model is depicted in this 1897 illustration?

J.J. Thomson's 'plum pudding model', which describes the atom as a spherical cloud of positive charge with negative electrons embedded throughout.

What did Ernest Rutherford discover from the alpha particle scattering experiment around 1910?

By firing positive alpha particles at a thin gold sheet, he observed that a small fraction (about 1 in 8000) was repelled, proving the atom consists mostly of empty space with a tiny, positively charged nucleus at the centre containing almost all the mass.

What modification did Niels Bohr introduce to the nuclear model of the atom in 1913?

He proposed that electrons orbit the nucleus at fixed distances within specific energy levels or shells.

What subatomic particles were discovered by Ernest Rutherford in 1918 and James Chadwick in 1932, respectively?

Rutherford discovered positive protons in the nucleus, and Chadwick discovered neutral neutrons in the nucleus.

What are the relative charges and masses of a proton, neutron, and electron?

Proton: charge $+1$, mass $1$; Neutron: charge $0$, mass $1$; Electron: charge $-1$, mass $\frac{1}{2000}$ or $0$ (too small to measure).

How is the number of neutrons in an atom calculated?

$$\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}$$

How many protons, electrons, and neutrons are present in an atom of sodium represented by $^{23}_{11}\text{Na}$?

$11$ protons, $11$ electrons, and $12$ neutrons ($23 - 11 = 12$).

What is the definition of isotopes?

Atoms of the same element that have the same number of protons (and electrons) but different numbers of neutrons.

What is the formula used to calculate the relative atomic mass ($A_r$) of an element from its isotopic data?

$$A_r = \frac{\sum (\text{isotope abundance} \times \text{isotope mass number})}{100}$$

If naturally occurring chlorine consists of $75\%$ $^{35}\text{Cl}$ and $25\%$ $^{37}\text{Cl}$, what is its relative atomic mass ($A_r$)?

$$A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = \frac{2625 + 925}{100} = 35.5$$

Silicon occurs naturally as $92\%$ $^{28}\text{Si}$, $5\%$ $^{29}\text{Si}$, and $3\%$ $^{30}\text{Si}$. What is its relative atomic mass ($A_r$)?

$$A_r = \frac{(92 \times 28) + (5 \times 29) + (3 \times 30)}{100} = \frac{2576 + 145 + 90}{100} = 28.1$$

What is a mixture, and what happens to the chemical properties of its components?

A mixture consists of two or more substances not chemically bonded together; the chemical properties of each individual substance remain unchanged.

What are the major volume percentages of gases present in dry air?

Nitrogen: $78.08\%$, Oxygen: $20.95\%$, Argon: $0.93\%$, and Other gases: $0.04\%$.

Which separation method is used to separate an insoluble solid from a liquid mixture?

Filtration (e.g., separating sand from water).

How do evaporation and crystallisation differ when separating a soluble solid from a liquid?

Evaporation is quicker and involves heating until all liquid is driven off, whereas crystallisation involves heating until crystals just start to form, then allowing the solvent to slowly evaporate to yield larger crystals.

How does simple distillation separate a liquid solvent from a dissolved solid?

The mixture is heated so the component with the lowest boiling point evaporates; its vapour passes into a condenser where it is cooled into a liquid and collected, leaving substances with higher boiling points behind in the flask.

What laboratory setup is shown in this diagram, and when is it preferred over simple distillation?

Fractional distillation; it is used when separating mixtures of liquids that have boiling points close together or mixtures containing many different liquids.

During fractional distillation of a mixture containing propanoic acid (boiling point $141\,^\circ\text{C}$) and butanoic acid (boiling point $164\,^\circ\text{C}$), which substance is collected first?

Propanoic acid, because it has the lower boiling point ($141\,^\circ\text{C}$).

Why can simple distillation NOT successfully separate a mixture of ethanol (boiling point $78\,^\circ\text{C}$) and propanol (boiling point $97\,^\circ\text{C}$)?

The boiling points of ethanol and propanol are too close together for simple distillation to separate them successfully.